Mathematics & Statistics (School of)http://hdl.handle.net/10023/282019-01-23T12:11:28Z2019-01-23T12:11:28ZA review of unmanned vehicles for the detection and monitoring of marine faunaVerfuss, Ursula K.Aniceto, Ana SofiaHarris, Danielle V.Gillespie, DouglasFielding, SophieJiménez, GuillermoJohnston, PhilSinclair, Rachael R.Sivertsen, AgnarSolbø, Stian A.Storvold, RuneBiuw, MartinWyatt, Royhttp://hdl.handle.net/10023/169242019-01-23T11:30:05Z2019-03-01T00:00:00ZRecent technology developments have turned present-day unmanned systems into realistic alternatives to traditional marine animal survey methods. Benefits include longer survey durations, improved mission safety, mission repeatability, and reduced operational costs. We review the present status of unmanned vehicles suitable for marine animal monitoring conducted in relation to industrial offshore activities, highlighting which systems are suitable for three main monitoring types: population, mitigation, and focal animal monitoring. We describe the technical requirements for each of these monitoring types and discuss the operational aspects. The selection of a specific sensor/platform combination depends critically on the target species and its behaviour. The technical specifications of unmanned platforms and sensors also need to be selected based on the surrounding conditions of a particular offshore project, such as the area of interest, the survey requirements and operational constraints.
This work was supported by the Joint Industry Programme on E&P Sound and Marine Life - Phase III. We would also like to thank the Research Centre for Arctic Exploration (ARCEx), funded by the Research Council of Norway (project #228107).
2019-03-01T00:00:00ZVerfuss, Ursula K.Aniceto, Ana SofiaHarris, Danielle V.Gillespie, DouglasFielding, SophieJiménez, GuillermoJohnston, PhilSinclair, Rachael R.Sivertsen, AgnarSolbø, Stian A.Storvold, RuneBiuw, MartinWyatt, RoyRecent technology developments have turned present-day unmanned systems into realistic alternatives to traditional marine animal survey methods. Benefits include longer survey durations, improved mission safety, mission repeatability, and reduced operational costs. We review the present status of unmanned vehicles suitable for marine animal monitoring conducted in relation to industrial offshore activities, highlighting which systems are suitable for three main monitoring types: population, mitigation, and focal animal monitoring. We describe the technical requirements for each of these monitoring types and discuss the operational aspects. The selection of a specific sensor/platform combination depends critically on the target species and its behaviour. The technical specifications of unmanned platforms and sensors also need to be selected based on the surrounding conditions of a particular offshore project, such as the area of interest, the survey requirements and operational constraints.Rigidity for sticky disksConnelly, RobertGortler, Steven J.Theran, Louis Simonhttp://hdl.handle.net/10023/168922019-01-21T11:30:04Z2019-01-14T00:00:00ZWe study the combinatorial and rigidity properties of disk packings with generic radii. We show that a packing of n disks in the plane with generic radii cannot have more than 2n − 3 pairs of disks in contact. The allowed motions of a packing preserve the disjointness of the disk interiors and tangency between pairs already in contact (modeling a collection of sticky disks). We show that if a packing has generic radii, then the allowed motions are all rigid body motions if and only if the packing has exactly 2n − 3 contacts. Our approach is to study the space of packings with a fixed contact graph. The main technical step is to show that this space is a smooth manifold, which is done via a connection to the Cauchy-Alexandrov stress lemma. Our methods also apply to jamming problems, in which contacts are allowed to break during a motion. We give a simple proof of a finite variant of a recent result of Connelly, et al. [1] on the number of contacts in a jammed packing of disks with generic radii.
2019-01-14T00:00:00ZConnelly, RobertGortler, Steven J.Theran, Louis SimonWe study the combinatorial and rigidity properties of disk packings with generic radii. We show that a packing of n disks in the plane with generic radii cannot have more than 2n − 3 pairs of disks in contact. The allowed motions of a packing preserve the disjointness of the disk interiors and tangency between pairs already in contact (modeling a collection of sticky disks). We show that if a packing has generic radii, then the allowed motions are all rigid body motions if and only if the packing has exactly 2n − 3 contacts. Our approach is to study the space of packings with a fixed contact graph. The main technical step is to show that this space is a smooth manifold, which is done via a connection to the Cauchy-Alexandrov stress lemma. Our methods also apply to jamming problems, in which contacts are allowed to break during a motion. We give a simple proof of a finite variant of a recent result of Connelly, et al. [1] on the number of contacts in a jammed packing of disks with generic radii.Density estimation of sound-producing terrestrial animals using single automatic acoustic recorders and distance samplingSebastián-González, EstherCamp, Richard J.Tanimoto, Ann M.de Oliveira, Priscilla M.Lima, Bruna B.Marques, Tiago A.Hart, Patrick J.http://hdl.handle.net/10023/168892019-01-21T11:30:04Z2018-12-01T00:00:00ZObtaining accurate information on the distribution, density, and abundance of animals is an important first step toward their conservation. Methodological approaches using automatic acoustic recorders for species that communicate acoustically are gaining increased interest because of their advantages over traditional sampling methods. In this study, we created and evaluated a protocol to estimate population density, which can be used to compute abundance of terrestrial sound-producing animals from single automatic acoustic recorders and using an automatic detection algorithm. The protocol uses cue rates from the target species, environmental conditions, and an estimate of the distance of the individual to the recorder based on the power of the received sound. We applied our protocol to estimate the density of a Hawaiian forest bird species (Hawaiˊi ˊAmakihi [Chlorodrepanis virens]) on the island of Hawaiˊi, USA. We validated our approach by comparing our density estimates with those calculated at the same stations using a traditional point-transect distance sampling method based on human observations. Overall density estimates based on recorded signals were lower than those based on human observations, but 95% confidence intervals of the two density estimates overlapped. This study presents a relatively simple but effective protocol for estimating animal density using single automatic acoustic recorders. Our protocol may easily be adapted to other sound-emitting terrestrial animals.
Financial support was provided by the NSF award #1345247 to D. Price, P. Hart, E. Stacy, and M. Takabayashi. ESG is funded by the Juan de la Cierva program from the Spanish Government (IJCI-2015-24947). TAM thanks partial support by CEAUL (funded by FCT - Fundação para a Ciência e a Tecnologia, Portugal, through the project UID/MAT/00006/2013). RJC is partially funded through the U.S. Geological Survey and the University of St. Andrews.
2018-12-01T00:00:00ZSebastián-González, EstherCamp, Richard J.Tanimoto, Ann M.de Oliveira, Priscilla M.Lima, Bruna B.Marques, Tiago A.Hart, Patrick J.Obtaining accurate information on the distribution, density, and abundance of animals is an important first step toward their conservation. Methodological approaches using automatic acoustic recorders for species that communicate acoustically are gaining increased interest because of their advantages over traditional sampling methods. In this study, we created and evaluated a protocol to estimate population density, which can be used to compute abundance of terrestrial sound-producing animals from single automatic acoustic recorders and using an automatic detection algorithm. The protocol uses cue rates from the target species, environmental conditions, and an estimate of the distance of the individual to the recorder based on the power of the received sound. We applied our protocol to estimate the density of a Hawaiian forest bird species (Hawaiˊi ˊAmakihi [Chlorodrepanis virens]) on the island of Hawaiˊi, USA. We validated our approach by comparing our density estimates with those calculated at the same stations using a traditional point-transect distance sampling method based on human observations. Overall density estimates based on recorded signals were lower than those based on human observations, but 95% confidence intervals of the two density estimates overlapped. This study presents a relatively simple but effective protocol for estimating animal density using single automatic acoustic recorders. Our protocol may easily be adapted to other sound-emitting terrestrial animals.Classification of animal dive tracks via automatic landmarking, principal components analysis and clusteringWalker, CameronMacKenzie, Monique LeaDonovan, Carl RobertHastie, Gordon DrummondQuick, Nicola JaneKidney, Darrenhttp://hdl.handle.net/10023/168612019-01-14T17:30:13Z2011-08-19T00:00:00ZThe behaviour of animals and their interactions with the environment can be inferred by tracking their movement. For this reason, biologgers are an important source of ecological data, but analysing the shape of the tracks they record is difficult. In this paper we present a technique for automatically determining landmarks that can be used to analyse the shape of animal tracks. The approach uses a parametric version of the SALSA algorithm to fit regression splines to 1‐dimensional curves in N dimensions (in practice N = 2 or 3). The knots of these splines are used as landmarks in a subsequent Principal Components Analysis, and the dives classified via agglomerative clustering. We demonstrate the efficacy of this algorithm on simulated 2‐dimensional dive data, and apply our method to real 3‐dimensional whale dive data from the Behavioral Response Study (BRS) in the Bahamas. The BRS is a series of experiments to quantify shifts in behavior due to SONAR. Our analysis of 3‐dimensional track data supports an alteration in the dive behavior post‐ensonification.
The BRS study was financially supported by the United States (U.S.) Office of Naval Research (www.onr.navy.mil) Grants N00014‐07‐10988, N00014‐07‐11023, N00014‐08‐10990; the U.S. Strategic Environmental Research and Development Program (www.serdp.org) Grant SI‐1539, the Environmental Readiness Division of the U.S. Navy (http://www.navy.mil/local/n45/), the U.S. Chief of Naval Operations Submarine Warfare Division (Undersea Surveillance), the U.S. National Oceanic and Atmospheric Administration (National Marine Fisheries Service, Office of Science and Technology) (http://www.st.nmfs.noaa.gov/), U.S. National Oceanic and Atmospheric Administration Ocean Acoustics Program (http://www.nmfs.noaa.gov/pr/acoustics/), and the Joint Industry Program on Sound and Marine Life of the International Association of Oil and Gas Producers (www.soundandmarinelife.org).
2011-08-19T00:00:00ZWalker, CameronMacKenzie, Monique LeaDonovan, Carl RobertHastie, Gordon DrummondQuick, Nicola JaneKidney, DarrenThe behaviour of animals and their interactions with the environment can be inferred by tracking their movement. For this reason, biologgers are an important source of ecological data, but analysing the shape of the tracks they record is difficult. In this paper we present a technique for automatically determining landmarks that can be used to analyse the shape of animal tracks. The approach uses a parametric version of the SALSA algorithm to fit regression splines to 1‐dimensional curves in N dimensions (in practice N = 2 or 3). The knots of these splines are used as landmarks in a subsequent Principal Components Analysis, and the dives classified via agglomerative clustering. We demonstrate the efficacy of this algorithm on simulated 2‐dimensional dive data, and apply our method to real 3‐dimensional whale dive data from the Behavioral Response Study (BRS) in the Bahamas. The BRS is a series of experiments to quantify shifts in behavior due to SONAR. Our analysis of 3‐dimensional track data supports an alteration in the dive behavior post‐ensonification.Observing the simulations : applying ZDI to 3D non-potential magnetic field simulationsLehmann, L. T.Hussain, G. A. J.Jardine, M. M.Mackay, D. H.Vidotto, A. A.http://hdl.handle.net/10023/168372019-01-10T16:30:06Z2019-03-11T00:00:00ZThe large-scale magnetic fields of stars can be obtained with the Zeeman-Doppler-Imaging (ZDI) technique, but their interpretation is still challenging as the contribution of the small-scale field or the reliability of the reconstructed field properties is still not fully understood. To quantify this, we use 3D non-potential magnetic field simulations for slowly rotating solar-like stars as inputs to test the capabilities of ZDI. These simulations are based on a flux transport model connected to a non-potential coronal evolution model using the observed solar flux emergence pattern. We first compare four field prescriptions regarding their reconstruction capabilities and investigate the influence of the spatial resolution of the input maps on the corresponding circularly polarised profiles. We then generate circularly polarised spectra based on our high resolution simulations of three stellar models with different activity levels, and reconstruct their large-scale magnetic fields using a non-potential ZDI code assuming two different stellar inclination angles. Our results show that the ZDI technique reconstructs the main features of slowly rotating solar-like stars but with $\sim\,$one order of magnitude less magnetic energy. The large-scale field morphologies are recovered up to harmonic modes $\ell \sim 5$, especially after averaging over several maps for each stellar model. While ZDI is not able to reproduce the input magnetic energy distributions across individual harmonic modes, the fractional energies across the modes are generally within $20\,\%$ agreement. The fraction of axisymmetric and toroidal field tends to be overestimated for stars with solar flux emergence patterns for more pole-on inclination angles.
2019-03-11T00:00:00ZLehmann, L. T.Hussain, G. A. J.Jardine, M. M.Mackay, D. H.Vidotto, A. A.The large-scale magnetic fields of stars can be obtained with the Zeeman-Doppler-Imaging (ZDI) technique, but their interpretation is still challenging as the contribution of the small-scale field or the reliability of the reconstructed field properties is still not fully understood. To quantify this, we use 3D non-potential magnetic field simulations for slowly rotating solar-like stars as inputs to test the capabilities of ZDI. These simulations are based on a flux transport model connected to a non-potential coronal evolution model using the observed solar flux emergence pattern. We first compare four field prescriptions regarding their reconstruction capabilities and investigate the influence of the spatial resolution of the input maps on the corresponding circularly polarised profiles. We then generate circularly polarised spectra based on our high resolution simulations of three stellar models with different activity levels, and reconstruct their large-scale magnetic fields using a non-potential ZDI code assuming two different stellar inclination angles. Our results show that the ZDI technique reconstructs the main features of slowly rotating solar-like stars but with $\sim\,$one order of magnitude less magnetic energy. The large-scale field morphologies are recovered up to harmonic modes $\ell \sim 5$, especially after averaging over several maps for each stellar model. While ZDI is not able to reproduce the input magnetic energy distributions across individual harmonic modes, the fractional energies across the modes are generally within $20\,\%$ agreement. The fraction of axisymmetric and toroidal field tends to be overestimated for stars with solar flux emergence patterns for more pole-on inclination angles.Contribution of observed multi frequency spectrum of Alfvén waves to coronal heatingPagano, PaoloDe Moortel, Inekehttp://hdl.handle.net/10023/168352019-01-13T00:34:14Z2019-01-07T00:00:00ZContext. Whilst there are observational indications that transverse MHD waves carry enough energy to maintain the thermal structure of the solar corona, it is not clear whether such energy can be efficiently and effectively converted into heating. Phase-mixing of Alfvén waves is considered a candidate mechanism, as it can develop transverse gradient where magnetic energy can be converted into thermal energy. However, phase-mixing is a process that crucially depends on the amplitude and period of the transverse oscillations, and only recently have we obtained a complete measurement of the power spectrum for transverse oscillations in the corona. Aims. We aim to investigate the heating generated by phase-mixing of transverse oscillations triggered by buffeting of a coronal loop that follows from the observed coronal power spectrum as well as the impact of these persistent oscillations on the structure of coronal loops. Methods. We consider a 3D MHD model of an active region coronal loop and we perturb its footpoints with a 2D horizontal driver that represents a random buffeting motion of the loop footpoints. Our driver is composed of 1000 pulses superimposed to generate the observed power spectrum. Results. We find that the heating supply from the observed power spectrum in the solar corona through phase mixing is not sufficient to maintain the million degree active region solar corona. We also find that the development of Kelvin-Helmholtz instabilities could be a common phenomenon in coronal loops that could affect their apparent life time. Conclusions. This study concludes that is unlikely that phase-mixing of Alfvén waves resulting from an observed power spectrum of transverse coronal loop oscillations can heat the active region solar corona. However, transverse waves could play an important role in the development of small scale structures.
2019-01-07T00:00:00ZPagano, PaoloDe Moortel, InekeContext. Whilst there are observational indications that transverse MHD waves carry enough energy to maintain the thermal structure of the solar corona, it is not clear whether such energy can be efficiently and effectively converted into heating. Phase-mixing of Alfvén waves is considered a candidate mechanism, as it can develop transverse gradient where magnetic energy can be converted into thermal energy. However, phase-mixing is a process that crucially depends on the amplitude and period of the transverse oscillations, and only recently have we obtained a complete measurement of the power spectrum for transverse oscillations in the corona. Aims. We aim to investigate the heating generated by phase-mixing of transverse oscillations triggered by buffeting of a coronal loop that follows from the observed coronal power spectrum as well as the impact of these persistent oscillations on the structure of coronal loops. Methods. We consider a 3D MHD model of an active region coronal loop and we perturb its footpoints with a 2D horizontal driver that represents a random buffeting motion of the loop footpoints. Our driver is composed of 1000 pulses superimposed to generate the observed power spectrum. Results. We find that the heating supply from the observed power spectrum in the solar corona through phase mixing is not sufficient to maintain the million degree active region solar corona. We also find that the development of Kelvin-Helmholtz instabilities could be a common phenomenon in coronal loops that could affect their apparent life time. Conclusions. This study concludes that is unlikely that phase-mixing of Alfvén waves resulting from an observed power spectrum of transverse coronal loop oscillations can heat the active region solar corona. However, transverse waves could play an important role in the development of small scale structures.Loggerhead turtle Caretta caretta density and abundance in Chesapeake Bay and the temperate ocean waters of the southern portion of the Mid-Atlantic BightBarco, Susan G.Burt, M. LouiseDiGiovanni, Robert A.Swingle, W. MarkWilliard, Amanda S.http://hdl.handle.net/10023/168332019-01-14T10:30:06Z2018-12-13T00:00:00ZWe conducted aerial surveys of sea turtles in 2011 and 2012, incorporating corrections for perception and availability bias in Chesapeake Bay and near-shore continental shelf waters of the Mid-Atlantic Bight off the US states of Virginia and Maryland. Results of these surveys and ancillary research to determine surface times for loggerhead turtles provide us with a new baseline population estimate for turtles in the region. Prior surveys were conducted in Chesapeake Bay in the mid-1980s and early 2000s, and in ocean waters in the late 1970s and early 1980s. Although comparison of density estimates not corrected for availability between prior surveys and this effort suggests that the population of sea turtles, especially loggerhead turtles, is higher than previous estimates, differences between surveys may be the result of survey methodologies and cannot be assumed to be true changes in density. Surface time for availability corrections was calculated using dive summaries from satellite telemetry on 27 loggerhead turtles tracked between 2011 and 2015. We calculated stratified seasonal availability corrections for bay and ocean waters based on assumed differences in turtle behavior and water clarity between the 2 habitats. For each habitat, we provided seasonal corrections for 3 detection depth bins (shallow, moderate, and deep) to account for differences in sub-surface detection ranges. Differences and trends toward differences among availability corrections underscore the need to better understand the many variables that affect surface time for sea turtles in temperate waters, and the effect that availability has on abundance and density estimates.
Funding was provided by the NOAA Species Recovery Grants to States program (Award #NA 47200033) issued to the Virginia Department of Game and Inland Fisheries which contracted with the Virginia Aquarium & Marine Science Center Foundation. Additional funding for tags and turtle capture was also provided by US Fleet Forces Command as well as the Virginia Aquarium Batten Collaborative Research Fund and Batten Professional Development Fund.
2018-12-13T00:00:00ZBarco, Susan G.Burt, M. LouiseDiGiovanni, Robert A.Swingle, W. MarkWilliard, Amanda S.We conducted aerial surveys of sea turtles in 2011 and 2012, incorporating corrections for perception and availability bias in Chesapeake Bay and near-shore continental shelf waters of the Mid-Atlantic Bight off the US states of Virginia and Maryland. Results of these surveys and ancillary research to determine surface times for loggerhead turtles provide us with a new baseline population estimate for turtles in the region. Prior surveys were conducted in Chesapeake Bay in the mid-1980s and early 2000s, and in ocean waters in the late 1970s and early 1980s. Although comparison of density estimates not corrected for availability between prior surveys and this effort suggests that the population of sea turtles, especially loggerhead turtles, is higher than previous estimates, differences between surveys may be the result of survey methodologies and cannot be assumed to be true changes in density. Surface time for availability corrections was calculated using dive summaries from satellite telemetry on 27 loggerhead turtles tracked between 2011 and 2015. We calculated stratified seasonal availability corrections for bay and ocean waters based on assumed differences in turtle behavior and water clarity between the 2 habitats. For each habitat, we provided seasonal corrections for 3 detection depth bins (shallow, moderate, and deep) to account for differences in sub-surface detection ranges. Differences and trends toward differences among availability corrections underscore the need to better understand the many variables that affect surface time for sea turtles in temperate waters, and the effect that availability has on abundance and density estimates.On the rate of convergence of ((||ƒ||p)¦(||ƒ||∞))p as p →∞Olsen, Lars Ole Ronnowhttp://hdl.handle.net/10023/168322019-01-10T14:30:04Z2019-02-01T00:00:00ZLet (X,ε,μ) be a measure space and let ƒ:X→ ℝ be a measurable function such that ||ƒ||p < ∞ for all p ≥ 1 and ||ƒ||∞ >0. In this paper, we describe the rate of convergence of ((||ƒ||p)¦(||ƒ||∞))p as p →∞.
2019-02-01T00:00:00ZOlsen, Lars Ole RonnowLet (X,ε,μ) be a measure space and let ƒ:X→ ℝ be a measurable function such that ||ƒ||p < ∞ for all p ≥ 1 and ||ƒ||∞ >0. In this paper, we describe the rate of convergence of ((||ƒ||p)¦(||ƒ||∞))p as p →∞.Modelling reassurances of clinicians with Hidden Markov modelsPopov, ValentinEllis-Robinson, AleshaHumphris, Geraldhttp://hdl.handle.net/10023/168292019-01-13T00:34:09Z2019-01-09T00:00:00ZBackground: A key element in the interaction between clinicians and patients with cancer is reassurance giving. Learning about the stochastic nature of reassurances as well as making inferential statements about the influence of covariates such as patient response and time spent on previous reassurances are of particular importance. Methods: We fit Hidden Markov Models (HMMs) to reassurance type from multiple time series of clinicians' reassurances, decoded from audio files of review consultations between patients with breast cancer and their therapeutic radiographer. Assuming a latent state process driving the observations process, HMMs naturally accommodate serial dependence in the data. Extensions to the baseline model such as including covariates as well as allowing for fixed effects for the different clinicians are straightforward to implement. Results: We found that clinicians undergo different states, in which they are more or less inclined to provide a particular type of reassurance. The states are very persistent, however switches occasionally occur. The lengthier the previous reassurance, the more likely the clinician is to stay in the current state. Conclusions: HMMs prove to be a valuable tool and provide important insights for practitioners. Trial registration: Trial Registration number: ClinicalTrials.gov: NCT02599506. Prospectively registered on 11th March 2015.
Generous support was received from the charity Breast Cancer Now (grant number: 6873)
2019-01-09T00:00:00ZPopov, ValentinEllis-Robinson, AleshaHumphris, GeraldBackground: A key element in the interaction between clinicians and patients with cancer is reassurance giving. Learning about the stochastic nature of reassurances as well as making inferential statements about the influence of covariates such as patient response and time spent on previous reassurances are of particular importance. Methods: We fit Hidden Markov Models (HMMs) to reassurance type from multiple time series of clinicians' reassurances, decoded from audio files of review consultations between patients with breast cancer and their therapeutic radiographer. Assuming a latent state process driving the observations process, HMMs naturally accommodate serial dependence in the data. Extensions to the baseline model such as including covariates as well as allowing for fixed effects for the different clinicians are straightforward to implement. Results: We found that clinicians undergo different states, in which they are more or less inclined to provide a particular type of reassurance. The states are very persistent, however switches occasionally occur. The lengthier the previous reassurance, the more likely the clinician is to stay in the current state. Conclusions: HMMs prove to be a valuable tool and provide important insights for practitioners. Trial registration: Trial Registration number: ClinicalTrials.gov: NCT02599506. Prospectively registered on 11th March 2015.Robustness of vortex populations in the two-dimensional inverse energy cascadeBurgess, Belle HelenScott, Richard Kirknesshttp://hdl.handle.net/10023/168252019-01-10T00:33:17Z2018-09-10T00:00:00ZWe study how the properties of forcing and dissipation affect the scaling behaviour of the vortex population in the two-dimensional turbulent inverse energy cascade. When the flow is forced at scales intermediate between the domain and dissipation scales, the growth rates of the largest vortex area and the spectral peak length scale are robust to all simulation parameters. For white-in-time forcing the number density distribution of vortex areas follows the scaling theory predictions of Burgess & Scott (J. Fluid Mech., vol. 811, 2017, pp. 742–756) and shows little sensitivity either to the forcing bandwidth or to the nature of the small-scale dissipation: both narrowband and broadband forcing generate nearly identical vortex populations, as do Laplacian diffusion and hyperdiffusion. The greatest differences arise in comparing simulations with correlated forcing to those with white-in-time forcing: in flows with correlated forcing the intermediate range in the vortex number density steepens significantly past the predicted scale-invariant A-1 scaling. We also study the impact of the forcing Reynolds number Rej, a measure of the relative importance of nonlinear terms and dissipation at the forcing scale, on vortex formation and the scaling of the number density. As Rej decreases, the flow changes from one dominated by intense circular vortices surrounded by filaments to a less structured flow in which vortex formation becomes progressively more suppressed and the filamentary nature of the surrounding vorticity field is lost. However, even at very small Rej, and in the absence of intense coherent vortex formation, regions of anomalously high vorticity merge and grow in area as predicted by the scaling theory, generating a three-part number density similar to that found at higher Rej. At late enough stages the aggregation process results in the formation of long-lived circular vortices, demonstrating a strong tendency to vortex formation, and via a route distinct from the axisymmetrization of forcing extrema seen at higher Rej. Our results establish coherent vortices as a robust feature of the two-dimensional inverse energy cascade, and provide clues as to the dynamical mechanisms shaping their statistics.
Funding: Leverhulme Early Career Fellowship from the Leverhulme Trust, the Natural Environment Research Council grant NE/M014983/1 (B.H.B.).
2018-09-10T00:00:00ZBurgess, Belle HelenScott, Richard KirknessWe study how the properties of forcing and dissipation affect the scaling behaviour of the vortex population in the two-dimensional turbulent inverse energy cascade. When the flow is forced at scales intermediate between the domain and dissipation scales, the growth rates of the largest vortex area and the spectral peak length scale are robust to all simulation parameters. For white-in-time forcing the number density distribution of vortex areas follows the scaling theory predictions of Burgess & Scott (J. Fluid Mech., vol. 811, 2017, pp. 742–756) and shows little sensitivity either to the forcing bandwidth or to the nature of the small-scale dissipation: both narrowband and broadband forcing generate nearly identical vortex populations, as do Laplacian diffusion and hyperdiffusion. The greatest differences arise in comparing simulations with correlated forcing to those with white-in-time forcing: in flows with correlated forcing the intermediate range in the vortex number density steepens significantly past the predicted scale-invariant A-1 scaling. We also study the impact of the forcing Reynolds number Rej, a measure of the relative importance of nonlinear terms and dissipation at the forcing scale, on vortex formation and the scaling of the number density. As Rej decreases, the flow changes from one dominated by intense circular vortices surrounded by filaments to a less structured flow in which vortex formation becomes progressively more suppressed and the filamentary nature of the surrounding vorticity field is lost. However, even at very small Rej, and in the absence of intense coherent vortex formation, regions of anomalously high vorticity merge and grow in area as predicted by the scaling theory, generating a three-part number density similar to that found at higher Rej. At late enough stages the aggregation process results in the formation of long-lived circular vortices, demonstrating a strong tendency to vortex formation, and via a route distinct from the axisymmetrization of forcing extrema seen at higher Rej. Our results establish coherent vortices as a robust feature of the two-dimensional inverse energy cascade, and provide clues as to the dynamical mechanisms shaping their statistics.Accounting for preferential sampling in species distribution modelsPennino, Maria GraziaParadinas, IosuIllian, Janine B.Muñoz, FacundoBellido, José MaríaLópez-Quílez, AntonioConesa, Davidhttp://hdl.handle.net/10023/167972019-01-13T00:34:12Z2018-12-26T00:00:00ZSpecies distribution models (SDMs) are now being widely used in ecology for management and conservation purposes across terrestrial, freshwater, and marine realms. The increasing interest in SDMs has drawn the attention of ecologists to spatial models and, in particular, to geostatistical models, which are used to associate observations of species occurrence or abundance with environmental covariates in a finite number of locations in order to predict where (and how much of) a species is likely to be present in unsampled locations. Standard geostatistical methodology assumes that the choice of sampling locations is independent of the values of the variable of interest. However, in natural environments, due to practical limitations related to time and financial constraints, this theoretical assumption is often violated. In fact, data commonly derive from opportunistic sampling (e.g., whale or bird watching), in which observers tend to look for a specific species in areas where they expect to find it. These are examples of what is referred to as preferential sampling, which can lead to biased predictions of the distribution of the species. The aim of this study is to discuss a SDM that addresses this problem and that it is more computationally efficient than existing MCMC methods. From a statistical point of view, we interpret the data as a marked point pattern, where the sampling locations form a point pattern and the measurements taken in those locations (i.e., species abundance or occurrence) are the associated marks. Inference and prediction of species distribution is performed using a Bayesian approach, and integrated nested Laplace approximation (INLA) methodology and software are used for model fitting to minimize the computational burden. We show that abundance is highly overestimated at low abundance locations when preferential sampling effects not accounted for, in both a simulated example and a practical application using fishery data. This highlights that ecologists should be aware of the potential bias resulting from preferential sampling and account for it in a model when a survey is based on non‐randomized and/or non‐systematic sampling.
D. C., A. L. Q. and F. M. would like to thank the Ministerio de Educación y Ciencia (Spain) for financial support (jointly financed by the European Regional Development Fund) via Research Grants MTM2013‐42323‐P and MTM2016‐77501‐P, and ACOMP/2015/202 from Generalitat Valenciana (Spain).
2018-12-26T00:00:00ZPennino, Maria GraziaParadinas, IosuIllian, Janine B.Muñoz, FacundoBellido, José MaríaLópez-Quílez, AntonioConesa, DavidSpecies distribution models (SDMs) are now being widely used in ecology for management and conservation purposes across terrestrial, freshwater, and marine realms. The increasing interest in SDMs has drawn the attention of ecologists to spatial models and, in particular, to geostatistical models, which are used to associate observations of species occurrence or abundance with environmental covariates in a finite number of locations in order to predict where (and how much of) a species is likely to be present in unsampled locations. Standard geostatistical methodology assumes that the choice of sampling locations is independent of the values of the variable of interest. However, in natural environments, due to practical limitations related to time and financial constraints, this theoretical assumption is often violated. In fact, data commonly derive from opportunistic sampling (e.g., whale or bird watching), in which observers tend to look for a specific species in areas where they expect to find it. These are examples of what is referred to as preferential sampling, which can lead to biased predictions of the distribution of the species. The aim of this study is to discuss a SDM that addresses this problem and that it is more computationally efficient than existing MCMC methods. From a statistical point of view, we interpret the data as a marked point pattern, where the sampling locations form a point pattern and the measurements taken in those locations (i.e., species abundance or occurrence) are the associated marks. Inference and prediction of species distribution is performed using a Bayesian approach, and integrated nested Laplace approximation (INLA) methodology and software are used for model fitting to minimize the computational burden. We show that abundance is highly overestimated at low abundance locations when preferential sampling effects not accounted for, in both a simulated example and a practical application using fishery data. This highlights that ecologists should be aware of the potential bias resulting from preferential sampling and account for it in a model when a survey is based on non‐randomized and/or non‐systematic sampling.Whale distribution in a breeding area : spatial models of habitat use and abundance of western South Atlantic humpback whalesBortolotto, Guilherme AugustoDanilewicz, DanielHammond, Philip StevenThomas, Leonard JosephZerbini, Alexandre Nhttp://hdl.handle.net/10023/167642019-01-06T03:19:32Z2017-12-27T00:00:00ZThe western South Atlantic humpback whale population was severely depleted by commercial whaling in the late 19th and 20th centuries, and today inhabits a human-impacted environment in its wintering grounds off the Brazilian coast. We identified distribution patterns related to environmental features and provide new estimates of population size, which can inform future management actions. We fitted spatial models to line transect data from 2 research cruises conducted in 2008 and 2012 to investigate (1) habitat use and (2) abundance of humpback whales wintering in the Brazilian continental shelf. Potential explanatory variables were year, depth, seabed slope, sea-surface temperature (SST), northing and easting, current speed, wind speed, distance to coastline and to the continental shelf break, and shelter (a combination of wind speed and SST categories). Whale density was higher in slower currents, at shorter distances to both the coastline and shelf break, and at SSTs between 24 and 25°C. The distribution of whales was also strongly related to shelter. For abundance estimation, easting and northing were included in the model instead of SST; estimates were 14264 whales (CV = 0.084) for 2008 and 20389 (CV = 0.071) for 2012. Environmental variables explained well the variation in whale density; higher density was found to the south of the Abrolhos Archipelago, and shelter seems to be important for these animals in their breeding area. Estimated distribution patterns presented here can be used to mitigate potential human-related impacts, such as supporting protection in the population’s core habitat near the Abrolhos Archipelago.
GA Bortolotto PhD is funded by the Brazilian National Council for Scientific and Technological Development (Conselho Nacional de Desenvolvimento Científico e Tecnológico, CNPq; Science Without Borders, scholarship #208203/2014 - 1). Cetacean International Society granted GA Bortolotto with small grants which contributed to the development of this study.
2017-12-27T00:00:00ZBortolotto, Guilherme AugustoDanilewicz, DanielHammond, Philip StevenThomas, Leonard JosephZerbini, Alexandre NThe western South Atlantic humpback whale population was severely depleted by commercial whaling in the late 19th and 20th centuries, and today inhabits a human-impacted environment in its wintering grounds off the Brazilian coast. We identified distribution patterns related to environmental features and provide new estimates of population size, which can inform future management actions. We fitted spatial models to line transect data from 2 research cruises conducted in 2008 and 2012 to investigate (1) habitat use and (2) abundance of humpback whales wintering in the Brazilian continental shelf. Potential explanatory variables were year, depth, seabed slope, sea-surface temperature (SST), northing and easting, current speed, wind speed, distance to coastline and to the continental shelf break, and shelter (a combination of wind speed and SST categories). Whale density was higher in slower currents, at shorter distances to both the coastline and shelf break, and at SSTs between 24 and 25°C. The distribution of whales was also strongly related to shelter. For abundance estimation, easting and northing were included in the model instead of SST; estimates were 14264 whales (CV = 0.084) for 2008 and 20389 (CV = 0.071) for 2012. Environmental variables explained well the variation in whale density; higher density was found to the south of the Abrolhos Archipelago, and shelter seems to be important for these animals in their breeding area. Estimated distribution patterns presented here can be used to mitigate potential human-related impacts, such as supporting protection in the population’s core habitat near the Abrolhos Archipelago.Generators and presentations for direct and wreath products of monoid actsMiller, Craighttp://hdl.handle.net/10023/167362019-01-13T02:35:25Z2018-12-17T00:00:00ZWe investigate the preservation of the properties of being finitely generated and finitely presented under both direct and wreath products of monoid acts. A monoid M is said to preserve property P in direct products if, for any two M-acts A and B, the direct product A x B has property P if and only if both A and B have property P. It is proved that the monoids M that preserve finite generation (resp. finitely presentability) in direct products are precisely those for which the diagonal M-act M x M is finitely generated (resp. finitely presented). We show that a wreath product A ≀ B is finitely generated if and only if both A and B are finitely generated. It is also proved that a necessary condition for A ≀ B to be finitely presented is that both A and B are finitely presented. Finally, we find some sufficient conditions for a wreath product to be finitely presented.
The author would like to thank the EPSRC for financial support.
2018-12-17T00:00:00ZMiller, CraigWe investigate the preservation of the properties of being finitely generated and finitely presented under both direct and wreath products of monoid acts. A monoid M is said to preserve property P in direct products if, for any two M-acts A and B, the direct product A x B has property P if and only if both A and B have property P. It is proved that the monoids M that preserve finite generation (resp. finitely presentability) in direct products are precisely those for which the diagonal M-act M x M is finitely generated (resp. finitely presented). We show that a wreath product A ≀ B is finitely generated if and only if both A and B are finitely generated. It is also proved that a necessary condition for A ≀ B to be finitely presented is that both A and B are finitely presented. Finally, we find some sufficient conditions for a wreath product to be finitely presented.Estimation of population size when capture probability depends on individual statesWorthington, HannahMcCrea, RachelKing, RuthGriffiths, Richardhttp://hdl.handle.net/10023/167352019-01-03T15:30:04Z2018-12-17T00:00:00ZWe develop a multi-state model to estimate the size of a closed population from capture–recapture studies. We consider the case where capture–recapture data are not of a simple binary form, but where the state of an individual is also recorded upon every capture as a discrete variable. The proposed multi-state model can be regarded as a generalisation of the commonly applied set of closed population models to a multi-state form. The model allows for heterogeneity within the capture probabilities associated with each state while also permitting individuals to move between the different discrete states. A closed-form expression for the likelihood is presented in terms of a set of sufficient statistics. The link between existing models for capture heterogeneity is established, and simulation is used to show that the estimate of population size can be biased when movement between states is not accounted for. The proposed unconditional approach is also compared to a conditional approach to assess estimation bias. The model derived in this paper is motivated by a real ecological data set on great crested newts, Triturus cristatus.
Funding: Carnegie Trust for the Universities of Scotland, UK Engineering and Physical Sciences Research Council (EP/10009171/1), UK Natural Environment Research Council (NE/J018473/1)
2018-12-17T00:00:00ZWorthington, HannahMcCrea, RachelKing, RuthGriffiths, RichardWe develop a multi-state model to estimate the size of a closed population from capture–recapture studies. We consider the case where capture–recapture data are not of a simple binary form, but where the state of an individual is also recorded upon every capture as a discrete variable. The proposed multi-state model can be regarded as a generalisation of the commonly applied set of closed population models to a multi-state form. The model allows for heterogeneity within the capture probabilities associated with each state while also permitting individuals to move between the different discrete states. A closed-form expression for the likelihood is presented in terms of a set of sufficient statistics. The link between existing models for capture heterogeneity is established, and simulation is used to show that the estimate of population size can be biased when movement between states is not accounted for. The proposed unconditional approach is also compared to a conditional approach to assess estimation bias. The model derived in this paper is motivated by a real ecological data set on great crested newts, Triturus cristatus.Dimension growth for iterated sumsetsFraser, JonathanHowroyd, Douglas CharlesYu, Hanhttp://hdl.handle.net/10023/167332019-01-13T02:35:25Z2018-12-17T00:00:00ZWe study dimensions of sumsets and iterated sumsets and provide natural conditions which guarantee that a set F⊆ℝ satisfies ^dim^BF+F>^dim^BF or even dimHnF→1. Our results apply to, for example, all uniformly perfect sets, which include Ahlfors–David regular sets. Our proofs rely on Hochman’s inverse theorem for entropy and the Assouad and lower dimensions play a critical role. We give several applications of our results including an Erdős–Volkmann type theorem for semigroups and new lower bounds for the box dimensions of distance sets for sets with small dimension.
Jonathan M. Fraser was financially supported by a Leverhulme Trust Research Fellowship (RF-2016-500) and an EPSRC Standard Grant (EP/R015104/1). Douglas C. Howroyd was financially supported by the EPSRC Doctoral Training Grant (EP/N509759/1). Han Yu was financially supported by the University of St Andrews.
2018-12-17T00:00:00ZFraser, JonathanHowroyd, Douglas CharlesYu, HanWe study dimensions of sumsets and iterated sumsets and provide natural conditions which guarantee that a set F⊆ℝ satisfies ^dim^BF+F>^dim^BF or even dimHnF→1. Our results apply to, for example, all uniformly perfect sets, which include Ahlfors–David regular sets. Our proofs rely on Hochman’s inverse theorem for entropy and the Assouad and lower dimensions play a critical role. We give several applications of our results including an Erdős–Volkmann type theorem for semigroups and new lower bounds for the box dimensions of distance sets for sets with small dimension.The Paris Observatory appoints a director in 1907Peres, TobyGomez Mena, JoseRobertson, Edmund FrederickFalconer, Isobel JessieO'Connor, John Josephhttp://hdl.handle.net/10023/167252018-12-23T00:35:25Z2018-12-12T00:00:00Z2018-12-12T00:00:00ZPeres, TobyGomez Mena, JoseRobertson, Edmund FrederickFalconer, Isobel JessieO'Connor, John JosephEvidence for downflows in the narrow plasma sheet of 2017 September 10 and their significance for flare reconnectionLongcope, DanaUnverferth, JohnKlein, CourtneyMcCarthy, MarikaPriest, Erichttp://hdl.handle.net/10023/167232019-01-13T00:34:11Z2018-12-04T00:00:00ZCurrent sheets are believed to form in the wakes of erupting flux ropes and to enable the magnetic reconnection responsible for an associated flare. Multiwavelength observations of an eruption on 2017 September 10 show a long, linear feature widely taken as evidence of a current sheet viewed edge-on. The relation between the high-temperature, high-density plasma thus observed and any current sheet is not yet entirely clear. We estimate the magnetic field strength surrounding the sheet and conclude that approximately one-third of all flux in the active region was opened by the eruption. Subsequently decreasing field strength suggests that the open flux closed down over the next several hours through reconnection at a rate Mx s−1. We find in AIA observations evidence of downward-moving, dark structures analogous to either supra-arcade downflows, more typically observed above flare arcades viewed face-on, or supra-arcade downflowing loops, previously reported in flares viewed in this perspective. These features suggest that the plasma sheet is composed of the magnetic flux retracting after being reconnected high above the arcade. We model flux tube retraction following reconnection to show that this process can generate high densities and temperatures as observed in the plasma sheet. The retracting flux tubes reach their highest temperatures at the end of their retraction, well below the site of reconnection, consistent with previous analysis of AIA and EIS data showing a peak in the plasma temperature near the base of this particular sheet.
This work was supported partly by a grant from NASA's Heliophysics Supporting Research (HSR) program and partly by a grant from NSF/AGS's Research Experiences for Undergraduates (REU) program.
2018-12-04T00:00:00ZLongcope, DanaUnverferth, JohnKlein, CourtneyMcCarthy, MarikaPriest, EricCurrent sheets are believed to form in the wakes of erupting flux ropes and to enable the magnetic reconnection responsible for an associated flare. Multiwavelength observations of an eruption on 2017 September 10 show a long, linear feature widely taken as evidence of a current sheet viewed edge-on. The relation between the high-temperature, high-density plasma thus observed and any current sheet is not yet entirely clear. We estimate the magnetic field strength surrounding the sheet and conclude that approximately one-third of all flux in the active region was opened by the eruption. Subsequently decreasing field strength suggests that the open flux closed down over the next several hours through reconnection at a rate Mx s−1. We find in AIA observations evidence of downward-moving, dark structures analogous to either supra-arcade downflows, more typically observed above flare arcades viewed face-on, or supra-arcade downflowing loops, previously reported in flares viewed in this perspective. These features suggest that the plasma sheet is composed of the magnetic flux retracting after being reconnected high above the arcade. We model flux tube retraction following reconnection to show that this process can generate high densities and temperatures as observed in the plasma sheet. The retracting flux tubes reach their highest temperatures at the end of their retraction, well below the site of reconnection, consistent with previous analysis of AIA and EIS data showing a peak in the plasma temperature near the base of this particular sheet.Magnetic helicity condensation and the solar cycleMackay, Duncan HendryDeVore, C. RichardAntiochos, SpiroYeates, Anthony Robinsonhttp://hdl.handle.net/10023/166942018-12-14T17:30:08Z2018-12-01T00:00:00ZSolar filaments exhibit a global chirality pattern where dextral/sinistral filaments, corresponding to negative/positive magnetic helicity, are dominant in the northern/southern hemisphere. This pattern is opposite to the sign of magnetic helicity injected by differential rotation along east-west oriented polarity inversion lines, posing a major conundrum for solar physics. A resolution of this problem is offered by the magnetic helicity condensation model of Antiochos (2013). To investigate the global consequences of helicity condensation for the hemispheric chirality pattern, we apply a temporally and spatially averaged statistical approximation of helicity condensation. Realistic magnetic field configurations in both the rising and declining phases of the solar cycle are simulated. For the helicity condensation process, we assume convective cells consisting of positive/negative vorticities in the northern/southern hemisphere, which inject negative/positive helicity. The magnitude of the vorticity is varied as a free parameter, corresponding to different rates of helicity injection. To reproduce the observed percentages of dominant and minority filament chiralities, we find that a vorticity of magnitude 2.5 x 10-6 s-1 is required. This rate, however, is insufficient to produce the observed unimodal profile of chirality with latitude. To achieve this, a vorticity of at least 5 x 10-6 s-1 is needed. Our results place a lower limit on the small-scale helicity injection required to dominate differential rotation and reproduce the observed hemispheric pattern. Future studies should aim to establish whether the helicity injection rate due to convective flows and/or flux emergence across all latitudes of the Sun is consistent with our results.
2018-12-01T00:00:00ZMackay, Duncan HendryDeVore, C. RichardAntiochos, SpiroYeates, Anthony RobinsonSolar filaments exhibit a global chirality pattern where dextral/sinistral filaments, corresponding to negative/positive magnetic helicity, are dominant in the northern/southern hemisphere. This pattern is opposite to the sign of magnetic helicity injected by differential rotation along east-west oriented polarity inversion lines, posing a major conundrum for solar physics. A resolution of this problem is offered by the magnetic helicity condensation model of Antiochos (2013). To investigate the global consequences of helicity condensation for the hemispheric chirality pattern, we apply a temporally and spatially averaged statistical approximation of helicity condensation. Realistic magnetic field configurations in both the rising and declining phases of the solar cycle are simulated. For the helicity condensation process, we assume convective cells consisting of positive/negative vorticities in the northern/southern hemisphere, which inject negative/positive helicity. The magnitude of the vorticity is varied as a free parameter, corresponding to different rates of helicity injection. To reproduce the observed percentages of dominant and minority filament chiralities, we find that a vorticity of magnitude 2.5 x 10-6 s-1 is required. This rate, however, is insufficient to produce the observed unimodal profile of chirality with latitude. To achieve this, a vorticity of at least 5 x 10-6 s-1 is needed. Our results place a lower limit on the small-scale helicity injection required to dominate differential rotation and reproduce the observed hemispheric pattern. Future studies should aim to establish whether the helicity injection rate due to convective flows and/or flux emergence across all latitudes of the Sun is consistent with our results.Title redactedErichson, N. Benjaminhttp://hdl.handle.net/10023/166932018-12-14T14:05:02Z2017-11-20T00:00:00Z2017-11-20T00:00:00ZErichson, N. BenjaminExact dimensionality and projection properties of Gaussian multiplicative chaos measuresFalconer, KennethJin, Xionghttp://hdl.handle.net/10023/166882018-12-30T00:40:43Z2018-12-12T00:00:00ZGiven a measure ν on a regular planar domain D, the Gaussian multiplicative chaos measure of ν studied in this paper is the random measure ^ν^ obtained as the limit of the exponential of the γ-parameter circle averages of the Gaussian free field on D weighted by ν. We investigate the dimensional and geometric properties of these random measures. We first show that if ν is a finite Borel measure on D with exact dimension α>0, then the associated GMC measure ^ν^ is non-degenerate and is almost surely exact dimensional with dimension α-γ2/2, provided γ2/2<α. We then show that if νt is a Hölder-continuously parameterized family of measures then the total mass of ^νt^ varies Hölder-continuously with t, provided that γ is sufficiently small. As an application we show that if γ<0.28, then, almost surely, the orthogonal projections of the γ-Liouville quantum gravity measure ^ν^ on a rotund convex domain D in all directions are simultaneously absolutely continuous with respect to Lebesgue measure with Hölder continuous densities. Furthermore, ^ν^ has positive Fourier dimension almost surely.
Paper originally entitled 'Hölder continuity of the Liouville Quantum Gravity measure'
2018-12-12T00:00:00ZFalconer, KennethJin, XiongGiven a measure ν on a regular planar domain D, the Gaussian multiplicative chaos measure of ν studied in this paper is the random measure ^ν^ obtained as the limit of the exponential of the γ-parameter circle averages of the Gaussian free field on D weighted by ν. We investigate the dimensional and geometric properties of these random measures. We first show that if ν is a finite Borel measure on D with exact dimension α>0, then the associated GMC measure ^ν^ is non-degenerate and is almost surely exact dimensional with dimension α-γ2/2, provided γ2/2<α. We then show that if νt is a Hölder-continuously parameterized family of measures then the total mass of ^νt^ varies Hölder-continuously with t, provided that γ is sufficiently small. As an application we show that if γ<0.28, then, almost surely, the orthogonal projections of the γ-Liouville quantum gravity measure ^ν^ on a rotund convex domain D in all directions are simultaneously absolutely continuous with respect to Lebesgue measure with Hölder continuous densities. Furthermore, ^ν^ has positive Fourier dimension almost surely.Collisionless distribution functions for force-free current sheets : using a pressure transformation to lower the plasma betaWilson, F.Neukirch, T.Allanson, O.http://hdl.handle.net/10023/166852018-12-16T00:38:18Z2018-06-14T00:00:00ZSo far, only one distribution function giving rise to a collisionless nonlinear force-free current sheet equilibrium allowing for a plasma beta less than one is known (Allanson et al., Phys. Plasmas, vol. 22 (10), 2015, 102116; Allanson et al., J. Plasma Phys., vol. 82 (3), 2016a, 905820306). This distribution function can only be expressed as an infinite series of Hermite functions with very slow convergence and this makes its practical use cumbersome. It is the purpose of this paper to present a general method that allows us to find distribution functions consisting of a finite number of terms (therefore easier to use in practice), but which still allow for current sheet equilibria that can, in principle, have an arbitrarily low plasma beta. The method involves using known solutions and transforming them into new solutions using transformations based on taking integer powers (N) of one component of the pressure tensor. The plasma beta of the current sheet corresponding to the transformed distribution functions can then, in principle, have values as low as 1/N. We present the general form of the distribution functions for arbitrary and then, as a specific example, discuss the case for N = 2 in detail.
We acknowledge the support of the Science and Technology Facilities Council via the consolidated grants ST/K000950/1 and ST/N000609/1 and the doctoral training grant ST/K502327/1 (O. A.), and the Natural Environment Research Council via grant no. NE/P017274/1 (Rad-Sat) (O. A.). F. W. and T. N. would also like to thank the University of St Andrews for general financial support.
2018-06-14T00:00:00ZWilson, F.Neukirch, T.Allanson, O.So far, only one distribution function giving rise to a collisionless nonlinear force-free current sheet equilibrium allowing for a plasma beta less than one is known (Allanson et al., Phys. Plasmas, vol. 22 (10), 2015, 102116; Allanson et al., J. Plasma Phys., vol. 82 (3), 2016a, 905820306). This distribution function can only be expressed as an infinite series of Hermite functions with very slow convergence and this makes its practical use cumbersome. It is the purpose of this paper to present a general method that allows us to find distribution functions consisting of a finite number of terms (therefore easier to use in practice), but which still allow for current sheet equilibria that can, in principle, have an arbitrarily low plasma beta. The method involves using known solutions and transforming them into new solutions using transformations based on taking integer powers (N) of one component of the pressure tensor. The plasma beta of the current sheet corresponding to the transformed distribution functions can then, in principle, have values as low as 1/N. We present the general form of the distribution functions for arbitrary and then, as a specific example, discuss the case for N = 2 in detail.The merger of geophysical vortices at finite Rossby and Froude numberReinaud, Jean N.Dritschel, David G.http://hdl.handle.net/10023/166292019-01-06T03:36:14Z2018-08-10T00:00:00ZWe investigate the merger of two co-rotating geophysical vortices at finite Rossby and Froude number. The initial conditions consist of two uniform potential vorticity vortices in near equilibrium and in a near 'balanced' state (i.e. with negligible emission of inertia-gravity wave radiation). We determine the critical merger distance between the two vortices. This distance is found to increase with the magnitude of the Rossby number: intense cyclones or intense anticyclones. Note that the Froude number is proportional to the Rossby number for the near equilibrium initial conditions considered. The critical merging distance also depends on the sign of the potential vorticity anomaly, which is positive for 'cyclones' and negative for 'anticyclones'. We show ageostrophic motions occurring at finite Rossby number tend to draw cyclones together but draw anticyclones apart. On the other hand, we show that anticyclones tend to deform more, in particular when subject to vertical shear (as when the vortices are vertically offset). These two effects compete. Overall, nearly aligned cyclones tend to merge from further apart than their anticyclonic counterparts, while vertically offset anticyclones merger from further apart than cyclones.
Partial support for this research has come from the UK Engineering and Physical Sciences Research Council (grant number EP/H001794/1).
2018-08-10T00:00:00ZReinaud, Jean N.Dritschel, David G.We investigate the merger of two co-rotating geophysical vortices at finite Rossby and Froude number. The initial conditions consist of two uniform potential vorticity vortices in near equilibrium and in a near 'balanced' state (i.e. with negligible emission of inertia-gravity wave radiation). We determine the critical merger distance between the two vortices. This distance is found to increase with the magnitude of the Rossby number: intense cyclones or intense anticyclones. Note that the Froude number is proportional to the Rossby number for the near equilibrium initial conditions considered. The critical merging distance also depends on the sign of the potential vorticity anomaly, which is positive for 'cyclones' and negative for 'anticyclones'. We show ageostrophic motions occurring at finite Rossby number tend to draw cyclones together but draw anticyclones apart. On the other hand, we show that anticyclones tend to deform more, in particular when subject to vertical shear (as when the vortices are vertically offset). These two effects compete. Overall, nearly aligned cyclones tend to merge from further apart than their anticyclonic counterparts, while vertically offset anticyclones merger from further apart than cyclones.Phases of physics in J.D. Forbes’ Dissertation Sixth for the Encyclopaedia Britannica (1856)Falconer, Isobel Jessiehttp://hdl.handle.net/10023/166182019-01-13T02:35:22Z2018-12-03T00:00:00ZThis paper takes James David Forbes’ Encyclopaedia Britannica entry, Dissertation Sixth, as a lens to examine physics as a cognitive, practical, and social, enterprise. Forbes wrote this survey of eighteenth- and nineteenth-century mathematical and physical sciences, in 1852-6, when British “physics” was at a pivotal point in its history, situated between a discipline identified by its mathematical methods – originating in France - and one identified by its university laboratory institutions. Contemporary encyclopaedias provided a nexus for publishers, the book trade, readers, and men of science, in the formation of physics as a field. Forbes was both a witness, whose account of the progress of physics or natural philosophy can be explored at face value, and an agent, who exploited the opportunity offered by the Encyclopaedia Britannica in the mid nineteenth century to enrol the broadly educated public, and scientific collective, illuminating the connection between the definition of physics and its forms of social practice. Forbes used the terms “physics” and “natural philosophy” interchangeably. He portrayed the field as progressed by the natural genius of great men, who curated the discipline within an associational culture that engendered true intellectual spirit. Although this societal mechanism was becoming ineffective, Forbes did not see university institutions as the way forward. Instead, running counter to his friend William Whewell, he advocated inclusion of the mechanical arts (engineering), and a strictly limited role for mathematics. He revealed tensions when the widely accepted discovery-based historiography conflicted with intellectual and moral worth, reflecting a nineteenth-century concern with spirit that cuts across twentieth-century questions about discipline and field.
2018-12-03T00:00:00ZFalconer, Isobel JessieThis paper takes James David Forbes’ Encyclopaedia Britannica entry, Dissertation Sixth, as a lens to examine physics as a cognitive, practical, and social, enterprise. Forbes wrote this survey of eighteenth- and nineteenth-century mathematical and physical sciences, in 1852-6, when British “physics” was at a pivotal point in its history, situated between a discipline identified by its mathematical methods – originating in France - and one identified by its university laboratory institutions. Contemporary encyclopaedias provided a nexus for publishers, the book trade, readers, and men of science, in the formation of physics as a field. Forbes was both a witness, whose account of the progress of physics or natural philosophy can be explored at face value, and an agent, who exploited the opportunity offered by the Encyclopaedia Britannica in the mid nineteenth century to enrol the broadly educated public, and scientific collective, illuminating the connection between the definition of physics and its forms of social practice. Forbes used the terms “physics” and “natural philosophy” interchangeably. He portrayed the field as progressed by the natural genius of great men, who curated the discipline within an associational culture that engendered true intellectual spirit. Although this societal mechanism was becoming ineffective, Forbes did not see university institutions as the way forward. Instead, running counter to his friend William Whewell, he advocated inclusion of the mechanical arts (engineering), and a strictly limited role for mathematics. He revealed tensions when the widely accepted discovery-based historiography conflicted with intellectual and moral worth, reflecting a nineteenth-century concern with spirit that cuts across twentieth-century questions about discipline and field.The interaction of two co-rotating quasi-geostrophic vortices in the vicinity of a surface buoyancy filamentReinaud, Jean Noelhttp://hdl.handle.net/10023/166162019-01-06T03:31:46Z2018-01-01T00:00:00ZIn this paper, we investigate the interaction between two like-signed quasi-geostrophic uniform potential vorticity internal vortices in the vicinity of a surface buoyancy anomaly filament in a three dimensional, stably stratified and rapidly rotating fluid. The surface buoyancy distribution locally modifies the pressure fields and generates a shear flow. We start the study by first considering the effects of a uniform linear horizontal shear on the binary vortex interaction. We confirm that a cooperative shear facilitates the merger of a pair of vortices while an adverse shear has the opposite effect. We next investigate the binary vortex interaction in thevicinity of the surface buoyancy filament explicitly. Here, not only the filament generates a shear flow, but it also responds dynamically to the forcing by the vortex pair. The filament destabilises and forms buoyancy billows at the surface. These billows interact with the internal vortices. In particular, a surface billow may pair with one of the internal vortices. In such cases, the like-signed internal vortex pair may separate if they are initially moderately distant from each other.
2018-01-01T00:00:00ZReinaud, Jean NoelIn this paper, we investigate the interaction between two like-signed quasi-geostrophic uniform potential vorticity internal vortices in the vicinity of a surface buoyancy anomaly filament in a three dimensional, stably stratified and rapidly rotating fluid. The surface buoyancy distribution locally modifies the pressure fields and generates a shear flow. We start the study by first considering the effects of a uniform linear horizontal shear on the binary vortex interaction. We confirm that a cooperative shear facilitates the merger of a pair of vortices while an adverse shear has the opposite effect. We next investigate the binary vortex interaction in thevicinity of the surface buoyancy filament explicitly. Here, not only the filament generates a shear flow, but it also responds dynamically to the forcing by the vortex pair. The filament destabilises and forms buoyancy billows at the surface. These billows interact with the internal vortices. In particular, a surface billow may pair with one of the internal vortices. In such cases, the like-signed internal vortex pair may separate if they are initially moderately distant from each other.Three-dimensional solutions of the magnetohydrostatic equations for rigidly rotating magnetospheres in cylindrical coordinatesWilson, FionaNeukirch, Thomashttp://hdl.handle.net/10023/165822019-01-06T03:31:42Z2018-01-01T00:00:00ZWe present new analytical three-dimensional solutions of the magnetohydrostatic equations, which are applicable to the co-rotating frame of reference outside a rigidly rotating cylindrical body, and have potential applications to planetary magnetospheres and stellar coronae. We consider the case with centrifugal force only, and use a transformation method in which the governing equation for the "pseudo-potential" (from which the magnetic field can be calculated) becomes the Laplace partial differential equation. The new solutions extend the set of previously found solutions to those of a "fractional multipole" nature, and offer wider possibilities for modelling than before. We consider some special cases, and present example solutions.
Funding: Science and Technology Facilities Council under grants ST/K000950/1 and ST/N000609/1.
2018-01-01T00:00:00ZWilson, FionaNeukirch, ThomasWe present new analytical three-dimensional solutions of the magnetohydrostatic equations, which are applicable to the co-rotating frame of reference outside a rigidly rotating cylindrical body, and have potential applications to planetary magnetospheres and stellar coronae. We consider the case with centrifugal force only, and use a transformation method in which the governing equation for the "pseudo-potential" (from which the magnetic field can be calculated) becomes the Laplace partial differential equation. The new solutions extend the set of previously found solutions to those of a "fractional multipole" nature, and offer wider possibilities for modelling than before. We consider some special cases, and present example solutions.No actual measurement ... was required : Maxwell and Cavendish's null method for the inverse square law of electrostaticsFalconer, Isobelhttp://hdl.handle.net/10023/165442019-01-06T03:08:20Z2017-12-02T00:00:00ZIn 1877 James Clerk Maxwell and his student Donald MacAlister refined Henry Cavendish’s 1773 null experiment demonstrating the absence of electricity inside a charged conductor. This null result was a mathematical prediction of the inverse square law of electrostatics, and both Cavendish and Maxwell took the experiment as verifying the law. However, Maxwell had already expressed absolute conviction in the law, based on results of Michael Faraday’s. So, what was the value to him of repeating Cavendish’s experiment? After assessing whether the law was as secure as he claimed, this paper explores its central importance to the electrical programme that Maxwell was pursuing. It traces the historical and conceptual re-orderings through which Maxwell established the law by constructing a tradition of null tests and asserting the superior accuracy of the method. Maxwell drew on his developing ‘doctrine of method’ to identify Cavendish’s experiment as a member of a wider class of null methods. By doing so, he appealed to the null practices of telegraph engineers, diverted attention from the flawed logic of the method, and sought to localise issues around the mapping of numbers onto instrumental indications, on the grounds that ‘no actual measurement … was required’.
2017-12-02T00:00:00ZFalconer, IsobelIn 1877 James Clerk Maxwell and his student Donald MacAlister refined Henry Cavendish’s 1773 null experiment demonstrating the absence of electricity inside a charged conductor. This null result was a mathematical prediction of the inverse square law of electrostatics, and both Cavendish and Maxwell took the experiment as verifying the law. However, Maxwell had already expressed absolute conviction in the law, based on results of Michael Faraday’s. So, what was the value to him of repeating Cavendish’s experiment? After assessing whether the law was as secure as he claimed, this paper explores its central importance to the electrical programme that Maxwell was pursuing. It traces the historical and conceptual re-orderings through which Maxwell established the law by constructing a tradition of null tests and asserting the superior accuracy of the method. Maxwell drew on his developing ‘doctrine of method’ to identify Cavendish’s experiment as a member of a wider class of null methods. By doing so, he appealed to the null practices of telegraph engineers, diverted attention from the flawed logic of the method, and sought to localise issues around the mapping of numbers onto instrumental indications, on the grounds that ‘no actual measurement … was required’.Weather-driven change in primary productivity explains variation in the amplitude of two herbivore population cycles in a boreal systemSchmidt, Joshua H.Rexstad, Eric A.Roland, Carl A.McIntyre, Carol L.MacCluskie, Margaret C.Flamme, Melanie J.http://hdl.handle.net/10023/165312019-01-06T03:32:07Z2018-02-01T00:00:00ZVertebrate populations throughout the circumpolar north often exhibit cyclic dynamics, and predation is generally considered to be a primary driver of these cycles in a variety of herbivore species. However, weather and climate play a role in entraining cycles over broad landscapes and may alter cyclic dynamics, although the mechanism by which these processes operate is uncertain. Experimental and observational work has suggested that weather influences primary productivity over multi-year time periods, suggesting a pathway through which weather and climate may influence cyclic herbivore dynamics. Using long-term monitoring data, we investigated the relationships among multi-year weather conditions, measures of primary productivity, and the abundance of two cyclic herbivore species: snowshoe hare and northern red-backed vole. We found that precipitation (rain and snow) and growing season temperatures were strongly associated with variation in primary productivity over multi-year time horizons. In turn, fourfold variation in the amplitude of both the hare and vole cycles observed in our study area corresponded to long-term changes in primary productivity. The congruence of our results for these two species suggests a general mechanism by which weather and climate might influence cyclic herbivore population dynamics. Our findings also suggested that the association between climate warming and the disappearance of cycles might be initiated by changes in primary productivity. This work provides an explanation for observed influences of weather and climate on primary productivity and population cycles and will help our collective understanding of how future climate warming may influence these ecological phenomena in the future.
2018-02-01T00:00:00ZSchmidt, Joshua H.Rexstad, Eric A.Roland, Carl A.McIntyre, Carol L.MacCluskie, Margaret C.Flamme, Melanie J.Vertebrate populations throughout the circumpolar north often exhibit cyclic dynamics, and predation is generally considered to be a primary driver of these cycles in a variety of herbivore species. However, weather and climate play a role in entraining cycles over broad landscapes and may alter cyclic dynamics, although the mechanism by which these processes operate is uncertain. Experimental and observational work has suggested that weather influences primary productivity over multi-year time periods, suggesting a pathway through which weather and climate may influence cyclic herbivore dynamics. Using long-term monitoring data, we investigated the relationships among multi-year weather conditions, measures of primary productivity, and the abundance of two cyclic herbivore species: snowshoe hare and northern red-backed vole. We found that precipitation (rain and snow) and growing season temperatures were strongly associated with variation in primary productivity over multi-year time horizons. In turn, fourfold variation in the amplitude of both the hare and vole cycles observed in our study area corresponded to long-term changes in primary productivity. The congruence of our results for these two species suggests a general mechanism by which weather and climate might influence cyclic herbivore population dynamics. Our findings also suggested that the association between climate warming and the disappearance of cycles might be initiated by changes in primary productivity. This work provides an explanation for observed influences of weather and climate on primary productivity and population cycles and will help our collective understanding of how future climate warming may influence these ecological phenomena in the future.Computational techniques in finite semigroup theoryWilson, Wilf A.http://hdl.handle.net/10023/165212018-11-21T17:17:15Z2019-06-25T00:00:00ZA semigroup is simply a set with an associative binary operation; computational semigroup theory is the branch of mathematics concerned with developing techniques for computing with semigroups, as well as investigating semigroups with the help of computers. This thesis explores both sides of computational semigroup theory, across several topics, especially in the finite case.
The central focus of this thesis is computing and describing maximal subsemigroups of finite semigroups. A maximal subsemigroup of a semigroup is a proper subsemigroup that is contained in no other proper subsemigroup. We present novel and useful algorithms for computing the maximal subsemigroups of an arbitrary finite semigroup, building on the paper of Graham, Graham, and Rhodes from 1968. In certain cases, the algorithms reduce to computing maximal subgroups of finite groups, and analysing graphs that capture information about the regular ℐ-classes of a semigroup. We use the framework underpinning these algorithms to describe the maximal subsemigroups of many families of finite transformation and diagram monoids. This reproduces and greatly extends a large amount of existing work in the literature, and allows us to easily see the common features between these maximal subsemigroups.
This thesis is also concerned with direct products of semigroups, and with a special class of semigroups known as Rees 0-matrix semigroups. We extend known results concerning the generating sets of direct products of semigroups; in doing so, we propose techniques for computing relatively small generating sets for certain kinds of direct products. Additionally, we characterise several features of Rees 0-matrix semigroups in terms of their underlying semigroups and matrices, such as their Green's relations and generating sets, and whether they are inverse. In doing so, we suggest new methods for computing Rees 0-matrix semigroups.
2019-06-25T00:00:00ZWilson, Wilf A.A semigroup is simply a set with an associative binary operation; computational semigroup theory is the branch of mathematics concerned with developing techniques for computing with semigroups, as well as investigating semigroups with the help of computers. This thesis explores both sides of computational semigroup theory, across several topics, especially in the finite case.
The central focus of this thesis is computing and describing maximal subsemigroups of finite semigroups. A maximal subsemigroup of a semigroup is a proper subsemigroup that is contained in no other proper subsemigroup. We present novel and useful algorithms for computing the maximal subsemigroups of an arbitrary finite semigroup, building on the paper of Graham, Graham, and Rhodes from 1968. In certain cases, the algorithms reduce to computing maximal subgroups of finite groups, and analysing graphs that capture information about the regular ℐ-classes of a semigroup. We use the framework underpinning these algorithms to describe the maximal subsemigroups of many families of finite transformation and diagram monoids. This reproduces and greatly extends a large amount of existing work in the literature, and allows us to easily see the common features between these maximal subsemigroups.
This thesis is also concerned with direct products of semigroups, and with a special class of semigroups known as Rees 0-matrix semigroups. We extend known results concerning the generating sets of direct products of semigroups; in doing so, we propose techniques for computing relatively small generating sets for certain kinds of direct products. Additionally, we characterise several features of Rees 0-matrix semigroups in terms of their underlying semigroups and matrices, such as their Green's relations and generating sets, and whether they are inverse. In doing so, we suggest new methods for computing Rees 0-matrix semigroups.Statistics of ambiguous rotationsArnold, R.Jupp, P. E.Schaeben, H.http://hdl.handle.net/10023/165112019-01-06T03:31:30Z2018-05-01T00:00:00ZThe orientation of a rigid object can be described by a rotation that transforms it into a standard position. For a symmetrical object the rotation is known only up to multiplication by an element of the symmetry group. Such ambiguous rotations arise in biomechanics, crystallography and seismology. We develop methods for analyzing data of this form. A test of uniformity is given. Parametric models for ambiguous rotations are presented, tests of location are considered, and a regression model is proposed. An example involving orientations of diopside crystals (which have symmetry of order 2) is used throughout to illustrate how our methods can be applied.
2018-05-01T00:00:00ZArnold, R.Jupp, P. E.Schaeben, H.The orientation of a rigid object can be described by a rotation that transforms it into a standard position. For a symmetrical object the rotation is known only up to multiplication by an element of the symmetry group. Such ambiguous rotations arise in biomechanics, crystallography and seismology. We develop methods for analyzing data of this form. A test of uniformity is given. Parametric models for ambiguous rotations are presented, tests of location are considered, and a regression model is proposed. An example involving orientations of diopside crystals (which have symmetry of order 2) is used throughout to illustrate how our methods can be applied.Heating effects from driven transverse and Alfvén waves in coronal loopsGuo, MingzheVan Doorsselaere, TomKarampelas, KostasLi, BoAntolin, PatrickDe Moortel, Inekehttp://hdl.handle.net/10023/165022019-01-14T13:30:05Z2019-01-10T00:00:00ZRecent numerical studies revealed that transverse motions of coronal loops can induce the Kelvin-Helmholtz Instability (KHI). This process could be important in coronal heating because it leads to dissipation of energy at small spatial-scale plasma interactions. Meanwhile, small amplitude decayless oscillations in coronal loops have been discovered recently in observations of SDO/AIA. We model such oscillations in coronal loops and study wave heating effects, considering a kink and Alfvén driver separately and a mixed driver at the bottom of flux tubes. Both the transverse and Alfvén oscillations can lead to the KHI. Meanwhile, the Alfvén oscillations established in loops will experience phase mixing. Both processes will generate small spatial-scale structures, which can help the dissipation of wave energy. Indeed, we observe the increase of internal energy and temperature in loop regions. The heating is more pronounced for the simulation containing the mixed kink and Alfvén driver. This means that the mixed wavemodes can lead to a more efficient energy dissipation in the turbulent state of the plasma and that the KHI eddies act as an agent to dissipate energy in other wave modes. Furthermore, we also obtained forward modelling results using the FoMo code. We obtained forward models which are very similar to the observations of decayless oscillations. Due to the limited resolution of instruments, neither Alfvén modes nor the fine structures are observable. Therefore, this numerical study shows that Alfvén modes probably can co-exist with kink modes, leading to enhanced heating.
Funding: This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No.724326 and No.647214) (IDM). P.A. acknowledges funding from his STFC Ernest Rutherford Fellowship (No. ST/R004285/1).
2019-01-10T00:00:00ZGuo, MingzheVan Doorsselaere, TomKarampelas, KostasLi, BoAntolin, PatrickDe Moortel, InekeRecent numerical studies revealed that transverse motions of coronal loops can induce the Kelvin-Helmholtz Instability (KHI). This process could be important in coronal heating because it leads to dissipation of energy at small spatial-scale plasma interactions. Meanwhile, small amplitude decayless oscillations in coronal loops have been discovered recently in observations of SDO/AIA. We model such oscillations in coronal loops and study wave heating effects, considering a kink and Alfvén driver separately and a mixed driver at the bottom of flux tubes. Both the transverse and Alfvén oscillations can lead to the KHI. Meanwhile, the Alfvén oscillations established in loops will experience phase mixing. Both processes will generate small spatial-scale structures, which can help the dissipation of wave energy. Indeed, we observe the increase of internal energy and temperature in loop regions. The heating is more pronounced for the simulation containing the mixed kink and Alfvén driver. This means that the mixed wavemodes can lead to a more efficient energy dissipation in the turbulent state of the plasma and that the KHI eddies act as an agent to dissipate energy in other wave modes. Furthermore, we also obtained forward modelling results using the FoMo code. We obtained forward models which are very similar to the observations of decayless oscillations. Due to the limited resolution of instruments, neither Alfvén modes nor the fine structures are observable. Therefore, this numerical study shows that Alfvén modes probably can co-exist with kink modes, leading to enhanced heating.Uphill acceleration in a spatially modulated electrostatic field particle acceleratorAlmansa, IBurton, DavidCairns, Robert AlanMarini, SPeter, EduardoRizzato, FelipeRussman, Fhttp://hdl.handle.net/10023/164682018-11-22T14:30:04Z2018-11-01T00:00:00ZSpatially modulated electrostatic fields can be designed to efficiently accelerate particles by exploring the relations between the amplitude, the phase velocity, the shape of the potential and the initial velocity of the particle. The acceleration process occurs when the value of the velocity excursions of the particle surpass the phase velocity of the carrier, as a resonant mechanism. The ponderomotive approximation based on the Lagrangian average is usually applied in this kind of system in non accelerating regimes. The mean dynamics of the particle is well described by this approximation far from resonance. However, the approximation fails to predict some interesting features of the model near resonance, such as the uphill acceleration phenomenon. A canonical perturbation theory is more accurate in these conditions. In this work we compare the results from the Lagrangian average and from a canonical perturbation theory, focusing in regions where the results of these two approaches differ from each other.
Funding: UK Science and Engineering Research Council grant EP/N028694/1.
2018-11-01T00:00:00ZAlmansa, IBurton, DavidCairns, Robert AlanMarini, SPeter, EduardoRizzato, FelipeRussman, FSpatially modulated electrostatic fields can be designed to efficiently accelerate particles by exploring the relations between the amplitude, the phase velocity, the shape of the potential and the initial velocity of the particle. The acceleration process occurs when the value of the velocity excursions of the particle surpass the phase velocity of the carrier, as a resonant mechanism. The ponderomotive approximation based on the Lagrangian average is usually applied in this kind of system in non accelerating regimes. The mean dynamics of the particle is well described by this approximation far from resonance. However, the approximation fails to predict some interesting features of the model near resonance, such as the uphill acceleration phenomenon. A canonical perturbation theory is more accurate in these conditions. In this work we compare the results from the Lagrangian average and from a canonical perturbation theory, focusing in regions where the results of these two approaches differ from each other.Incorporating animal movement with distance sampling and spatial capture-recaptureGlennie, Richardhttp://hdl.handle.net/10023/164672018-11-15T09:16:10Z2018-12-06T00:00:00ZDistance sampling and spatial capture-recapture are statistical methods to estimate the
number of animals in a wild population based on encounters between these animals and
scientific detectors. Both methods estimate the probability an animal is detected during a
survey, but do not explicitly model animal movement.
The primary challenge is that animal movement in these surveys is unobserved; one must
average over all possible paths each animal could have travelled during the survey. In this
thesis, a general statistical model, with distance sampling and spatial capture-recapture
as special cases, is presented that explicitly incorporates animal movement. An efficient
algorithm to integrate over all possible movement paths, based on quadrature and hidden
Markov modelling, is given to overcome the computational obstacles.
For distance sampling, simulation studies and case studies show that incorporating animal
movement can reduce the bias in estimated abundance found in conventional models and
expand application of distance sampling to surveys that violate the assumption of no animal
movement. For spatial capture-recapture, continuous-time encounter records are used to
make detailed inference on where animals spend their time during the survey. In surveys
conducted in discrete occasions, maximum likelihood models that allow for mobile activity
centres are presented to account for transience, dispersal, and heterogeneous space use.
These methods provide an alternative when animal movement causes bias in standard methods and the opportunity to gain richer inference on how animals move, where they spend
their time, and how they interact.
2018-12-06T00:00:00ZGlennie, RichardDistance sampling and spatial capture-recapture are statistical methods to estimate the
number of animals in a wild population based on encounters between these animals and
scientific detectors. Both methods estimate the probability an animal is detected during a
survey, but do not explicitly model animal movement.
The primary challenge is that animal movement in these surveys is unobserved; one must
average over all possible paths each animal could have travelled during the survey. In this
thesis, a general statistical model, with distance sampling and spatial capture-recapture
as special cases, is presented that explicitly incorporates animal movement. An efficient
algorithm to integrate over all possible movement paths, based on quadrature and hidden
Markov modelling, is given to overcome the computational obstacles.
For distance sampling, simulation studies and case studies show that incorporating animal
movement can reduce the bias in estimated abundance found in conventional models and
expand application of distance sampling to surveys that violate the assumption of no animal
movement. For spatial capture-recapture, continuous-time encounter records are used to
make detailed inference on where animals spend their time during the survey. In surveys
conducted in discrete occasions, maximum likelihood models that allow for mobile activity
centres are presented to account for transience, dispersal, and heterogeneous space use.
These methods provide an alternative when animal movement causes bias in standard methods and the opportunity to gain richer inference on how animals move, where they spend
their time, and how they interact.Effects of neonicotinoids on bees: an invalid experimentBailey, R. A.Greenwood, J. J. D.http://hdl.handle.net/10023/164442019-01-06T03:32:21Z2018-01-01T00:00:00ZWe use a recent study on the effects of neonicotinoids on bees as a concrete example to reinforce the advice of OEPP/EPPO (2010) that strong inference is impossible if there is no true replication and that analyses based on pseudoreplication are invalid.
2018-01-01T00:00:00ZBailey, R. A.Greenwood, J. J. D.We use a recent study on the effects of neonicotinoids on bees as a concrete example to reinforce the advice of OEPP/EPPO (2010) that strong inference is impossible if there is no true replication and that analyses based on pseudoreplication are invalid.Techniques for estimating the size of low-density gopher tortoise populationsStober, Jonathan M.Prieto-Gonzalez, RocioSmith, Lora L.Marques, Tiago A.Thomas, Lenhttp://hdl.handle.net/10023/164342019-01-06T03:38:17Z2017-12-01T00:00:00ZGopher tortoises (Gopherus polyphemus) are candidates for range-wide listing as threatened under the U.S. Endangered Species Act. Reliable population estimates are important to inform policy and management for recovery of the species. Line transect distance sampling has been adopted as the preferred method to estimate population size. However, when tortoise density is low, it can be challenging to obtain enough tortoise observations to reliably estimate the probability of detection, a vital component of the method. We suggest a modification to the method based on counting usable tortoise burrows (more abundant than tortoises) and separately accounting for the proportion of burrows occupied by tortoises. The increased sample size of burrows can outweigh the additional uncertainty induced by the need to account for the proportion of burrows occupied. We demonstrate the method using surveys conducted within a 13,118-ha portion of the Gopher Tortoise Habitat Management Unit at Fort Gordon Army Installation, Georgia. We used a systematic random design to obtain more precise estimates, using a newly developed systematic variance estimator. Individual transects had a spatially efficient design (pseudocircuits), which greatly improved sampling efficiency on this large site. Estimated burrow density was 0.091 ± 0.011 burrows/ha (CV = 12.6%, 95% CI = 0.071–0.116), with 25% of burrows occupied by a tortoise (CV = 14.4%), yielding a tortoise density of 0.023 ± 0.004 tortoise/ha (CV = 19.0%, 95% CI = 0.016–0.033) and a population estimate of 297 tortoises (95% CI = 210–433). These techniques are applicable to other studies and species. Surveying burrows or nests, rather than animals, can produce more reliable estimates when it leads to a significantly larger sample of detections and when the occupancy status can reliably be ascertained. Systematic line transect survey designs give better precision and are practical to implement and analyze.
Partial support by CEAUL (funded by Fundação para a Ciência e a Tecnologia, Portugal, through the project UID/MAT/00006/2013) (TAM).
2017-12-01T00:00:00ZStober, Jonathan M.Prieto-Gonzalez, RocioSmith, Lora L.Marques, Tiago A.Thomas, LenGopher tortoises (Gopherus polyphemus) are candidates for range-wide listing as threatened under the U.S. Endangered Species Act. Reliable population estimates are important to inform policy and management for recovery of the species. Line transect distance sampling has been adopted as the preferred method to estimate population size. However, when tortoise density is low, it can be challenging to obtain enough tortoise observations to reliably estimate the probability of detection, a vital component of the method. We suggest a modification to the method based on counting usable tortoise burrows (more abundant than tortoises) and separately accounting for the proportion of burrows occupied by tortoises. The increased sample size of burrows can outweigh the additional uncertainty induced by the need to account for the proportion of burrows occupied. We demonstrate the method using surveys conducted within a 13,118-ha portion of the Gopher Tortoise Habitat Management Unit at Fort Gordon Army Installation, Georgia. We used a systematic random design to obtain more precise estimates, using a newly developed systematic variance estimator. Individual transects had a spatially efficient design (pseudocircuits), which greatly improved sampling efficiency on this large site. Estimated burrow density was 0.091 ± 0.011 burrows/ha (CV = 12.6%, 95% CI = 0.071–0.116), with 25% of burrows occupied by a tortoise (CV = 14.4%), yielding a tortoise density of 0.023 ± 0.004 tortoise/ha (CV = 19.0%, 95% CI = 0.016–0.033) and a population estimate of 297 tortoises (95% CI = 210–433). These techniques are applicable to other studies and species. Surveying burrows or nests, rather than animals, can produce more reliable estimates when it leads to a significantly larger sample of detections and when the occupancy status can reliably be ascertained. Systematic line transect survey designs give better precision and are practical to implement and analyze.Some isomorphism results for Thompson-like groups Vn(G) Bleak, CollinDonoven, CaseyJonusas, Juliushttp://hdl.handle.net/10023/164132019-01-06T02:58:39Z2017-11-08T00:00:00ZWe find some perhaps surprising isomorphism results for the groups {Vn(G)}, where Vn(G) is a supergroup of the Higman–Thompson group Vn for n ∈ N and G ≤ Sn, the symmetric group on n points. These groups, introduced by Farley and Hughes, are the groups generated by Vn and the tree automorphisms [α]g defined as follows. For each g ∈ G and each node α in the infinite rooted n-ary tree, the automorphisms [α]g acts iteratively as g on the child leaves of α and every descendent of α. In particular, we show that Vn ≅ Vn(G) if and only if G is semiregular (acts freely on n points), as well as some additional sufficient conditions for isomorphisms between other members of this family of groups. Essential tools in the above work are a study of the dynamics of the action of elements of Vn(G) on the Cantor space, Rubin’s Theorem, and transducers from Grigorchuk, Nekrashevych, and Suschanskiĭ’s rational group on the n-ary alphabet.
2017-11-08T00:00:00ZBleak, CollinDonoven, CaseyJonusas, JuliusWe find some perhaps surprising isomorphism results for the groups {Vn(G)}, where Vn(G) is a supergroup of the Higman–Thompson group Vn for n ∈ N and G ≤ Sn, the symmetric group on n points. These groups, introduced by Farley and Hughes, are the groups generated by Vn and the tree automorphisms [α]g defined as follows. For each g ∈ G and each node α in the infinite rooted n-ary tree, the automorphisms [α]g acts iteratively as g on the child leaves of α and every descendent of α. In particular, we show that Vn ≅ Vn(G) if and only if G is semiregular (acts freely on n points), as well as some additional sufficient conditions for isomorphisms between other members of this family of groups. Essential tools in the above work are a study of the dynamics of the action of elements of Vn(G) on the Cantor space, Rubin’s Theorem, and transducers from Grigorchuk, Nekrashevych, and Suschanskiĭ’s rational group on the n-ary alphabet.Finiteness properties of direct products of algebraic structuresMayr, P.Ruskuc, Nikolahttp://hdl.handle.net/10023/164052019-01-06T03:31:22Z2018-01-15T00:00:00ZWe consider the preservation of properties of being finitely generated, being finitely presented and being residually finite under direct products in the context of different types of algebraic structures. The structures considered include Mal’cev algebras (including groups, rings and other classical algebras, as well as loops), idempotent algebras (including lattices), semigroups, and algebras in congruence modular varieties. We aim to identify as broadclasses as possible in which the ‘expected’ preservation results ( A × B satisfies property P if and only if A and B satisfy P) hold, and to exhibit ways in which they may fail outside those classes.
2018-01-15T00:00:00ZMayr, P.Ruskuc, NikolaWe consider the preservation of properties of being finitely generated, being finitely presented and being residually finite under direct products in the context of different types of algebraic structures. The structures considered include Mal’cev algebras (including groups, rings and other classical algebras, as well as loops), idempotent algebras (including lattices), semigroups, and algebras in congruence modular varieties. We aim to identify as broadclasses as possible in which the ‘expected’ preservation results ( A × B satisfies property P if and only if A and B satisfy P) hold, and to exhibit ways in which they may fail outside those classes.Dimensions of sets which uniformly avoid arithmetic progressionsFraser, Jonathan MacDonaldSaito, KotaYu, Hanhttp://hdl.handle.net/10023/163742018-11-02T00:48:19Z2017-11-02T00:00:00ZWe provide estimates for the dimensions of sets in ℝ which uniformly avoid finite arithmetic progressions (APs). More precisely, we say F uniformly avoids APs of length k≥3 if there is an ϵ>0 such that one cannot find an AP of length k and gap length Δ>0 inside the ϵΔ neighbourhood of F. Our main result is an explicit upper bound for the Assouad (and thus Hausdorff) dimension of such sets in terms of k and ϵ. In the other direction, we provide examples of sets which uniformly avoid APs of a given length but still have relatively large Hausdorff dimension. We also consider higher dimensional analogues of these problems, where APs are replaced with arithmetic patches lying in a hyperplane. As a consequence, we obtain a discretized version of a “reverse Kakeya problem:” we show that if the dimension of a set in ℝd is sufficiently large, then it closely approximates APs in every direction.
2017-11-02T00:00:00ZFraser, Jonathan MacDonaldSaito, KotaYu, HanWe provide estimates for the dimensions of sets in ℝ which uniformly avoid finite arithmetic progressions (APs). More precisely, we say F uniformly avoids APs of length k≥3 if there is an ϵ>0 such that one cannot find an AP of length k and gap length Δ>0 inside the ϵΔ neighbourhood of F. Our main result is an explicit upper bound for the Assouad (and thus Hausdorff) dimension of such sets in terms of k and ϵ. In the other direction, we provide examples of sets which uniformly avoid APs of a given length but still have relatively large Hausdorff dimension. We also consider higher dimensional analogues of these problems, where APs are replaced with arithmetic patches lying in a hyperplane. As a consequence, we obtain a discretized version of a “reverse Kakeya problem:” we show that if the dimension of a set in ℝd is sufficiently large, then it closely approximates APs in every direction.Dimensions of equilibrium measures on a class of planar self-affine setsFraser, Jonathan MacDonaldJordan, ThomasJurga, Nataliahttp://hdl.handle.net/10023/163612018-11-11T00:50:31Z2018-06-11T00:00:00ZWe study equilibrium measures (Käenmäki measure) supported on self-affine sets generated by a finite collection of diagonal and anti-diagonal matrices acting on the plane and satisfying the strong separation property. Our main result is that such measures are exact dimensional and the dimension satisfies the Ledrappier-Young formula, which gives an explicit expression for the dimension in terms of the entropy and Lyapunov exponents as well as the dimension of a coordinate projection of the measure. In particular, we do this by showing that the Käenmäki measure is equal to the sum of (the pushforwards) of two Gibbs measures on an associated subshift of finite type.
Funding: JMF was financially supported by a Leverhulme Trust Research Fellowship (RF-2016-500).
2018-06-11T00:00:00ZFraser, Jonathan MacDonaldJordan, ThomasJurga, NataliaWe study equilibrium measures (Käenmäki measure) supported on self-affine sets generated by a finite collection of diagonal and anti-diagonal matrices acting on the plane and satisfying the strong separation property. Our main result is that such measures are exact dimensional and the dimension satisfies the Ledrappier-Young formula, which gives an explicit expression for the dimension in terms of the entropy and Lyapunov exponents as well as the dimension of a coordinate projection of the measure. In particular, we do this by showing that the Käenmäki measure is equal to the sum of (the pushforwards) of two Gibbs measures on an associated subshift of finite type.Estimating cetacean density and abundance in the Central and Western Mediterranean Sea through aerial surveys : implications for managementPanigada, SimoneLauriano, GiancarloDonovan, GregPierantonio, NinoCañadas, AnaVázquez, José AntonioBurt, Louisehttp://hdl.handle.net/10023/163272019-01-06T03:16:40Z2017-07-01T00:00:00ZSystematic, effective monitoring of animal population parameters underpins successful conservation strategy and wildlife management, but it is often neglected in many regions, including much of the Mediterranean Sea. Nonetheless, a series of systematic multispecies aerial surveys was carried out in the seas around Italy to gather important baseline information on cetacean occurrence, distribution and abundance. The monitored areas included the Pelagos Sanctuary, the Tyrrhenian Sea, portions of the Seas of Corsica and Sardinia, the Ionian Seas as well as the Gulf of Taranto. Overall, approximately 48,000 km were flown in either spring, summer and winter between 2009–2014, covering an area of 444,621 km2. The most commonly observed species were the striped dolphin and the fin whale, with 975 and 83 recorded sightings, respectively. Other sighted cetacean species were the common bottlenose dolphin, the Risso's dolphin, the sperm whale, the pilot whale and the Cuvier's beaked whale. Uncorrected model- and design-based estimates of density and abundance for striped dolphins and fin whales were produced, resulting in a best estimate (model-based) of around 95,000 striped dolphins (CV=11.6%; 95% CI=92,900–120,300) occurring in the Pelagos Sanctuary, Central Tyrrhenian and Western Seas of Corsica and Sardinia combined area in summer 2010. Estimates were also obtained for each individual study region and year. An initial attempt to estimate perception bias for striped dolphins is also provided. The preferred summer 2010 uncorrected best estimate (design-based) for the same areas for fin whales was around 665 (CV=33.1%; 95% CI=350–1,260). Estimates are also provided for the individual study regions and years. The results represent baseline data to develop efficient, long-term, systematic monitoring programmes, essential to evaluate trends, as required by a number of national and international frameworks, and stress the need to ensure that surveys are undertaken regularly and at a sufficient spatial scale. The management implications of the results are discussed also in light of a possible decline of fin whales abundance over the period from the mid-1990s to the present. Further work to understand changes in distribution and to allow for improved spatial models is emphasized.
The equipment for data collection was that used during the EU LIFE Nature programme, project LIFE04NAT/GB/000245 project ‘‘Small Cetaceans in the European Atlantic and North Sea (SCANS-II)’’, funded in 2006.
2017-07-01T00:00:00ZPanigada, SimoneLauriano, GiancarloDonovan, GregPierantonio, NinoCañadas, AnaVázquez, José AntonioBurt, LouiseSystematic, effective monitoring of animal population parameters underpins successful conservation strategy and wildlife management, but it is often neglected in many regions, including much of the Mediterranean Sea. Nonetheless, a series of systematic multispecies aerial surveys was carried out in the seas around Italy to gather important baseline information on cetacean occurrence, distribution and abundance. The monitored areas included the Pelagos Sanctuary, the Tyrrhenian Sea, portions of the Seas of Corsica and Sardinia, the Ionian Seas as well as the Gulf of Taranto. Overall, approximately 48,000 km were flown in either spring, summer and winter between 2009–2014, covering an area of 444,621 km2. The most commonly observed species were the striped dolphin and the fin whale, with 975 and 83 recorded sightings, respectively. Other sighted cetacean species were the common bottlenose dolphin, the Risso's dolphin, the sperm whale, the pilot whale and the Cuvier's beaked whale. Uncorrected model- and design-based estimates of density and abundance for striped dolphins and fin whales were produced, resulting in a best estimate (model-based) of around 95,000 striped dolphins (CV=11.6%; 95% CI=92,900–120,300) occurring in the Pelagos Sanctuary, Central Tyrrhenian and Western Seas of Corsica and Sardinia combined area in summer 2010. Estimates were also obtained for each individual study region and year. An initial attempt to estimate perception bias for striped dolphins is also provided. The preferred summer 2010 uncorrected best estimate (design-based) for the same areas for fin whales was around 665 (CV=33.1%; 95% CI=350–1,260). Estimates are also provided for the individual study regions and years. The results represent baseline data to develop efficient, long-term, systematic monitoring programmes, essential to evaluate trends, as required by a number of national and international frameworks, and stress the need to ensure that surveys are undertaken regularly and at a sufficient spatial scale. The management implications of the results are discussed also in light of a possible decline of fin whales abundance over the period from the mid-1990s to the present. Further work to understand changes in distribution and to allow for improved spatial models is emphasized.Online resource for the history of astronomyTalbot, GemmaShand, IanFalconer, IsobelRobertson, EdmundO'Connor, Johnhttp://hdl.handle.net/10023/163142019-01-06T03:38:46Z2018-02-01T00:00:00Z2018-02-01T00:00:00ZTalbot, GemmaShand, IanFalconer, IsobelRobertson, EdmundO'Connor, JohnThe moist parcel-in-cell method for modelling moist convectionDritschel, D. G.Böing, S. J.Parker, D. J.Blyth, A. M.http://hdl.handle.net/10023/162862019-01-06T03:36:20Z2018-10-04T00:00:00ZWe describe a promising alternative approach to modelling moist convection and cloud development in the atmosphere. Rather than using a conventional grid‐based approach, we use Lagrangian “parcels” to represent key dynamical and thermodynamical variables. In the prototype model considered, parcels carry vorticity, mass, specific humidity, and liquid‐water potential temperature. In this first study, we ignore precipitation, and many of these parcel “attributes” remain unchanged (i.e. are materially conserved). While the vorticity does change following the parcel motion, the vorticity tendency is readily computed and, crucially, unwanted numerical diffusion can be avoided. The model, called “Moist Parcel‐In‐Cell” (MPIC), is a hybrid approach which uses both parcels and a fixed underlying grid for efficiency: advection (here moving parcels) is Lagrangian whereas inversion (determining the velocity field) is Eulerian. The parcel‐based representation of key variables has several advantages: (a) it allows an explicit subgrid representation; (b) it provides a velocity field which is undamped by numerical diffusion all the way down to the grid scale; (c) it does away with the need for eddy viscosity parametrizations and, in their place, it provides for a natural subgrid parcel mixing; (d) it is exactly conservative (i.e. there can be no net loss or gain of any theoretically conserved attribute); and (e) it dispenses with the need to have separate equations for each conserved parcel attribute; attributes are simply labels carried by each parcel. Moreover, the latter advantage increases as more attributes are added, such as the distributions of microphysical properties, chemical composition and aerosol loading.
The authors gratefully acknowledge support for this research from the EPSRC Maths Foresees Network. The numerical model development was carried out under the grant “A prototype vortex-in-cell algorithm for modelling moist convection” from March to October 2016. Steven Böing is partially funded through the NERC/Met Office Joint Programme on Understanding and Representing Atmospheric Convection across Scales (GENESIS, grant number NE/N013840/1). Doug Parker is supported by a Royal Society Wolfson Research Merit Award.
2018-10-04T00:00:00ZDritschel, D. G.Böing, S. J.Parker, D. J.Blyth, A. M.We describe a promising alternative approach to modelling moist convection and cloud development in the atmosphere. Rather than using a conventional grid‐based approach, we use Lagrangian “parcels” to represent key dynamical and thermodynamical variables. In the prototype model considered, parcels carry vorticity, mass, specific humidity, and liquid‐water potential temperature. In this first study, we ignore precipitation, and many of these parcel “attributes” remain unchanged (i.e. are materially conserved). While the vorticity does change following the parcel motion, the vorticity tendency is readily computed and, crucially, unwanted numerical diffusion can be avoided. The model, called “Moist Parcel‐In‐Cell” (MPIC), is a hybrid approach which uses both parcels and a fixed underlying grid for efficiency: advection (here moving parcels) is Lagrangian whereas inversion (determining the velocity field) is Eulerian. The parcel‐based representation of key variables has several advantages: (a) it allows an explicit subgrid representation; (b) it provides a velocity field which is undamped by numerical diffusion all the way down to the grid scale; (c) it does away with the need for eddy viscosity parametrizations and, in their place, it provides for a natural subgrid parcel mixing; (d) it is exactly conservative (i.e. there can be no net loss or gain of any theoretically conserved attribute); and (e) it dispenses with the need to have separate equations for each conserved parcel attribute; attributes are simply labels carried by each parcel. Moreover, the latter advantage increases as more attributes are added, such as the distributions of microphysical properties, chemical composition and aerosol loading. A short review of the distribution of short-beaked common dolphins (Delphinus delphis) in the central and eastern North Atlantic with an abundance estimate for part of this areaCanadas, Ana MariaDonovan, Greg PDesportes, GenevièveBorchers, David Louishttp://hdl.handle.net/10023/162802018-12-16T00:37:05Z2009-01-01T00:00:00ZThis paper uses data from 3 programmes: (1) the North Atlantic Sightings Surveys (NASS) surveys undertaken throughout much of the central and eastern North Atlantic north of about 40° N in 1987, 1989, 1995 and 2001; (2) the MICA-93 programme; and (3) the north eastern Atlantic segment of the Small Cetacean Abundance in the North Sea (SCANS) survey in 1994. The data from all surveys were used to examine the distribution of common dolphins in the NE Atlantic. No sightings were made north of 57° N. An initial attempt to examine distribution against 4 potential non biological explanatory variables was made. A simple interpretation of the preliminary analyses presented here is that the primary areas for groups of common dolphins were in waters over 15° C and depths of 400-1,000 m (there does appear a link with shelf features), between around 49°-55° N especially between 20°-30°W. An illustrative example of spatial modelling is presented. Only for 1 year (and part of the total survey area) were there sufficient data to attempt to estimate abundance: 1995. The estimated abundance in the W Block of the NASS-95 Faroese survey was 273,159 (cv=0.26; 95% CI=153,392-435,104) short-beaked common dolphins. This estimate is corrected for animals missed on the trackline (g(0)) and for responsive movement.
2009-01-01T00:00:00ZCanadas, Ana MariaDonovan, Greg PDesportes, GenevièveBorchers, David LouisThis paper uses data from 3 programmes: (1) the North Atlantic Sightings Surveys (NASS) surveys undertaken throughout much of the central and eastern North Atlantic north of about 40° N in 1987, 1989, 1995 and 2001; (2) the MICA-93 programme; and (3) the north eastern Atlantic segment of the Small Cetacean Abundance in the North Sea (SCANS) survey in 1994. The data from all surveys were used to examine the distribution of common dolphins in the NE Atlantic. No sightings were made north of 57° N. An initial attempt to examine distribution against 4 potential non biological explanatory variables was made. A simple interpretation of the preliminary analyses presented here is that the primary areas for groups of common dolphins were in waters over 15° C and depths of 400-1,000 m (there does appear a link with shelf features), between around 49°-55° N especially between 20°-30°W. An illustrative example of spatial modelling is presented. Only for 1 year (and part of the total survey area) were there sufficient data to attempt to estimate abundance: 1995. The estimated abundance in the W Block of the NASS-95 Faroese survey was 273,159 (cv=0.26; 95% CI=153,392-435,104) short-beaked common dolphins. This estimate is corrected for animals missed on the trackline (g(0)) and for responsive movement.Minke whale abundance estimation from the NASS 1987 and 2001 aerial cue–counting surveys taking appropriate account of distance estimation errorsBorchers, David LouisPike, Daniel GGunnlaugsson, ThorvaldurVíkingsson, Gísli Ahttp://hdl.handle.net/10023/162792018-12-16T00:37:04Z2009-01-01T00:00:00ZWe estimate the abundance of minke whales (Balaenoptera acutorostrata) from the Icelandic coastal shelf aerial surveys carried out as part of the 1987 and 2001 North Atlantic Sightings Surveys (NASS). In the case of the 1987 survey, the probability of detecting animals at distance zero (g(0)) is very close to 1 but there is substantial random measurement error in estimating distances. To estimate abundance from these data, we use methods which assume g(0)=1 but which includea distance measurement error model. In the case of the 2001 survey, measurement errors were sufficiently small to be negligible, and we use double platform methods which estimate g(0) and assume no measurement error to estimate abundance. From the 1987 survey, we estimate abundance to be 24,532 animals, with 95% CI (13,399; 44,916). From the 2001 NASS survey data, minke whale abundance is estimated to be 43,633 animals, with 95% CI (30,148; 63,149).
2009-01-01T00:00:00ZBorchers, David LouisPike, Daniel GGunnlaugsson, ThorvaldurVíkingsson, Gísli AWe estimate the abundance of minke whales (Balaenoptera acutorostrata) from the Icelandic coastal shelf aerial surveys carried out as part of the 1987 and 2001 North Atlantic Sightings Surveys (NASS). In the case of the 1987 survey, the probability of detecting animals at distance zero (g(0)) is very close to 1 but there is substantial random measurement error in estimating distances. To estimate abundance from these data, we use methods which assume g(0)=1 but which includea distance measurement error model. In the case of the 2001 survey, measurement errors were sufficiently small to be negligible, and we use double platform methods which estimate g(0) and assume no measurement error to estimate abundance. From the 1987 survey, we estimate abundance to be 24,532 animals, with 95% CI (13,399; 44,916). From the 2001 NASS survey data, minke whale abundance is estimated to be 43,633 animals, with 95% CI (30,148; 63,149).Embedding right-angled Artin groups into Brin-Thompson groupsBelk, JamesBleak, CollinMatucci, Francescohttp://hdl.handle.net/10023/162772018-12-30T00:40:53Z2018-07-26T00:00:00ZWe prove that every finitely-generated right-angled Artin group can be embedded into some Brin-Thompson group nV. It follows that many other groups can be embedded into some nV (e.g., any finite extension of any of Haglund and Wise's special groups), and that various decision problems involving subgroups of nV are unsolvable.
2018-07-26T00:00:00ZBelk, JamesBleak, CollinMatucci, FrancescoWe prove that every finitely-generated right-angled Artin group can be embedded into some Brin-Thompson group nV. It follows that many other groups can be embedded into some nV (e.g., any finite extension of any of Haglund and Wise's special groups), and that various decision problems involving subgroups of nV are unsolvable.Broadening of the differential emission measure by multi-shelled and turbulent loopsVan Doorsselaere, TomAntolin, PatrickKarampelas, Kostashttp://hdl.handle.net/10023/162752018-12-07T12:30:06Z2018-12-01T00:00:00ZContext. Broad differential emission measure (DEM) distributions in the corona are a sign of multi-thermal plasma along the line-of-sight.Traditionally, this is interpreted as evidence of multi-stranded loops. Recently, however, it has been shown that multi-stranded loops are unlikely to exist in the solar corona, because of their instability to transverse perturbations. Aims. We aim to test if loop models subject to the Transverse Wave-Induced Kelvin-Helmholtz (TWIKH) instability result in broad DEMs, potentially explaining the observations. We took simulation snapshots and compute the numerical DEM. Moreover, we performed forward-modelling in the relevant AIA channels before reconstructing the DEM. Results. We find that turbulent loop models broaden their initial DEM, because of the turbulent mixing. The width of the DEM is determined by the initial temperature contrast with the exterior. Conclusions. We conclude that impulsively excited loop models have a rather narrow DEM, but that continuously driven models result in broad DEMs that are comparable to the observations.
Funding: UK Science and Technology Facilities Council (Consolidated Grant ST/K000950/1), the European Union Horizon 2020 research and innovation programme (grant agreement No. 647214) and STFC Ernest Rutherford Fellowship (grant agreement No. ST/R004285/1) (PA).
2018-12-01T00:00:00ZVan Doorsselaere, TomAntolin, PatrickKarampelas, KostasContext. Broad differential emission measure (DEM) distributions in the corona are a sign of multi-thermal plasma along the line-of-sight.Traditionally, this is interpreted as evidence of multi-stranded loops. Recently, however, it has been shown that multi-stranded loops are unlikely to exist in the solar corona, because of their instability to transverse perturbations. Aims. We aim to test if loop models subject to the Transverse Wave-Induced Kelvin-Helmholtz (TWIKH) instability result in broad DEMs, potentially explaining the observations. We took simulation snapshots and compute the numerical DEM. Moreover, we performed forward-modelling in the relevant AIA channels before reconstructing the DEM. Results. We find that turbulent loop models broaden their initial DEM, because of the turbulent mixing. The width of the DEM is determined by the initial temperature contrast with the exterior. Conclusions. We conclude that impulsively excited loop models have a rather narrow DEM, but that continuously driven models result in broad DEMs that are comparable to the observations.Surveying abundance and stand type associations of Formica aquilonia and F. lugubris (Hymenoptera: Formicidae) nest mounds over an extensive area : Trialing a novel methodBorkin, KerrySummers, RonThomas, Lenhttp://hdl.handle.net/10023/162602019-01-06T02:34:19Z2012-01-03T00:00:00ZRed wood ants are ecologically important members of woodland communities, and some species are of conservation concern. They occur commonly only in certain habitats in Britain, but there is limited knowledge of their numbers and distribution. This study provided baseline information at a key locality (Abernethy Forest, 37 km2) in the central Highlands of Scotland and trialed a new method of surveying red wood ant density and stand type associations: a distance sampling line transect survey of nests. This method is efficient because it allows an observer to quickly survey a large area either side of transect lines, without having to assume that all nests are detected. Instead, data collected on the distance of nests from the line are used to estimate probability of detection and the effective transect width, using the free software "Distance". Surveys took place in August and September 2003 along a total of 71.2 km of parallel, equally-spaced transects. One hundred and forty-four red wood ant nests were located, comprising 89 F. aquilonia (Yarrow, 1955) and 55 F. lugubris (Zetterstedt, 1838) nests. Estimated densities were 1.13 nests per hectare (95% CI 0.74-1.73) for F. aquilonia and 0.83 nests per hectare (95% CI 0.32-2.17) for F. lugubris. These translated to total estimated nest numbers of 4,200 (95% CI 2,700-6,400) and 3,100 (95% CI 1,200-8,100), respectively, for the whole forest. Indices of stand selection indicated that F. aquilonia had some positive association with old-growth and F. lugubris with younger stands (stem exclusion stage). No nests were found in areas that had been clear-felled, and ploughed and planted in the 1970s-1990s. The pattern of stand type association and hence distribution of F. aquilonia and F. lugubris may be due to the differing ability to disperse (F. lugubris is the faster disperser) and compete (F. aquilonia is competitively superior). We recommend using line transect sampling for extensive surveys of ants that construct nest mounds to estimate abundance and stand type association.
2012-01-03T00:00:00ZBorkin, KerrySummers, RonThomas, LenRed wood ants are ecologically important members of woodland communities, and some species are of conservation concern. They occur commonly only in certain habitats in Britain, but there is limited knowledge of their numbers and distribution. This study provided baseline information at a key locality (Abernethy Forest, 37 km2) in the central Highlands of Scotland and trialed a new method of surveying red wood ant density and stand type associations: a distance sampling line transect survey of nests. This method is efficient because it allows an observer to quickly survey a large area either side of transect lines, without having to assume that all nests are detected. Instead, data collected on the distance of nests from the line are used to estimate probability of detection and the effective transect width, using the free software "Distance". Surveys took place in August and September 2003 along a total of 71.2 km of parallel, equally-spaced transects. One hundred and forty-four red wood ant nests were located, comprising 89 F. aquilonia (Yarrow, 1955) and 55 F. lugubris (Zetterstedt, 1838) nests. Estimated densities were 1.13 nests per hectare (95% CI 0.74-1.73) for F. aquilonia and 0.83 nests per hectare (95% CI 0.32-2.17) for F. lugubris. These translated to total estimated nest numbers of 4,200 (95% CI 2,700-6,400) and 3,100 (95% CI 1,200-8,100), respectively, for the whole forest. Indices of stand selection indicated that F. aquilonia had some positive association with old-growth and F. lugubris with younger stands (stem exclusion stage). No nests were found in areas that had been clear-felled, and ploughed and planted in the 1970s-1990s. The pattern of stand type association and hence distribution of F. aquilonia and F. lugubris may be due to the differing ability to disperse (F. lugubris is the faster disperser) and compete (F. aquilonia is competitively superior). We recommend using line transect sampling for extensive surveys of ants that construct nest mounds to estimate abundance and stand type association.A comparison of inferential methods for highly nonlinear state space models in ecology and epidemiologyFasiolo, MatteoPya, NatalyaWood, Simon N.http://hdl.handle.net/10023/162502019-01-06T03:07:16Z2016-01-01T00:00:00ZHighly nonlinear, chaotic or near chaotic, dynamic models are important in fields such as ecology and epidemiology: for example, pest species and diseases often display highly nonlinear dynamics. However, such models are problematic from the point of view of statistical inference. The defining feature of chaotic and near chaotic systems is extreme sensitivity to small changes in system states and parameters, and this can interfere with inference. There are twomain classes ofmethods for circumventing these difficulties: information reduction approaches, such as Approximate Bayesian Computation or Synthetic Likelihood, and state space methods, such as Particle Markov chain Monte Carlo, Iterated Filtering or Parameter Cascading. The purpose of this article is to compare the methods in order to reach conclusions about how to approach inference with such models in practice. We show that neither class of methods is universally superior to the other. We show that state space methods can suffer multimodality problems in settings with low process noise or model misspecification, leading to bias toward stable dynamics and high process noise. Information reduction methods avoid this problem, but, under the correct model and with sufficient process noise, state space methods lead to substantially sharper inference than information reduction methods. More practically, there are also differences in the tuning requirements of different methods. Our overall conclusion is that model development and checking should probably be performed using an information reduction method with low tuning requirements, while for final inference it is likely to be better to switch to a state space method, checking results against the information reduction approach.
Most of this work was undertaken at the University of Bath, where M.F. was a Ph.D. student, and it was supported in part by EPSRC Grants EP/I000917 and EP/K005251/1.
2016-01-01T00:00:00ZFasiolo, MatteoPya, NatalyaWood, Simon N.Highly nonlinear, chaotic or near chaotic, dynamic models are important in fields such as ecology and epidemiology: for example, pest species and diseases often display highly nonlinear dynamics. However, such models are problematic from the point of view of statistical inference. The defining feature of chaotic and near chaotic systems is extreme sensitivity to small changes in system states and parameters, and this can interfere with inference. There are twomain classes ofmethods for circumventing these difficulties: information reduction approaches, such as Approximate Bayesian Computation or Synthetic Likelihood, and state space methods, such as Particle Markov chain Monte Carlo, Iterated Filtering or Parameter Cascading. The purpose of this article is to compare the methods in order to reach conclusions about how to approach inference with such models in practice. We show that neither class of methods is universally superior to the other. We show that state space methods can suffer multimodality problems in settings with low process noise or model misspecification, leading to bias toward stable dynamics and high process noise. Information reduction methods avoid this problem, but, under the correct model and with sufficient process noise, state space methods lead to substantially sharper inference than information reduction methods. More practically, there are also differences in the tuning requirements of different methods. Our overall conclusion is that model development and checking should probably be performed using an information reduction method with low tuning requirements, while for final inference it is likely to be better to switch to a state space method, checking results against the information reduction approach.A uniform dimension result for two-dimensional fractional multiplicative processesJin, Xionghttp://hdl.handle.net/10023/162492019-01-06T02:37:10Z2014-03-01T00:00:00ZWe show that a two-dimensional fractional multiplicative process has a uniform Hausdorff dimension result if and only if the two parameters of the process coincide.
2014-03-01T00:00:00ZJin, XiongWe show that a two-dimensional fractional multiplicative process has a uniform Hausdorff dimension result if and only if the two parameters of the process coincide.Crambled : a Shiny application to enable intuitive resolution of conflicting cellularity estimatesLynch, Andyhttp://hdl.handle.net/10023/162482019-01-06T03:19:51Z2015-12-07T00:00:00ZIt is now commonplace to investigate tumour samples using whole-genome sequencing, and some commonly performed tasks are the estimation of cellularity (or sample purity), the genome-wide profiling of copy numbers, and the assessment of sub-clonal behaviours. Several tools are available to undertake these tasks, but often give conflicting results - not least because there is often genuine uncertainty due to a lack of model identifiability. Presented here is a tool, "Crambled", that allows for an intuitive visual comparison of the conflicting solutions. Crambled is implemented as a Shiny application within R, and is accompanied by example images from two use cases (one tumour sample with matched normal sequencing, and one standalone cell line example) as well as functions to generate the necessary images from any sequencing data set. Through the use of Crambled, a user may gain insight into why each tool has offered its given solution and combined with a knowledge of the disease being studied can choose between the competing solutions in an informed manner.
2015-12-07T00:00:00ZLynch, AndyIt is now commonplace to investigate tumour samples using whole-genome sequencing, and some commonly performed tasks are the estimation of cellularity (or sample purity), the genome-wide profiling of copy numbers, and the assessment of sub-clonal behaviours. Several tools are available to undertake these tasks, but often give conflicting results - not least because there is often genuine uncertainty due to a lack of model identifiability. Presented here is a tool, "Crambled", that allows for an intuitive visual comparison of the conflicting solutions. Crambled is implemented as a Shiny application within R, and is accompanied by example images from two use cases (one tumour sample with matched normal sequencing, and one standalone cell line example) as well as functions to generate the necessary images from any sequencing data set. Through the use of Crambled, a user may gain insight into why each tool has offered its given solution and combined with a knowledge of the disease being studied can choose between the competing solutions in an informed manner.Friedrich Steinle, Exploratory Experiments: Ampère, Faraday and the Origins of Electrodynamics. Translated by Alex Levine. Pittsburgh, PA: University of Pittsburgh Press, 2016. Pp. 494. ISBN 978-0-8229-4450-8. $65.00 (hardcover)Falconer, Isobel Jessiehttp://hdl.handle.net/10023/162272018-12-02T00:49:08Z2018-06-01T00:00:00Z2018-06-01T00:00:00ZFalconer, Isobel JessieJohn W. Arthur, Brilliant Lives: the Clerk Maxwells and the Scottish EnlightenmentFalconer, Isobel Jessiehttp://hdl.handle.net/10023/162262018-12-10T09:37:55Z2017-11-01T00:00:00Z2017-11-01T00:00:00ZFalconer, Isobel JessieSequencing of prostate cancers identifies new cancer genes, routes of progression and drug targetsWedge, David C.Gundem, GunesMitchell, ThomasWoodstock, Dan J.Martincorena, InigoGhori, MohammedZamora, JorgeButler, AdamWhitaker, HayleyKote-Jarai, ZsofiaAlexandrov, Lumil B.Van Loo, PeterMassie, Charlie E.Dentro, StefanWarren, Anne Y.Verrill, ClareBerney, Dan M.Dennis, NeningMerson, SueHawkins, SteveHowat, WilliamLu, Yong-JieLambert, AdamKay, JonathanKremeyer, BarbaraKaraszi, KatalinCamacho, NiedzicaLuxton, HayleyMarsden, LukeEdwards, SandraMatthews, LucyBo, ValeriaLeongamornlert, DanielMcLaren, StuartNg, AnthonyYu, YongweiZhang, HongweiDadaev, TokhirThomas, SarahEaston, Douglas F.Ahmed, MahbublBancroft, ElizabethFisher, CyrilLivni, NaomiNicol, DavidTavaré, SimonGill, PelvenderGreenman, ChristopherKhoo, VincentVan As, NicholasKumar, PardeepOgden, ChristopherCahill, DeclanThompson, AlanMayer, ErikRowe, EdwardDudderidge, TimGnanapragasam, VincentShah, Nimish C.Raine, KeiranJones, DavidMenzies, AndrewStebbings, LucyTeague, JonHazell, StevenCorbishley, CathyCamCaP Study Groupde Bono, JohannAttard, GerhardtIsaacs, WilliamVisakorpi, TapioFraser, MichaelBoutros, Paul C.Bristow, Robert G.Workman, PaulSander, ChrisThe TCGA ConsortiumHamdy, Freddie C.Futreal, AndrewMcDermott, UltanAl-Lazikani, BissanLynch, AndyBova, G. StevenFoster, Christopher S.Brewer, Daniel S.Neal, David E.Cooper, Colin S.Eeles, Rosalind A.http://hdl.handle.net/10023/162192019-01-13T02:35:11Z2018-04-16T00:00:00ZProstate cancer represents a substantial clinical challenge because it is difficult to predict outcome and advanced disease is often fatal. We sequenced the whole genomes of 112 primary and metastatic prostate cancer samples. From joint analysis of these cancers with those from previous studies (930 cancers in total), we found evidence for 22 previously unidentified putative driver genes harboring coding mutations, as well as evidence for NEAT1 and FOXA1 acting as drivers through noncoding mutations. Through the temporal dissection of aberrations, we identified driver mutations specifically associated with steps in the progression of prostate cancer, establishing, for example, loss of CHD1 and BRCA2 as early events in cancer development of ETS fusion-negative cancers. Computational chemogenomic (canSAR) analysis of prostate cancer mutations identified 11 targets of approved drugs, 7 targets of investigational drugs, and 62 targets of compounds that may be active and should be considered candidates for future clinical trials.
2018-04-16T00:00:00ZWedge, David C.Gundem, GunesMitchell, ThomasWoodstock, Dan J.Martincorena, InigoGhori, MohammedZamora, JorgeButler, AdamWhitaker, HayleyKote-Jarai, ZsofiaAlexandrov, Lumil B.Van Loo, PeterMassie, Charlie E.Dentro, StefanWarren, Anne Y.Verrill, ClareBerney, Dan M.Dennis, NeningMerson, SueHawkins, SteveHowat, WilliamLu, Yong-JieLambert, AdamKay, JonathanKremeyer, BarbaraKaraszi, KatalinCamacho, NiedzicaLuxton, HayleyMarsden, LukeEdwards, SandraMatthews, LucyBo, ValeriaLeongamornlert, DanielMcLaren, StuartNg, AnthonyYu, YongweiZhang, HongweiDadaev, TokhirThomas, SarahEaston, Douglas F.Ahmed, MahbublBancroft, ElizabethFisher, CyrilLivni, NaomiNicol, DavidTavaré, SimonGill, PelvenderGreenman, ChristopherKhoo, VincentVan As, NicholasKumar, PardeepOgden, ChristopherCahill, DeclanThompson, AlanMayer, ErikRowe, EdwardDudderidge, TimGnanapragasam, VincentShah, Nimish C.Raine, KeiranJones, DavidMenzies, AndrewStebbings, LucyTeague, JonHazell, StevenCorbishley, CathyCamCaP Study Groupde Bono, JohannAttard, GerhardtIsaacs, WilliamVisakorpi, TapioFraser, MichaelBoutros, Paul C.Bristow, Robert G.Workman, PaulSander, ChrisThe TCGA ConsortiumHamdy, Freddie C.Futreal, AndrewMcDermott, UltanAl-Lazikani, BissanLynch, AndyBova, G. StevenFoster, Christopher S.Brewer, Daniel S.Neal, David E.Cooper, Colin S.Eeles, Rosalind A.Prostate cancer represents a substantial clinical challenge because it is difficult to predict outcome and advanced disease is often fatal. We sequenced the whole genomes of 112 primary and metastatic prostate cancer samples. From joint analysis of these cancers with those from previous studies (930 cancers in total), we found evidence for 22 previously unidentified putative driver genes harboring coding mutations, as well as evidence for NEAT1 and FOXA1 acting as drivers through noncoding mutations. Through the temporal dissection of aberrations, we identified driver mutations specifically associated with steps in the progression of prostate cancer, establishing, for example, loss of CHD1 and BRCA2 as early events in cancer development of ETS fusion-negative cancers. Computational chemogenomic (canSAR) analysis of prostate cancer mutations identified 11 targets of approved drugs, 7 targets of investigational drugs, and 62 targets of compounds that may be active and should be considered candidates for future clinical trials.The role of flux cancellation in eruptions from bipolar ARsYardley, S. L.Green, L. M.Driel-Gesztelyi, L. VanWilliams, D. R.Mackay, D. H.http://hdl.handle.net/10023/161732019-01-06T03:38:39Z2018-10-10T00:00:00ZThe physical processes or trigger mechanisms that lead to the eruption of coronal mass ejections (CMEs), the largest eruptive phenomenon in the heliosphere, are still undetermined. Low-altitude magnetic reconnection associated with flux cancellation appears to play an important role in CME occurrence as it can form an eruptive configuration and reduce the magnetic flux that contributes to the overlying, stabilizing field. We conduct the first comprehensive study of 20 small bipolar ARs (ARs) in order to probe the role of flux cancellation as an eruption trigger mechanism. We categorize eruptions from the bipolar regions into three types related to location, and find that the type of eruption produced depends on the evolutionary stage of the AR. In addition, we find that ARs that form eruptive structures by flux cancellation (low-altitude reconnection) had, on average, lower flux cancellation rates than the AR sample as a whole. Therefore, while flux cancellation plays a key role, by itself it is insufficient for the production of an eruption. The results provide supporting evidence that although flux cancellation in a sheared arcade may be able to build an eruptive configuration, a successful eruption depends upon the removal of sufficient overlying and stabilizing field. Convergence of the bipole polarities also appears to be present in regions that produce an eruption. These findings have important implications for understanding the physical processes that occur on our Sun in relation to CMEs and for space weather forecasting.
2018-10-10T00:00:00ZYardley, S. L.Green, L. M.Driel-Gesztelyi, L. VanWilliams, D. R.Mackay, D. H.The physical processes or trigger mechanisms that lead to the eruption of coronal mass ejections (CMEs), the largest eruptive phenomenon in the heliosphere, are still undetermined. Low-altitude magnetic reconnection associated with flux cancellation appears to play an important role in CME occurrence as it can form an eruptive configuration and reduce the magnetic flux that contributes to the overlying, stabilizing field. We conduct the first comprehensive study of 20 small bipolar ARs (ARs) in order to probe the role of flux cancellation as an eruption trigger mechanism. We categorize eruptions from the bipolar regions into three types related to location, and find that the type of eruption produced depends on the evolutionary stage of the AR. In addition, we find that ARs that form eruptive structures by flux cancellation (low-altitude reconnection) had, on average, lower flux cancellation rates than the AR sample as a whole. Therefore, while flux cancellation plays a key role, by itself it is insufficient for the production of an eruption. The results provide supporting evidence that although flux cancellation in a sheared arcade may be able to build an eruptive configuration, a successful eruption depends upon the removal of sufficient overlying and stabilizing field. Convergence of the bipole polarities also appears to be present in regions that produce an eruption. These findings have important implications for understanding the physical processes that occur on our Sun in relation to CMEs and for space weather forecasting.Role of extracellular matrix and microenvironment in regulation of tumor growth and LAR-mediated invasion in glioblastomaKim, YangjinKang, HyunjiPowathil, GibinKim, HyeongiTrucu, DumitruLee, WanhoLawler, SeanChaplain, Markhttp://hdl.handle.net/10023/161602019-01-06T03:38:30Z2018-10-04T00:00:00ZThe cellular dispersion and therapeutic control of glioblastoma, the most aggressive type of primary brain cancer, depends critically on the migration patterns after surgery and intracellular responses of the individual cancer cells in response to external biochemical cues in the microenvironment. Recent studies have shown that miR-451 regulates downstream molecules including AMPK/CAB39/MARK and mTOR to determine the balance between rapid proliferation and invasion in response to metabolic stress in the harsh tumor microenvironment. Surgical removal of the main tumor is inevitably followed by recurrence of the tumor due to inaccessibility of dispersed tumor cells in normal brain tissue. In order to address this complex process of cell proliferation and invasion and its response to conventional treatment, we propose a mathematical model that analyzes the intracellular dynamics of the miR-451-AMPK- mTOR-cell cycle signaling pathway within a cell. The model identifies a key mechanism underlying the molecular switches between proliferative phase and migratory phase in response to metabolic stress in response to fluctuating glucose levels. We show how up- or down-regulation of components in these pathways affects the key cellular decision to infiltrate or proliferate in a complex microenvironment in the absence and presence of time delays and stochastic noise. Glycosylated chondroitin sulfate proteoglycans (CSPGs), a major component of the extracellular matrix (ECM) in the brain, contribute to the physical structure of the local brain microenvironment but also induce or inhibit glioma invasion by regulating the dynamics of the CSPG receptor LAR as well as the spatiotemporal activation status of resident astrocytes and tumor-associated microglia. Using a multi-scale mathematical model, we investigate a CSPG-induced switch between invasive and non-invasive tumors through the coordination of ECM-cell adhesion and dynamic changes in stromal cells. We show that the CSPG-rich microenvironment is associated with non-invasive tumor lesions through LAR-CSGAG binding while the absence of glycosylated CSPGs induce the critical glioma invasion. We illustrate how high molecular weight CSPGs can regulate the exodus of local reactive astrocytes from the main tumor lesion, leading to encapsulation of non-invasive tumor and inhibition of tumor invasion. These different CSPG conditions also change the spatial profiles of ramified and activated microglia. The complex distribution of CSPGs in the tumor microenvironment can determine the nonlinear invasion behaviors of glioma cells, which suggests the need for careful therapeutic strategies.
This paper was supported by Konkuk University in 2014.
2018-10-04T00:00:00ZKim, YangjinKang, HyunjiPowathil, GibinKim, HyeongiTrucu, DumitruLee, WanhoLawler, SeanChaplain, MarkThe cellular dispersion and therapeutic control of glioblastoma, the most aggressive type of primary brain cancer, depends critically on the migration patterns after surgery and intracellular responses of the individual cancer cells in response to external biochemical cues in the microenvironment. Recent studies have shown that miR-451 regulates downstream molecules including AMPK/CAB39/MARK and mTOR to determine the balance between rapid proliferation and invasion in response to metabolic stress in the harsh tumor microenvironment. Surgical removal of the main tumor is inevitably followed by recurrence of the tumor due to inaccessibility of dispersed tumor cells in normal brain tissue. In order to address this complex process of cell proliferation and invasion and its response to conventional treatment, we propose a mathematical model that analyzes the intracellular dynamics of the miR-451-AMPK- mTOR-cell cycle signaling pathway within a cell. The model identifies a key mechanism underlying the molecular switches between proliferative phase and migratory phase in response to metabolic stress in response to fluctuating glucose levels. We show how up- or down-regulation of components in these pathways affects the key cellular decision to infiltrate or proliferate in a complex microenvironment in the absence and presence of time delays and stochastic noise. Glycosylated chondroitin sulfate proteoglycans (CSPGs), a major component of the extracellular matrix (ECM) in the brain, contribute to the physical structure of the local brain microenvironment but also induce or inhibit glioma invasion by regulating the dynamics of the CSPG receptor LAR as well as the spatiotemporal activation status of resident astrocytes and tumor-associated microglia. Using a multi-scale mathematical model, we investigate a CSPG-induced switch between invasive and non-invasive tumors through the coordination of ECM-cell adhesion and dynamic changes in stromal cells. We show that the CSPG-rich microenvironment is associated with non-invasive tumor lesions through LAR-CSGAG binding while the absence of glycosylated CSPGs induce the critical glioma invasion. We illustrate how high molecular weight CSPGs can regulate the exodus of local reactive astrocytes from the main tumor lesion, leading to encapsulation of non-invasive tumor and inhibition of tumor invasion. These different CSPG conditions also change the spatial profiles of ramified and activated microglia. The complex distribution of CSPGs in the tumor microenvironment can determine the nonlinear invasion behaviors of glioma cells, which suggests the need for careful therapeutic strategies.Entrapping of a vortex pair interacting with a fixed point vortex revisited. I. Point vorticesKoshel, Konstantin V.Reinaud, Jean N.Riccardi, GiogioRyzhov, Eugene A.http://hdl.handle.net/10023/161422019-01-06T03:37:42Z2018-09-28T00:00:00ZThe problem of a pair of point vortices impinging on a fixed point vortex of arbitrary strengths [E. Ryzhov and K. Koshel, “Dynamics of a vortex pair interacting with a fixed point vortex,” Europhys. Lett. 102, 44004 (2013)] is revisited and investigated comprehensively. Although the motion of a pair of point vortices is established to be regular, the model presents a plethora of possible bounded and unbounded solutions with complicated vortex trajectories. The initial classification [E. Ryzhov and K. Koshel, “Dynamics of a vortex pair interacting with a fixed point vortex,” Europhys. Lett. 102, 44004 (2013)] revealed that a pair could be compelled to perform bounded or unbounded motion without giving a full classification of either of those dynamical regimes. The present work capitalizes upon the previous results and introduces a finer classification with a multitude of possible regimes of motion. The regimes of bounded motion for the vortex pair entrapped near the fixed vortex or of unbounded motion, when the vortex pair moves away from the fixed vortex, can be categorized by varying the two governing parameters: (i) the ratio of the distances between the pair’s vortices and the fixed vortex and (ii) the ratio of the strengths of the vortices of the pair and the strength of the fixed vortex. In particular, a bounded motion regime where one of the pair’s vortices does not rotate about the fixed vortex is revealed. In this case, only one of the pair’s vortices rotates about the fixed vortex, while the other oscillates at a certain distance. Extending the results obtained with the point-vortex model to an equivalent model of finite size vortices is the focus of Paper II [J. N. Reinaud et al., “Entrapping of a vortex pair interacting with a fixed point vortex revisited. II. Finite size vortices and the effect of deformation,” Phys. Fluids 30, 096604 (2018)].
The reported study was partially supported by the POI FEB RAS Program “Mathematical simulation and analysis of dynamical processes in the ocean” (No. 117030110034-7) and by the Russian Foundation for Basic Research, Project No. 17-05-00035. E.A.R. was partially supported by the NERC Grant No. NE/R011567/1. Thework of K.V.K. in obtaining the analytical estimates was supported by the Russian Scientific Foundation, Project No. 16-17-10025.
2018-09-28T00:00:00ZKoshel, Konstantin V.Reinaud, Jean N.Riccardi, GiogioRyzhov, Eugene A.The problem of a pair of point vortices impinging on a fixed point vortex of arbitrary strengths [E. Ryzhov and K. Koshel, “Dynamics of a vortex pair interacting with a fixed point vortex,” Europhys. Lett. 102, 44004 (2013)] is revisited and investigated comprehensively. Although the motion of a pair of point vortices is established to be regular, the model presents a plethora of possible bounded and unbounded solutions with complicated vortex trajectories. The initial classification [E. Ryzhov and K. Koshel, “Dynamics of a vortex pair interacting with a fixed point vortex,” Europhys. Lett. 102, 44004 (2013)] revealed that a pair could be compelled to perform bounded or unbounded motion without giving a full classification of either of those dynamical regimes. The present work capitalizes upon the previous results and introduces a finer classification with a multitude of possible regimes of motion. The regimes of bounded motion for the vortex pair entrapped near the fixed vortex or of unbounded motion, when the vortex pair moves away from the fixed vortex, can be categorized by varying the two governing parameters: (i) the ratio of the distances between the pair’s vortices and the fixed vortex and (ii) the ratio of the strengths of the vortices of the pair and the strength of the fixed vortex. In particular, a bounded motion regime where one of the pair’s vortices does not rotate about the fixed vortex is revealed. In this case, only one of the pair’s vortices rotates about the fixed vortex, while the other oscillates at a certain distance. Extending the results obtained with the point-vortex model to an equivalent model of finite size vortices is the focus of Paper II [J. N. Reinaud et al., “Entrapping of a vortex pair interacting with a fixed point vortex revisited. II. Finite size vortices and the effect of deformation,” Phys. Fluids 30, 096604 (2018)].A spatiotemporal multispecies model of a semicontinuous responseJones-Todd, Charlotte M.Swallow, BenIllian, Janine B.Toms, Mikehttp://hdl.handle.net/10023/161392019-01-06T03:20:33Z2018-04-01T00:00:00ZAs accessible and potentially vulnerable species high up in the food chain, birds are often used as indicator species to highlight changes in ecosystems. This study focuses on multiple spatially dependent relationships between a raptor (sparrowhawk), a potential prey species (house sparrow) and a sympatric species (collared doves) in space and time. We construct a complex spatiotemporal latent Gaussian model to incorporate both predator–prey and sympatric relationships, which is novel in two ways. First, different types of species interactions are represented by a shared spatiotemporal random effect, which extends existing approaches to multivariate spatial modelling through the use of a joint latent modelling approach. Second, we use a delta–gamma model to capture the semicontinuous nature of the data to model the binary and continuous sections of the response jointly. The results indicate that sparrowhawks have a localized effect on the presence of house sparrows, which could indicate that house sparrows avoid sites where sparrowhawks are present.
BS was part funded by Engineering and Physical Sciences Research Council–Natural Environment Research Council grant EP/10009171/1.
2018-04-01T00:00:00ZJones-Todd, Charlotte M.Swallow, BenIllian, Janine B.Toms, MikeAs accessible and potentially vulnerable species high up in the food chain, birds are often used as indicator species to highlight changes in ecosystems. This study focuses on multiple spatially dependent relationships between a raptor (sparrowhawk), a potential prey species (house sparrow) and a sympatric species (collared doves) in space and time. We construct a complex spatiotemporal latent Gaussian model to incorporate both predator–prey and sympatric relationships, which is novel in two ways. First, different types of species interactions are represented by a shared spatiotemporal random effect, which extends existing approaches to multivariate spatial modelling through the use of a joint latent modelling approach. Second, we use a delta–gamma model to capture the semicontinuous nature of the data to model the binary and continuous sections of the response jointly. The results indicate that sparrowhawks have a localized effect on the presence of house sparrows, which could indicate that house sparrows avoid sites where sparrowhawks are present.Entrapping of a vortex pair interacting with a fixed point vortex revisited. II. Finite size vortices and the effect of deformationReinaud, Jean N.Koshel, Konstantin V.Ryzhov, Eugene A.http://hdl.handle.net/10023/161172019-01-06T03:38:27Z2018-09-28T00:00:00ZWe investigate the evolution of a pair of two-dimensional, opposite-signed, finite-size vortices interacting with a fixed point vortex. The present paper builds on the accompanying study by Koshel et al. [“Entrapping of a vortex pair interacting with a fixed point vortex revisited. I. Point vortices,” Phys. Fluids 30 , 096603 (2018)] focusing on the motion of a pair of point vortices impinging on a fixed point vortex. Here, by contrast, the pair of opposite-signed finite-size vortices, or vortex dipole for simplicity, can deform. This deformation has an impact on the dynamics. We show that, as expected, finite size vortices behave like point vortices if they are distant enough from each other. This allows one to recover the rich and diverse set of possible trajectories for the dipole. This includes the regimes of intricate bounded motion when the finite-size vortices remain stable near the fixed vortex for a long time. On the other hand, we show that large finite-size vortices can deform significantly and deviate from the trajectories of equivalent point vortices. When the shear that the vortices induce on each other is large enough, the finite size vortices may break into smaller structures or may even be completely strained out.
The reported study was partially supported the POI FEB RAS Program “Mathematical simulation and analysis of dynamical processes in the ocean” (No. 117030110034-7) and by the Russian Foundation for Basic Research, Project No. 17-05-00035. E.A.R. was partially supported by NERC Grant No. NE/R011567/1. The work of K.V.K. in comparing trajectories of the point vortices and the finite-size ones was supported by the Russian Scientific Foundation, Project No. 16-17-10025.
2018-09-28T00:00:00ZReinaud, Jean N.Koshel, Konstantin V.Ryzhov, Eugene A.We investigate the evolution of a pair of two-dimensional, opposite-signed, finite-size vortices interacting with a fixed point vortex. The present paper builds on the accompanying study by Koshel et al. [“Entrapping of a vortex pair interacting with a fixed point vortex revisited. I. Point vortices,” Phys. Fluids 30 , 096603 (2018)] focusing on the motion of a pair of point vortices impinging on a fixed point vortex. Here, by contrast, the pair of opposite-signed finite-size vortices, or vortex dipole for simplicity, can deform. This deformation has an impact on the dynamics. We show that, as expected, finite size vortices behave like point vortices if they are distant enough from each other. This allows one to recover the rich and diverse set of possible trajectories for the dipole. This includes the regimes of intricate bounded motion when the finite-size vortices remain stable near the fixed vortex for a long time. On the other hand, we show that large finite-size vortices can deform significantly and deviate from the trajectories of equivalent point vortices. When the shear that the vortices induce on each other is large enough, the finite size vortices may break into smaller structures or may even be completely strained out.A structured population model of clonal selection in acute leukemias with multiple maturation stagesLorenzi, TommasoMarciniak-Czochra, AnnaStiehl, Thomashttp://hdl.handle.net/10023/161032018-12-23T00:31:35Z2018-09-28T00:00:00ZRecent progress in genetic techniques has shed light on the complex coevolution of malignant cell clones in leukemias. However, several aspects of clonal selection still remain unclear. In this paper, we present a multicompartmental continuously structured population model of selection dynamics in acute leukemias, which consists of a system of coupled integrodifferential equations. Compared to classical models formulated in terms of ordinary differential equations, our model can be handled analytically in a more efficient way. Exploiting the analytical tractability of our model, we investigate how clonal selection is shaped by the self-renewal fraction and the proliferation rate of leukemic cells at different maturation stages. We integrate the results of our analyses with numerical solutions of a calibrated version of the model based on real patient data. In summary, our mathematical results formalise the biological notion that clonal selection is driven by the self-renewal fraction of leukemic stem cells and the clones that possess the highest value of this parameter will ultimately be selected. Moreover, we demonstrate that the self-renewal fraction and the proliferation rate of non-stem cells do not have a substantial impact on clonal selection. Taken together, our results indicate that interclonal variability in the self-renewal fraction of leukemic stem cells provides the necessary substrate for clonal selection to act on.
TS and AM-C were supported by research funding from the German Research Foundation DFG (SFB 873; subproject B08). TL gratefully acknowledges support from Heidelberg Graduate School (HGS).
2018-09-28T00:00:00ZLorenzi, TommasoMarciniak-Czochra, AnnaStiehl, ThomasRecent progress in genetic techniques has shed light on the complex coevolution of malignant cell clones in leukemias. However, several aspects of clonal selection still remain unclear. In this paper, we present a multicompartmental continuously structured population model of selection dynamics in acute leukemias, which consists of a system of coupled integrodifferential equations. Compared to classical models formulated in terms of ordinary differential equations, our model can be handled analytically in a more efficient way. Exploiting the analytical tractability of our model, we investigate how clonal selection is shaped by the self-renewal fraction and the proliferation rate of leukemic cells at different maturation stages. We integrate the results of our analyses with numerical solutions of a calibrated version of the model based on real patient data. In summary, our mathematical results formalise the biological notion that clonal selection is driven by the self-renewal fraction of leukemic stem cells and the clones that possess the highest value of this parameter will ultimately be selected. Moreover, we demonstrate that the self-renewal fraction and the proliferation rate of non-stem cells do not have a substantial impact on clonal selection. Taken together, our results indicate that interclonal variability in the self-renewal fraction of leukemic stem cells provides the necessary substrate for clonal selection to act on.Existence and non-existence results for strong external difference familiesHuczynska, SophiePaterson, Maura B.http://hdl.handle.net/10023/160492019-01-06T03:21:04Z2018-01-01T00:00:00ZWe consider strong external difference families (SEDFs); these are external difference families satisfying additional conditions on the patterns of external differences that occur, and were first defined in the context of classifying optimal strong algebraic manipulation detection codes. We establish new necessary conditions for the existence of ( n , m , k , λ ) -SEDFs; in particular giving a near-complete treatment of the λ = 2 case. For the case m = 2 , we obtain a structural characterization for partition type SEDFs (of maximum possible k and λ), showing that these correspond to Paley partial difference sets. We also prove a version of our main result for generalized SEDFs, establishing non-trivial necessary conditions for their existence.
The first author is supported by a Research Incentive Grant from The Carnegie Trust for the Universities of Scotland (Grant No. 70582).
2018-01-01T00:00:00ZHuczynska, SophiePaterson, Maura B.We consider strong external difference families (SEDFs); these are external difference families satisfying additional conditions on the patterns of external differences that occur, and were first defined in the context of classifying optimal strong algebraic manipulation detection codes. We establish new necessary conditions for the existence of ( n , m , k , λ ) -SEDFs; in particular giving a near-complete treatment of the λ = 2 case. For the case m = 2 , we obtain a structural characterization for partition type SEDFs (of maximum possible k and λ), showing that these correspond to Paley partial difference sets. We also prove a version of our main result for generalized SEDFs, establishing non-trivial necessary conditions for their existence.Understanding the population consequences of disturbancePirotta, EnricoBooth, Cormac G.Costa, Daniel P.Fleishman, EricaKraus, Scott D.Lusseau, DavidMoretti, DavidNew, Leslie F.Schick, Robert S.Schwarz, Lisa K.Simmons, Samantha E.Thomas, LenTyack, Peter L.Weise, Michael J.Wells, Randall S.Harwood, Johnhttp://hdl.handle.net/10023/160412019-01-06T03:38:29Z2018-09-12T00:00:00ZManaging the nonlethal effects of disturbance on wildlife populations has been a long-term goal for decision makers, managers, and ecologists, and assessment of these effects is currently required by European Union and United States legislation. However, robust assessment of these effects is challenging. The management of human activities that have nonlethal effects on wildlife is a specific example of a fundamental ecological problem: how to understand the population-level consequences of changes in the behavior or physiology of individual animals that are caused by external stressors. In this study, we review recent applications of a conceptual framework for assessing and predicting these consequences for marine mammal populations. We explore the range of models that can be used to formalize the approach and we identify critical research gaps. We also provide a decision tree that can be used to select the most appropriate model structure given the available data. Synthesis and applications: The implementation of this framework has moved the focus of discussion of the management of nonlethal disturbances on marine mammal populations away from a rhetorical debate about defining negligible impact and toward a quantitative understanding of long-term population-level effects. Here we demonstrate the framework's general applicability to other marine and terrestrial systems and show how it can support integrated modeling of the proximate and ultimate mechanisms that regulate trait-mediated, indirect interactions in ecological communities, that is, the nonconsumptive effects of a predator or stressor on a species' behavior, physiology, or life history.
This review was supported by Office of Naval Research grant N00014‐16‐1‐2858: “PCoD+: Developing widely‐applicable models of the population consequences of disturbance.” PLT and DL acknowledge support from the MASTS pooling initiative (Marine Alliance for Science and Technology for Scotland; supported by the Scottish Funding Council, grant reference HR09011, and contributing institutions) and PLT acknowledges support from ONR grant N00014‐15‐1‐2553.
2018-09-12T00:00:00ZPirotta, EnricoBooth, Cormac G.Costa, Daniel P.Fleishman, EricaKraus, Scott D.Lusseau, DavidMoretti, DavidNew, Leslie F.Schick, Robert S.Schwarz, Lisa K.Simmons, Samantha E.Thomas, LenTyack, Peter L.Weise, Michael J.Wells, Randall S.Harwood, JohnManaging the nonlethal effects of disturbance on wildlife populations has been a long-term goal for decision makers, managers, and ecologists, and assessment of these effects is currently required by European Union and United States legislation. However, robust assessment of these effects is challenging. The management of human activities that have nonlethal effects on wildlife is a specific example of a fundamental ecological problem: how to understand the population-level consequences of changes in the behavior or physiology of individual animals that are caused by external stressors. In this study, we review recent applications of a conceptual framework for assessing and predicting these consequences for marine mammal populations. We explore the range of models that can be used to formalize the approach and we identify critical research gaps. We also provide a decision tree that can be used to select the most appropriate model structure given the available data. Synthesis and applications: The implementation of this framework has moved the focus of discussion of the management of nonlethal disturbances on marine mammal populations away from a rhetorical debate about defining negligible impact and toward a quantitative understanding of long-term population-level effects. Here we demonstrate the framework's general applicability to other marine and terrestrial systems and show how it can support integrated modeling of the proximate and ultimate mechanisms that regulate trait-mediated, indirect interactions in ecological communities, that is, the nonconsumptive effects of a predator or stressor on a species' behavior, physiology, or life history.Lloyd's mirror effect in fin whale calls and its use to infer the depth of vocalizing animalsPereira, AndreiaHarris, DanielleTyack, PeterMatias, Luishttp://hdl.handle.net/10023/159662019-01-06T03:30:34Z2017-01-01T00:00:00ZThe ocean acoustic Lloyd's Mirror effect (LME) is produced by interference between the direct-path and the sea surface phase-reversed reflection of a sound as observed at a receiver. It results in a frequencydependent interference pattern that can be observed in a spectrogram. Many studies have found variations of spectral characteristics of the 20 Hz 'regular' fin whale call, which seem to reflect geographic differences. However, variability of spectral measurements may occur due to the LME. Using a bout of regular calls with estimated ranges, our study aimed to: 1) show and analyze differences of call features due to the LME; and 2) estimate the depth of the vocalizing whale. The composite spectrogram showed that different spectral characteristics of the calls could be identified within the same bout. We developed transmission loss models considering the LME for a fin whale call generated close to the surface and recorded at the sea bottom by an instrument. Our results suggested that some differences measured in fin whale calls could be related to the LME. Inference of depths of calling whale was not straightforward and needed to be assessed at a finer temporal scale than the full bout of calls.
2017-01-01T00:00:00ZPereira, AndreiaHarris, DanielleTyack, PeterMatias, LuisThe ocean acoustic Lloyd's Mirror effect (LME) is produced by interference between the direct-path and the sea surface phase-reversed reflection of a sound as observed at a receiver. It results in a frequencydependent interference pattern that can be observed in a spectrogram. Many studies have found variations of spectral characteristics of the 20 Hz 'regular' fin whale call, which seem to reflect geographic differences. However, variability of spectral measurements may occur due to the LME. Using a bout of regular calls with estimated ranges, our study aimed to: 1) show and analyze differences of call features due to the LME; and 2) estimate the depth of the vocalizing whale. The composite spectrogram showed that different spectral characteristics of the calls could be identified within the same bout. We developed transmission loss models considering the LME for a fin whale call generated close to the surface and recorded at the sea bottom by an instrument. Our results suggested that some differences measured in fin whale calls could be related to the LME. Inference of depths of calling whale was not straightforward and needed to be assessed at a finer temporal scale than the full bout of calls.Some group presentations with few defining relationsGill, David Michaelhttp://hdl.handle.net/10023/159642018-09-04T14:10:06Z1990-01-01T00:00:00ZWe consider two classes of groups with two generators and three relations. One class has a similar presentation to groups considered in the paper by C.M. Campbell and R.M. Thomas, ‘On (2,n)-Groups related to Fibonacci Groups’, (Israel J. Math., 58), with one generator of order three instead of order two . We attempt to find the order of these groups and in one case find groups which have the alternating group A₅ as a subgroup of index equal to the order of the second generator of the group. Questions remain as to the order of some of the other groups.
The second class has already been considered in the paper 'Some families of finite groups having two generators and two relations' by C.M. Campbell , H.S.M. Coxeter and E.F. Robertson, (Proc. R. Soc. London A. 357, 423-438 (1977)), in which a formula for the orders of these groups was found. We attempt to find simpler formulae based on recurrence relations for subclasses and write Maple programs to enable us to do this. We also find a formula, again based on recurrence relations, for an upper bound for the orders of the groups.
1990-01-01T00:00:00ZGill, David MichaelWe consider two classes of groups with two generators and three relations. One class has a similar presentation to groups considered in the paper by C.M. Campbell and R.M. Thomas, ‘On (2,n)-Groups related to Fibonacci Groups’, (Israel J. Math., 58), with one generator of order three instead of order two . We attempt to find the order of these groups and in one case find groups which have the alternating group A₅ as a subgroup of index equal to the order of the second generator of the group. Questions remain as to the order of some of the other groups.
The second class has already been considered in the paper 'Some families of finite groups having two generators and two relations' by C.M. Campbell , H.S.M. Coxeter and E.F. Robertson, (Proc. R. Soc. London A. 357, 423-438 (1977)), in which a formula for the orders of these groups was found. We attempt to find simpler formulae based on recurrence relations for subclasses and write Maple programs to enable us to do this. We also find a formula, again based on recurrence relations, for an upper bound for the orders of the groups.Self-similar approach for rotating magnetohydrodynamic solar and astrophysical structuresLuna, M.Priest, E.Moreno-Insertis, F.http://hdl.handle.net/10023/159632019-01-06T03:38:23Z2018-08-20T00:00:00ZRotating magnetic structures are common in astrophysics, from vortex tubes and tornadoes in the Sun all the way to jets in different astrophysical systems. The physics of these objects often combine inertial, magnetic, gas pressure, and gravitational terms. Also, they often show approximate symmetries that help simplify the otherwise rather intractable equations governing their morphology and evolution. Here we propose a general formulation of the equations assuming axisymmetry and a self-similar form for all variables: in spherical coordinates ( r , θ , φ ), the magnetic field and plasma velocity are taken to be of the form B = f(θ)/rn and v = g(θ)/rm, with corresponding expressions for the scalar variables like pressure and density. Solutions are obtained for potential, force-free, and non-force-free magnetic configurations. Potential field solutions can be found for all values of n . Nonpotential force-free solutions possess an azimuthal component Bφ and exist only for n ≥ 2; the resulting structures are twisted and have closed field lines but are not collimated around the system axis. In the non-force-free case, including gas pressure, the magnetic field lines acquire an additional curvature to compensate for an outward pointing pressure gradient force. We have also considered a pure rotation situation with no gravity, in the zero- β limit: the solution has cylindrical geometry and twisted magnetic field lines. The latter solutions can be helpful in producing a collimated magnetic field structure; but they exist only when n < 0 and m < 0: for applications they must be matched to an external system at a finite distance from the origin.
Support by the Spanish Ministry of Economy and Competitiveness through project AYA2014-55078-P is acknowledged. M.L. also acknowledges support from the International Space Science Institute (ISSI) to the Team 374 on “Solving the Prominence Paradox” led by Nicolas Labrosse.
2018-08-20T00:00:00ZLuna, M.Priest, E.Moreno-Insertis, F.Rotating magnetic structures are common in astrophysics, from vortex tubes and tornadoes in the Sun all the way to jets in different astrophysical systems. The physics of these objects often combine inertial, magnetic, gas pressure, and gravitational terms. Also, they often show approximate symmetries that help simplify the otherwise rather intractable equations governing their morphology and evolution. Here we propose a general formulation of the equations assuming axisymmetry and a self-similar form for all variables: in spherical coordinates ( r , θ , φ ), the magnetic field and plasma velocity are taken to be of the form B = f(θ)/rn and v = g(θ)/rm, with corresponding expressions for the scalar variables like pressure and density. Solutions are obtained for potential, force-free, and non-force-free magnetic configurations. Potential field solutions can be found for all values of n . Nonpotential force-free solutions possess an azimuthal component Bφ and exist only for n ≥ 2; the resulting structures are twisted and have closed field lines but are not collimated around the system axis. In the non-force-free case, including gas pressure, the magnetic field lines acquire an additional curvature to compensate for an outward pointing pressure gradient force. We have also considered a pure rotation situation with no gravity, in the zero- β limit: the solution has cylindrical geometry and twisted magnetic field lines. The latter solutions can be helpful in producing a collimated magnetic field structure; but they exist only when n < 0 and m < 0: for applications they must be matched to an external system at a finite distance from the origin.Bimodal or quadrimodal? Statistical tests for the shape of fault patternsHealy, DavidJupp, Peterhttp://hdl.handle.net/10023/159622019-01-06T03:38:22Z2018-08-22T00:00:00ZNatural fault patterns formed in response to a single tectonic event often display significant variation in their orientation distribution. The cause of this variation is the subject of some debate: it could be "noise" on underlying conjugate (or bimodal) fault patterns or it could be intrinsic "signal" from an underlying polymodal (e.g. quadrimodal) pattern. In this contribution, we present new statistical tests to assess the probability of a fault pattern having two (bimodal, or conjugate) or four (quadrimodal) underlying modes and orthorhombic symmetry. We use the eigenvalues of the second- and fourth-rank orientation tensors, derived from the direction cosines of the poles to the fault planes, as the basis for our tests. Using a combination of the existing fabric eigenvalue (or modified Flinn) plot and our new tests, we can discriminate reliably between bimodal (conjugate) and quadrimodal fault patterns. We validate our tests using synthetic fault orientation datasets constructed from multimodal Watson distributions and then assess six natural fault datasets from outcrops and earthquake focal plane solutions. We show that five out of six of these natural datasets are probably quadrimodal and orthorhombic. The tests have been implemented in the R language and a link is given to the authors' source code.
David Healy gratefully acknowledges receipt of NERC grant NE/N003063/1 and thanks the School of Geosciences at the University of Aberdeen for accommodating a period of research study leave, during which time this paper was written.
2018-08-22T00:00:00ZHealy, DavidJupp, PeterNatural fault patterns formed in response to a single tectonic event often display significant variation in their orientation distribution. The cause of this variation is the subject of some debate: it could be "noise" on underlying conjugate (or bimodal) fault patterns or it could be intrinsic "signal" from an underlying polymodal (e.g. quadrimodal) pattern. In this contribution, we present new statistical tests to assess the probability of a fault pattern having two (bimodal, or conjugate) or four (quadrimodal) underlying modes and orthorhombic symmetry. We use the eigenvalues of the second- and fourth-rank orientation tensors, derived from the direction cosines of the poles to the fault planes, as the basis for our tests. Using a combination of the existing fabric eigenvalue (or modified Flinn) plot and our new tests, we can discriminate reliably between bimodal (conjugate) and quadrimodal fault patterns. We validate our tests using synthetic fault orientation datasets constructed from multimodal Watson distributions and then assess six natural fault datasets from outcrops and earthquake focal plane solutions. We show that five out of six of these natural datasets are probably quadrimodal and orthorhombic. The tests have been implemented in the R language and a link is given to the authors' source code.Modeling subsurface hydrology in floodplainsEvans, Cristina M.Dritschel, David G.Singer, Michael B.http://hdl.handle.net/10023/159392019-01-06T03:34:48Z2018-03-01T00:00:00ZSoil-moisture patterns in floodplains are highly dynamic, owing to the complex relationships between soil properties, climatic conditions at the surface, and the position of the water table. Given this complexity, along with climate change scenarios in many regions, there is a need for a model to investigate the implications of different conditions on water availability to riparian vegetation. We present a model, HaughFlow, which is able to predict coupled water movement in the vadose and phreatic zones of hydraulically connected floodplains. Model output was calibrated and evaluated at 6 sites in Australia to identify key patterns in subsurface hydrology. This study identifies the importance of the capillary fringe in vadose zone hydrology due to its water storage capacity and creation of conductive pathways. Following peaks in water table elevation, water can be stored in the capillary fringe for up to months (depending on the soil properties). This water can provide a critical resource for vegetation that is unable to access the water table. When water table peaks coincide with heavy rainfall events, the capillary fringe can support saturation of the entire soil profile. HaughFlow is used to investigate the water availability to riparian vegetation, producing daily output of water content in the soil over decadal time periods within different depth ranges. These outputs can be summarised to support scientific investigations of plant-water relations, as well as in management applications.
Evans would also like to acknowledge the generous funding provided by the Natural Environment Research Council (NERC) and the University of St Andrews 600 Fund, without which this work would not have been possible. Singer was supported by funding from NSF EAR #700555. The data output files are hosted online by the NERC Environmental Information Data Centre [Evans et al., 2018].
2018-03-01T00:00:00ZEvans, Cristina M.Dritschel, David G.Singer, Michael B.Soil-moisture patterns in floodplains are highly dynamic, owing to the complex relationships between soil properties, climatic conditions at the surface, and the position of the water table. Given this complexity, along with climate change scenarios in many regions, there is a need for a model to investigate the implications of different conditions on water availability to riparian vegetation. We present a model, HaughFlow, which is able to predict coupled water movement in the vadose and phreatic zones of hydraulically connected floodplains. Model output was calibrated and evaluated at 6 sites in Australia to identify key patterns in subsurface hydrology. This study identifies the importance of the capillary fringe in vadose zone hydrology due to its water storage capacity and creation of conductive pathways. Following peaks in water table elevation, water can be stored in the capillary fringe for up to months (depending on the soil properties). This water can provide a critical resource for vegetation that is unable to access the water table. When water table peaks coincide with heavy rainfall events, the capillary fringe can support saturation of the entire soil profile. HaughFlow is used to investigate the water availability to riparian vegetation, producing daily output of water content in the soil over decadal time periods within different depth ranges. These outputs can be summarised to support scientific investigations of plant-water relations, as well as in management applications.The role of TET-mediated DNA hydroxymethylation in prostate cancerSmeets, E.Lynch, A. G.Prekovic, S.Van den Broeck, T.Moris, LHelsen, C.Joniau, S.Claessens, F.Massie, C. E.http://hdl.handle.net/10023/159372019-01-06T03:20:41Z2018-02-15T00:00:00ZTen-eleven translocation (TET) proteins are recently characterized dioxygenases that regulate demethylation by oxidizing 5-methylcytosine to 5-hydroxymethylcytosine and further derivatives. The recent finding that 5hmC is also a stable and independent epigenetic modification indicates that these proteins play an important role in diverse physiological and pathological processes such as neural and tumor development. Both the genomic distribution of (hydroxy)methylation and the expression and activity of TET proteins are dysregulated in a wide range of cancers including prostate cancer. Up to now it is still unknown how changes in TET and 5(h)mC profiles are related to the pathogenesis of prostate cancer. In this review, we explore recent advances in the current understanding of how TET expression and function are regulated in development and cancer. Furthermore, we look at the impact on 5hmC in prostate cancer and the potential underlying mechanisms. Finally, we tried to summarize the latest techniques for detecting and quantifying global and locus-specific 5hmC levels of genomic DNA.
Massie C. is funded by an ERC grant (337905) and acknowledges support of the University of Cambridge, the Cancer Research UK Cambridge Centre and Hutchison Whampoa Limited. Claessens F. and Joniau S. hold grants from Fonds Wetenschappelijk Onderzoek-Vlaanderen (GOA9816N, G.0684.12N, G.0830.13N). Van den Broeck T. is supported by a PhD fellowship from Fonds Wetenschappelijk Onderzoek-Vlaanderen (11ZO616N). This work was also supported by the KU Leuven (GOA/15/017) and Kom op tegen Kanker.
2018-02-15T00:00:00ZSmeets, E.Lynch, A. G.Prekovic, S.Van den Broeck, T.Moris, LHelsen, C.Joniau, S.Claessens, F.Massie, C. E.Ten-eleven translocation (TET) proteins are recently characterized dioxygenases that regulate demethylation by oxidizing 5-methylcytosine to 5-hydroxymethylcytosine and further derivatives. The recent finding that 5hmC is also a stable and independent epigenetic modification indicates that these proteins play an important role in diverse physiological and pathological processes such as neural and tumor development. Both the genomic distribution of (hydroxy)methylation and the expression and activity of TET proteins are dysregulated in a wide range of cancers including prostate cancer. Up to now it is still unknown how changes in TET and 5(h)mC profiles are related to the pathogenesis of prostate cancer. In this review, we explore recent advances in the current understanding of how TET expression and function are regulated in development and cancer. Furthermore, we look at the impact on 5hmC in prostate cancer and the potential underlying mechanisms. Finally, we tried to summarize the latest techniques for detecting and quantifying global and locus-specific 5hmC levels of genomic DNA.Automorphism groups of linearly ordered structures and endomorphisms of the ordered set ( Q ,≤) of rational numbersMcPhee, Jillian DawnMitchell, James DavidQuick, Martynhttp://hdl.handle.net/10023/159242019-01-13T00:33:15Z2018-08-28T00:00:00ZWe investigate the structure of the monoid of endomorphisms of the ordered set ( Q ,≤) of rational numbers. We show that for any countable linearly ordered set Ω, there are uncountably many maximal subgroups of End( Q ,≤) isomorphic to the automorphism group of Ω. We characterise those subsets X of Q that arise as a retract in ( Q ,≤) in terms of topological information concerning X. Finally, we establish that a countable group arises as the automorphism group of a countable linearly ordered set, and hence as a maximal subgroup of End( Q ,≤), if and only if it is free abelian of finite rank.
2018-08-28T00:00:00ZMcPhee, Jillian DawnMitchell, James DavidQuick, MartynWe investigate the structure of the monoid of endomorphisms of the ordered set ( Q ,≤) of rational numbers. We show that for any countable linearly ordered set Ω, there are uncountably many maximal subgroups of End( Q ,≤) isomorphic to the automorphism group of Ω. We characterise those subsets X of Q that arise as a retract in ( Q ,≤) in terms of topological information concerning X. Finally, we establish that a countable group arises as the automorphism group of a countable linearly ordered set, and hence as a maximal subgroup of End( Q ,≤), if and only if it is free abelian of finite rank.Title redactedMillar, Colin Pearsonhttp://hdl.handle.net/10023/159092018-08-30T13:38:27Z2017-06-23T00:00:00Z2017-06-23T00:00:00ZMillar, Colin PearsonDecision problems in groups of homeomorphisms of Cantor spaceOlukoya, Feyisayohttp://hdl.handle.net/10023/158852018-08-29T07:35:16Z2018-12-06T00:00:00ZThe Thompson groups $F, T$ and $V$ are important groups in geometric group theory: $T$ and $V$ being the first discovered examples of finitely presented infinite simple groups. There are many generalisations of these groups including, for $n$ and $r$ natural numbers and $1 < r < n$, the groups $F_{n}$, $T_{n,r}$ and $G_{n,r}$ ($T ≅ T_{2,1}$ and $V ≅ G_{2,1}$). Automorphisms of $F$ and $T$ were characterised in the seminal paper of Brin ([16]) and, later on, Brin and Guzman ([17]) investigate automorphisms of $T_{n, n-1}$ and $F_{n}$ for $n>2$. However, their techniques give no information about automorphisms of $G_{n,r}$.
The second chapter of this thesis is dedicated to characterising the automorphisms of $G_{n,r}$. Presenting results of the author's article [10], we show that automorphisms of $G_{n,r}$ are homeomorphisms of Cantor space induced by transducers (finite state machines) which satisfy a strong synchronizing condition.
In the rest of Chapter 2 and early sections of Chapter 3 we investigate the group $\out{G_{n,r}}$ of outer automorphisms of $G_{n,r}$. Presenting results of the forthcoming article [6] of the author's, we show that there is a subgroup $\hn{n}$ of $\out{G_{n,r}}$, independent of $r$, which is isomorphic to the group of automorphisms of the one-sided shift dynamical system. Most of Chapter 3 is devoted to the order problem in $\hn{n}$ and is based on [44]. We give necessary and sufficient conditions for an element of $\hn{n}$ to have finite order, although these do not yield a decision procedure.
Given an automorphism $\phi$ of a group $G$, two elements $f, g ∈ G$ are said to be $\phi$-twisted conjugate to one another if for some $h ∈ G$, $g = h⁻¹ f (h)\phi$. This defines an equivalence relation on $G$ and $G$ is said to have the $\rfty$ property if it has infinitely many $\phi$-twisted conjugacy classes for all automorphisms $\phi ∈ \aut{G}$. In the final chapter we show, using the description of $\aut{G_{n,r}}$, that for certain automorphisms, $G_{n,r}$ has infinitely many twisted conjugacy classes. We also show that for certain $\phi ∈ \aut{G_{2,1}}$ the problem of deciding when two elements of $G_{2,1}$ are $\phi$-twisted conjugate to one another is soluble.
2018-12-06T00:00:00ZOlukoya, FeyisayoThe Thompson groups $F, T$ and $V$ are important groups in geometric group theory: $T$ and $V$ being the first discovered examples of finitely presented infinite simple groups. There are many generalisations of these groups including, for $n$ and $r$ natural numbers and $1 < r < n$, the groups $F_{n}$, $T_{n,r}$ and $G_{n,r}$ ($T ≅ T_{2,1}$ and $V ≅ G_{2,1}$). Automorphisms of $F$ and $T$ were characterised in the seminal paper of Brin ([16]) and, later on, Brin and Guzman ([17]) investigate automorphisms of $T_{n, n-1}$ and $F_{n}$ for $n>2$. However, their techniques give no information about automorphisms of $G_{n,r}$.
The second chapter of this thesis is dedicated to characterising the automorphisms of $G_{n,r}$. Presenting results of the author's article [10], we show that automorphisms of $G_{n,r}$ are homeomorphisms of Cantor space induced by transducers (finite state machines) which satisfy a strong synchronizing condition.
In the rest of Chapter 2 and early sections of Chapter 3 we investigate the group $\out{G_{n,r}}$ of outer automorphisms of $G_{n,r}$. Presenting results of the forthcoming article [6] of the author's, we show that there is a subgroup $\hn{n}$ of $\out{G_{n,r}}$, independent of $r$, which is isomorphic to the group of automorphisms of the one-sided shift dynamical system. Most of Chapter 3 is devoted to the order problem in $\hn{n}$ and is based on [44]. We give necessary and sufficient conditions for an element of $\hn{n}$ to have finite order, although these do not yield a decision procedure.
Given an automorphism $\phi$ of a group $G$, two elements $f, g ∈ G$ are said to be $\phi$-twisted conjugate to one another if for some $h ∈ G$, $g = h⁻¹ f (h)\phi$. This defines an equivalence relation on $G$ and $G$ is said to have the $\rfty$ property if it has infinitely many $\phi$-twisted conjugacy classes for all automorphisms $\phi ∈ \aut{G}$. In the final chapter we show, using the description of $\aut{G_{n,r}}$, that for certain automorphisms, $G_{n,r}$ has infinitely many twisted conjugacy classes. We also show that for certain $\phi ∈ \aut{G_{2,1}}$ the problem of deciding when two elements of $G_{2,1}$ are $\phi$-twisted conjugate to one another is soluble.Topological 2-generation of automorphism groups of countable ultrahomogeneous graphsJonusas, JuliusMitchell, J. D.http://hdl.handle.net/10023/158612019-01-06T03:03:17Z2017-07-01T00:00:00ZA countable graph is ultrahomogeneous if every isomorphism between finite induced subgraphs can be extended to an automorphism. Woodrow and Lachlan showed that there are essentially four types of such countably infinite graphs: The random graph, infinite disjoint unions of complete graphs Kn with n ò Nvertices, the Kn-free graphs, finite unions of the infinite complete graph K, and duals of such graphs. The groups Aut(λ) of automorphisms of such graphs λ have a natural topology, which is compatible with multiplication and inversion, i.e. The groupsAut(λ) are topological groups.We consider the problem of finding minimally generated dense subgroups of the groups Aut(λ) where λ is ultrahomogeneous.We show that if λ is ultrahomogeneous, then Aut(λ) has 2-generated dense subgroups, and that under certain conditions given f ò Aut(λ) there exists g ò Aut(λ) such that the subgroup generated by f and g is dense. We also show that, roughly speaking, g can be chosen with a high degree of freedom. For example, if λ is either an infinite disjoint union of Kn or a finite union of Kω, then g can be chosen to have any given finite set of orbit representatives.
2017-07-01T00:00:00ZJonusas, JuliusMitchell, J. D.A countable graph is ultrahomogeneous if every isomorphism between finite induced subgraphs can be extended to an automorphism. Woodrow and Lachlan showed that there are essentially four types of such countably infinite graphs: The random graph, infinite disjoint unions of complete graphs Kn with n ò Nvertices, the Kn-free graphs, finite unions of the infinite complete graph K, and duals of such graphs. The groups Aut(λ) of automorphisms of such graphs λ have a natural topology, which is compatible with multiplication and inversion, i.e. The groupsAut(λ) are topological groups.We consider the problem of finding minimally generated dense subgroups of the groups Aut(λ) where λ is ultrahomogeneous.We show that if λ is ultrahomogeneous, then Aut(λ) has 2-generated dense subgroups, and that under certain conditions given f ò Aut(λ) there exists g ò Aut(λ) such that the subgroup generated by f and g is dense. We also show that, roughly speaking, g can be chosen with a high degree of freedom. For example, if λ is either an infinite disjoint union of Kn or a finite union of Kω, then g can be chosen to have any given finite set of orbit representatives.Groups in Galway and Groups St Andrews conferencesCampbell, Colin Matthewhttp://hdl.handle.net/10023/158512018-11-15T17:30:04Z2018-05-23T00:00:00Z2018-05-23T00:00:00ZCampbell, Colin MatthewEvolution of cancer cell populations under cytotoxic therapy and treatment optimisation : insight from a phenotype-structured modelAlmeida, LuisBagnerini, PatriziaFabrini, GiuliaHughes, Barry D.Lorenzi, Tommasohttp://hdl.handle.net/10023/158502018-12-02T00:48:58Z2018-08-03T00:00:00ZWe consider a phenotype-structured model of evolutionary dynamics in a population of cancer cells exposed to the action of a cytotoxic drug. The model consists of a nonlocal parabolic equation governing the evolution of the cell population density function. We develop a novel method for constructing exact solutions of the model equation, which allows for a systematic investigation of the way in which the size and the phenotypic composition of the cell population vary in response to variations of the drug dose and other evolutionary parameters. Moreover, we address numerical optimal control for a calibrated version of the model based on biological data from the existing literature, with the goal being to identify the drug delivery schedule that makes it possible to minimise either the population size at the end of the treatment or the time average of the population size during the course of treatment. The results obtained challenge the notion that traditional high-dose therapy represents a ‘one-fits-all solution’ in anticancer therapy by showing that the continuous administration of a relatively low dose of the cytotoxic drug performs more closely to the optimal dosing regimen to minimise the average size of the cancer cell population during the course of treatment.
BDH acknowledges support from the Australian Research Council (DP140100339). LA and TL gratefully acknowledge support of the project PICS-CNRS no. 07688.
2018-08-03T00:00:00ZAlmeida, LuisBagnerini, PatriziaFabrini, GiuliaHughes, Barry D.Lorenzi, TommasoWe consider a phenotype-structured model of evolutionary dynamics in a population of cancer cells exposed to the action of a cytotoxic drug. The model consists of a nonlocal parabolic equation governing the evolution of the cell population density function. We develop a novel method for constructing exact solutions of the model equation, which allows for a systematic investigation of the way in which the size and the phenotypic composition of the cell population vary in response to variations of the drug dose and other evolutionary parameters. Moreover, we address numerical optimal control for a calibrated version of the model based on biological data from the existing literature, with the goal being to identify the drug delivery schedule that makes it possible to minimise either the population size at the end of the treatment or the time average of the population size during the course of treatment. The results obtained challenge the notion that traditional high-dose therapy represents a ‘one-fits-all solution’ in anticancer therapy by showing that the continuous administration of a relatively low dose of the cytotoxic drug performs more closely to the optimal dosing regimen to minimise the average size of the cancer cell population during the course of treatment.Connecting the large- and the small-scale magnetic fields of solar-like starsLehmann, L. T.Jardine, M. M.Mackay, D. H.Vidotto, A. A.http://hdl.handle.net/10023/158372019-01-06T03:38:09Z2018-08-21T00:00:00ZA key question in understanding the observed magnetic field topologies of cool stars is the link between the small- and the large-scalemagnetic field and the influence of the stellar parameters on the magnetic field topology. We examine various simulated stars to connect the small scale with the observable large-scale field. The highly resolved 3D simulations we used couple a flux transport model with a non-potential coronal model using a magnetofrictional technique. The surface magnetic field of these simulations is decomposed into spherical harmonics which enables us to analyse the magnetic field topologies on a wide range of length scales and to filter the large-scale magnetic field for a direct comparison with the observations. We show that the large-scale field of the self-consistent simulations fits the observed solar-like stars and is mainly set up by the global dipolar field and the large-scale properties of the flux pattern, e.g. the averaged latitudinal position of the emerging small-scale field and its global polarity pattern. The stellar parameter flux emergence rate, differential rotation, and meridional flow affect the large-scale magnetic field topology. An increased flux emergence rate increases the magnetic flux in all field components and an increased differential rotation increases the toroidal field fraction by decreasing the poloidal field. The meridional flow affects the distribution of the magnetic energy across the spherical harmonic modes.
LTL acknowledges support from the Scottish Universities Physics Alliance (SUPA) prize studentship and the University of St Andrews Higgs studentship. MMJ acknowledges the support of the Science & Technology Facilities Council (STFC) (ST/M001296/1).
2018-08-21T00:00:00ZLehmann, L. T.Jardine, M. M.Mackay, D. H.Vidotto, A. A.A key question in understanding the observed magnetic field topologies of cool stars is the link between the small- and the large-scalemagnetic field and the influence of the stellar parameters on the magnetic field topology. We examine various simulated stars to connect the small scale with the observable large-scale field. The highly resolved 3D simulations we used couple a flux transport model with a non-potential coronal model using a magnetofrictional technique. The surface magnetic field of these simulations is decomposed into spherical harmonics which enables us to analyse the magnetic field topologies on a wide range of length scales and to filter the large-scale magnetic field for a direct comparison with the observations. We show that the large-scale field of the self-consistent simulations fits the observed solar-like stars and is mainly set up by the global dipolar field and the large-scale properties of the flux pattern, e.g. the averaged latitudinal position of the emerging small-scale field and its global polarity pattern. The stellar parameter flux emergence rate, differential rotation, and meridional flow affect the large-scale magnetic field topology. An increased flux emergence rate increases the magnetic flux in all field components and an increased differential rotation increases the toroidal field fraction by decreasing the poloidal field. The meridional flow affects the distribution of the magnetic energy across the spherical harmonic modes.Determining the behavioural dose-response relationship of marine mammals to air gun noise and source proximityDunlop, Rebecca A.Noad, Michael J.McCauley, Robert D.Scott-Hayward, Lindesay Alexandra SarahKniest, EricSlade, RobertPaton, DavidCato, Douglas H.http://hdl.handle.net/10023/158272019-01-06T03:20:44Z2017-08-16T00:00:00ZThe effect of various anthropogenic sources of noise (e.g. sonar, seismic surveys) on the behaviour of marine mammals is sometimes quantified as a dosetextendashresponse relationship, where the probability of an animal behaviourally 'responding' (e.g. avoiding the source) increases with 'dose' (or received level of noise). To do this, however, requires a definition of a 'significant' response (avoidance), which can be difficult to quantify. There is also the potential that the animal 'avoids' not only the source of noise but also the vessel operating the source, complicating the relationship. The proximity of the source is an important variable to consider in the response, yet difficult to account for given that received level and proximity are highly correlated. This study used the behavioural response of humpback whales to noise from two different air gun arrays (20 and 140 cubic inch air gun array) to determine whether a dosetextendashresponse relationship existed. To do this, a measure of avoidance of the source was developed, and the magnitude (rather than probability) of this response was tested against dose. The proximity to the source, and the vessel itself, was included within the one-analysis model. Humpback whales were more likely to avoid the air gun arrays (but not the controls) within 3 km of the source at levels over 140 re. 1 μPa2 s-1, meaning that both the proximity and the received level were important factors and the relationship between dose (received level) and response is not a simple one.
Funding was provided as part of Joint Industry Programme on E&P Sound and Marine Life, managed by the International Association of Oil & Gas Producers (IOGP). The principal contributing companies to the programme are BG group, BHP Billiton, Chevron, ConocoPhillips, Eni, ExxonMobil, IAGC, Santos, Statoil and Woodside. The US Bureau of Ocean Energy Management (BOEM), Origin Energy, Beach Energy and AWE Limited provided support specifically for the BRAHSS study.
2017-08-16T00:00:00ZDunlop, Rebecca A.Noad, Michael J.McCauley, Robert D.Scott-Hayward, Lindesay Alexandra SarahKniest, EricSlade, RobertPaton, DavidCato, Douglas H.The effect of various anthropogenic sources of noise (e.g. sonar, seismic surveys) on the behaviour of marine mammals is sometimes quantified as a dosetextendashresponse relationship, where the probability of an animal behaviourally 'responding' (e.g. avoiding the source) increases with 'dose' (or received level of noise). To do this, however, requires a definition of a 'significant' response (avoidance), which can be difficult to quantify. There is also the potential that the animal 'avoids' not only the source of noise but also the vessel operating the source, complicating the relationship. The proximity of the source is an important variable to consider in the response, yet difficult to account for given that received level and proximity are highly correlated. This study used the behavioural response of humpback whales to noise from two different air gun arrays (20 and 140 cubic inch air gun array) to determine whether a dosetextendashresponse relationship existed. To do this, a measure of avoidance of the source was developed, and the magnitude (rather than probability) of this response was tested against dose. The proximity to the source, and the vessel itself, was included within the one-analysis model. Humpback whales were more likely to avoid the air gun arrays (but not the controls) within 3 km of the source at levels over 140 re. 1 μPa2 s-1, meaning that both the proximity and the received level were important factors and the relationship between dose (received level) and response is not a simple one.A cancellation nanoflare model for solar chromospheric and coronal heatingPriest, E. R.Chitta, L. P.Syntelis, P.http://hdl.handle.net/10023/158262019-01-06T03:38:04Z2018-08-01T00:00:00ZNanoflare models for heating the solar corona usually assume magnetic braiding and reconnection as the source of the energy. However, recent observations at record spatial resolution from the SUNRISE balloon mission suggest that photospheric magnetic flux cancellation is much more common than previously realized. We therefore examine the possibility of three-dimensional reconnection driven by flux cancellation as a cause of chromospheric and coronal heating. In particular, we estimate how the heights and amount of energy release produced by flux cancellation depend on flux size, flux cancellation speed, and overlying field strength.
L.P.C. received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 707837. The German contribution to SUNRISE and its reflight was funded by the Max Planck Foundation, the Strategic Innovations Fund of the President of the Max Planck Society (MPG), DLR, and private donations by supporting members of the Max Planck Society. The Spanish contribution was funded by the Ministerio de Economía y Competitividad under Projects ESP2013-47349-C6 and ESP2014-56169-C6, partially using European FEDER funds.
2018-08-01T00:00:00ZPriest, E. R.Chitta, L. P.Syntelis, P.Nanoflare models for heating the solar corona usually assume magnetic braiding and reconnection as the source of the energy. However, recent observations at record spatial resolution from the SUNRISE balloon mission suggest that photospheric magnetic flux cancellation is much more common than previously realized. We therefore examine the possibility of three-dimensional reconnection driven by flux cancellation as a cause of chromospheric and coronal heating. In particular, we estimate how the heights and amount of energy release produced by flux cancellation depend on flux size, flux cancellation speed, and overlying field strength.Recurrent CME-like eruptions in emerging flux regions. I. On the mechanism of eruptionsSyntelis, P.Archontis, V.Tsinganos, K.http://hdl.handle.net/10023/158252019-01-06T03:38:04Z2017-11-21T00:00:00ZWe report on three-dimensional (3D) magnetohydrodynamic (MHD) simulations of recurrent eruptions in emerging flux regions. We find that reconnection of sheared field lines, along the polarity inversion line of an emerging bipolar region, leads to the formation of a new magnetic structure, which adopts the shape of a magnetic flux rope (FR) during its rising motion. Initially, the FR undergoes a slow-rise phase and, eventually, it experiences a fast-rise phase and ejective eruption toward the outer solar atmosphere. In total, four eruptions occur during the evolution of the system. For the first eruption, our analysis indicates that the torus instability initiates the eruption and that tether-cutting reconnection of the field lines, which envelop the FR, triggers the rapid acceleration of the eruptive field. For the following eruptions, we conjecture that it is the interplay between tether-cutting reconnection and torus instability that causes the onset of the various phases. We show the 3D shape of the erupting fields, focusing more on how magnetic field lines reconnect during the eruptions. We find that when the envelope field lines reconnect mainly with themselves, hot and dense plasma is transferred closer to the core of the erupting FR. The same area appears to be cooler and less dense when the envelope field lines reconnect with neighboring sheared field lines. The plasma density and temperature distribution, together with the rising speeds, energies, and size of the erupting fields, indicate that they may account for small-scale (mini) coronal mass ejections.
This project has received funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1. This research has been co-financed by the European Union (European Social Fund—ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF)—Research Funding Program: Thales Investing in Knowledge Society through the European Social Fund. The authors acknowledge support by the Royal Society.
2017-11-21T00:00:00ZSyntelis, P.Archontis, V.Tsinganos, K.We report on three-dimensional (3D) magnetohydrodynamic (MHD) simulations of recurrent eruptions in emerging flux regions. We find that reconnection of sheared field lines, along the polarity inversion line of an emerging bipolar region, leads to the formation of a new magnetic structure, which adopts the shape of a magnetic flux rope (FR) during its rising motion. Initially, the FR undergoes a slow-rise phase and, eventually, it experiences a fast-rise phase and ejective eruption toward the outer solar atmosphere. In total, four eruptions occur during the evolution of the system. For the first eruption, our analysis indicates that the torus instability initiates the eruption and that tether-cutting reconnection of the field lines, which envelop the FR, triggers the rapid acceleration of the eruptive field. For the following eruptions, we conjecture that it is the interplay between tether-cutting reconnection and torus instability that causes the onset of the various phases. We show the 3D shape of the erupting fields, focusing more on how magnetic field lines reconnect during the eruptions. We find that when the envelope field lines reconnect mainly with themselves, hot and dense plasma is transferred closer to the core of the erupting FR. The same area appears to be cooler and less dense when the envelope field lines reconnect with neighboring sheared field lines. The plasma density and temperature distribution, together with the rising speeds, energies, and size of the erupting fields, indicate that they may account for small-scale (mini) coronal mass ejections.Evolution of the transverse density structure of oscillating coronal loops inferred by forward modelling of EUV intensityGoddard, Christopher RhysAntolin, PatrickPascoe, David J.http://hdl.handle.net/10023/158162019-01-06T03:38:02Z2018-08-22T00:00:00ZRecent developments in the observation and modelling of kink oscillations of coronal loops have led to heightened interest over the last few years. The modification of the Transverse Density Profile (TDP) of oscillating coronal loops by non-linear effects, in particular the Kelvin-Helmholtz Instability (KHI), is investigated. How this evolution may be detected is established, in particular, when the KHI vortices may not be observed directly. A model for the loop's TDP is used which includes a finite inhomogeneous layer and homogeneous core, with a linear transition between them. The evolution of the loop's transverse intensity profile from numerical simulations of kink oscillations is analysed. Bayesian inference and forward modelling techniques are applied to infer the evolution of the TDP from the intensity profiles, in a manner which may be applied to observations. The strongest observational evidence for the development of the KHI is found to be a widening of the loop's inhomogeneous layer, which may be inferred for sufficiently well resolved loops, i.e > 15 data points across the loop. The main signatures when observing the core of the loop (for this specific loop model) during the oscillation are: a widening inhomogeneous layer, decreasing intensity, an unchanged radius, and visible fine transverse structuring when the resolution is sufficient. The appearance of these signatures are delayed for loops with wider inhomogeneous layers, and quicker for loops oscillating at higher amplitudes. These cases should also result in stronger observational signatures, with visible transverse structuring appearing for wide loops observed at SDO/AIA resolution.
This work was supported by the European Research Council (ERC) under the SeismoSun Research Project No. 321141 (CRG, DJP) and by the British Council via the Institutional Links Programme (Project 277352569 - Seismology of Solar Coronal Active Regions) (CRG). DJP has received funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 724326). P.A. has received funding from the UK Science and Technology Facilities Council (Consolidated Grant ST/K000950/1), the European Union Horizon 2020 research and innovation programme (grant agreement No. 647214) and his STFC Ernest Rutherford Fellowship (grant agreement No. ST/R004285/1).
2018-08-22T00:00:00ZGoddard, Christopher RhysAntolin, PatrickPascoe, David J.Recent developments in the observation and modelling of kink oscillations of coronal loops have led to heightened interest over the last few years. The modification of the Transverse Density Profile (TDP) of oscillating coronal loops by non-linear effects, in particular the Kelvin-Helmholtz Instability (KHI), is investigated. How this evolution may be detected is established, in particular, when the KHI vortices may not be observed directly. A model for the loop's TDP is used which includes a finite inhomogeneous layer and homogeneous core, with a linear transition between them. The evolution of the loop's transverse intensity profile from numerical simulations of kink oscillations is analysed. Bayesian inference and forward modelling techniques are applied to infer the evolution of the TDP from the intensity profiles, in a manner which may be applied to observations. The strongest observational evidence for the development of the KHI is found to be a widening of the loop's inhomogeneous layer, which may be inferred for sufficiently well resolved loops, i.e > 15 data points across the loop. The main signatures when observing the core of the loop (for this specific loop model) during the oscillation are: a widening inhomogeneous layer, decreasing intensity, an unchanged radius, and visible fine transverse structuring when the resolution is sufficient. The appearance of these signatures are delayed for loops with wider inhomogeneous layers, and quicker for loops oscillating at higher amplitudes. These cases should also result in stronger observational signatures, with visible transverse structuring appearing for wide loops observed at SDO/AIA resolution.Modelling the broadband propagation of marine mammal echolocation clicks for click-based population density estimatesvon Benda-Beckmann, AlexanderThomas, Leonard JosephTyack, Peter LloydAinslie, Michaelhttp://hdl.handle.net/10023/158152019-01-06T03:34:36Z2018-02-01T00:00:00ZPassive acoustic monitoring with widely-dispersed hydrophones has been suggested as a cost-effective method to monitor population densities of echolocating marine mammals. This requires an estimate of the area around each receiver over which vocalizations are detected—the “effective detection area” (EDA). In the absence of auxiliary measurements enabling estimation of the EDA, it can be modelled instead. Common simplifying model assumptions include approximating the spectrum of clicks by flat energy spectra, and neglecting the frequency-dependence of sound absorption within the click bandwidth (narrowband assumption), rendering the problem amenable to solution using the sonar equation. Here, it is investigated how these approximations affect the estimated EDA and their potential for biasing the estimated density. EDA was estimated using the passive sonar equation, and by applying detectors to simulated clicks injected into measurements of background noise. By comparing model predictions made using these two approaches for different spectral energy distributions of echolocation clicks, but identical click source energy level and detector settings, EDA differed by up to a factor of 2 for Blainville's beaked whales. Both methods predicted relative density bias due to narrowband assumptions ranged from 5% to more than 100%, depending on the species, detector settings, and noise conditions.
Funding: U.S. Office of Naval Research (ONR Grant No. N00014-14-1-0409); P.L.T. acknowledges funding received from the MASTS pooling initiative (The Marine Alliance for Science and Technology for Scotland). MASTS is funded by the Scottish Funding Council (Grant No. HR09011) and contributing institutions.
2018-02-01T00:00:00Zvon Benda-Beckmann, AlexanderThomas, Leonard JosephTyack, Peter LloydAinslie, MichaelPassive acoustic monitoring with widely-dispersed hydrophones has been suggested as a cost-effective method to monitor population densities of echolocating marine mammals. This requires an estimate of the area around each receiver over which vocalizations are detected—the “effective detection area” (EDA). In the absence of auxiliary measurements enabling estimation of the EDA, it can be modelled instead. Common simplifying model assumptions include approximating the spectrum of clicks by flat energy spectra, and neglecting the frequency-dependence of sound absorption within the click bandwidth (narrowband assumption), rendering the problem amenable to solution using the sonar equation. Here, it is investigated how these approximations affect the estimated EDA and their potential for biasing the estimated density. EDA was estimated using the passive sonar equation, and by applying detectors to simulated clicks injected into measurements of background noise. By comparing model predictions made using these two approaches for different spectral energy distributions of echolocation clicks, but identical click source energy level and detector settings, EDA differed by up to a factor of 2 for Blainville's beaked whales. Both methods predicted relative density bias due to narrowband assumptions ranged from 5% to more than 100%, depending on the species, detector settings, and noise conditions.Global non-potential magnetic models of the solar corona during the March 2015 eclipseYeates, A. R.Amari, TaharContopoulos, IoannisFeng, XueshangMackay, D. H.Mikić, ZoranWiegelmann, ThomasHutton, JosephLowder, Christopher A.Morgan, HuwPetrie, GordonRachmeler, L. A.Upton, Lisa A.Canou, AurelienChopin, PierreDowns, CooperDruckmüller, MiloslavLinker, Jon A.Seaton, Daniel B.Török, Tiborhttp://hdl.handle.net/10023/158122019-01-06T03:38:01Z2018-08-01T00:00:00ZSeven different models are applied to the same problem of simulating the Sun’s coronal magnetic field during the solar eclipse on 2015 March 20. All of the models are non-potential, allowing for free magnetic energy, but the associated electric currents are developed in significantly different ways. This is not a direct comparison of the coronal modelling techniques, in that the different models also use different photospheric boundary conditions, reflecting the range of approaches currently used in the community. Despite the significant differences, the results show broad agreement in the overall magnetic topology. Among those models with significant volume currents in much of the corona, there is general agreement that the ratio of total to potential magnetic energy should be approximately 1.4. However, there are significant differences in the electric current distributions; while static extrapolations are best able to reproduce active regions, they are unable to recover sheared magnetic fields in filament channels using currently available vector magnetogram data. By contrast, time-evolving simulations can recover the filament channel fields at the expense of not matching the observed vector magnetic fields within active regions. We suggest that, at present, the best approach may be a hybrid model using static extrapolations but with additional energization informed by simplified evolution models. This is demonstrated by one of the models.
Funding: UK Science and Technology Facilities Council (STFC) (ARY, DHM).
2018-08-01T00:00:00ZYeates, A. R.Amari, TaharContopoulos, IoannisFeng, XueshangMackay, D. H.Mikić, ZoranWiegelmann, ThomasHutton, JosephLowder, Christopher A.Morgan, HuwPetrie, GordonRachmeler, L. A.Upton, Lisa A.Canou, AurelienChopin, PierreDowns, CooperDruckmüller, MiloslavLinker, Jon A.Seaton, Daniel B.Török, TiborSeven different models are applied to the same problem of simulating the Sun’s coronal magnetic field during the solar eclipse on 2015 March 20. All of the models are non-potential, allowing for free magnetic energy, but the associated electric currents are developed in significantly different ways. This is not a direct comparison of the coronal modelling techniques, in that the different models also use different photospheric boundary conditions, reflecting the range of approaches currently used in the community. Despite the significant differences, the results show broad agreement in the overall magnetic topology. Among those models with significant volume currents in much of the corona, there is general agreement that the ratio of total to potential magnetic energy should be approximately 1.4. However, there are significant differences in the electric current distributions; while static extrapolations are best able to reproduce active regions, they are unable to recover sheared magnetic fields in filament channels using currently available vector magnetogram data. By contrast, time-evolving simulations can recover the filament channel fields at the expense of not matching the observed vector magnetic fields within active regions. We suggest that, at present, the best approach may be a hybrid model using static extrapolations but with additional energization informed by simplified evolution models. This is demonstrated by one of the models.Solvable quotients of subdirect products of perfect groups are nilpotentKearnes, KeithMayr, PeterRuskuc, Nikhttp://hdl.handle.net/10023/157962019-01-06T03:37:45Z2018-12-01T00:00:00ZWe prove the statement in the title and exhibit examples of quotients of arbitrary nilpotency class. This answers a question by Holt.
The first two authors were supported by the National Science Foundation under Grant No. DMS 1500254.
2018-12-01T00:00:00ZKearnes, KeithMayr, PeterRuskuc, NikWe prove the statement in the title and exhibit examples of quotients of arbitrary nilpotency class. This answers a question by Holt.Spatial modeling with R-INLA : a reviewBakka, HaakonRue, HåvardFuglstad, Geir-arneRiebler, AndreaBolin, DavidIllian, JanineKrainski, EliasSimpson, DanielLindgren, Finnhttp://hdl.handle.net/10023/157872019-01-06T03:37:28Z2018-07-05T00:00:00ZComing up with Bayesian models for spatial data is easy, but performing inference with them can be challenging. Writing fast inference code for a complex spatial model with realistically‐sized datasets from scratch is time‐consuming, and if changes are made to the model, there is little guarantee that the code performs well. The key advantages of R‐INLA are the ease with which complex models can be created and modified, without the need to write complex code, and the speed at which inference can be done even for spatial problems with hundreds of thousands of observations. R‐INLA handles latent Gaussian models, where fixed effects, structured and unstructured Gaussian random effects are combined linearly in a linear predictor, and the elements of the linear predictor are observed through one or more likelihoods. The structured random effects can be both standard areal model such as the Besag and the BYM models, and geostatistical models from a subset of the Matérn Gaussian random fields. In this review, we discuss the large success of spatial modeling with R‐INLA and the types of spatial models that can be fitted, we give an overview of recent developments for areal models, and we give an overview of the stochastic partial differential equation (SPDE) approach and some of the ways it can be extended beyond the assumptions of isotropy and separability. In particular, we describe how slight changes to the SPDE approach leads to straight‐forward approaches for nonstationary spatial models and nonseparable space–time models.
2018-07-05T00:00:00ZBakka, HaakonRue, HåvardFuglstad, Geir-arneRiebler, AndreaBolin, DavidIllian, JanineKrainski, EliasSimpson, DanielLindgren, FinnComing up with Bayesian models for spatial data is easy, but performing inference with them can be challenging. Writing fast inference code for a complex spatial model with realistically‐sized datasets from scratch is time‐consuming, and if changes are made to the model, there is little guarantee that the code performs well. The key advantages of R‐INLA are the ease with which complex models can be created and modified, without the need to write complex code, and the speed at which inference can be done even for spatial problems with hundreds of thousands of observations. R‐INLA handles latent Gaussian models, where fixed effects, structured and unstructured Gaussian random effects are combined linearly in a linear predictor, and the elements of the linear predictor are observed through one or more likelihoods. The structured random effects can be both standard areal model such as the Besag and the BYM models, and geostatistical models from a subset of the Matérn Gaussian random fields. In this review, we discuss the large success of spatial modeling with R‐INLA and the types of spatial models that can be fitted, we give an overview of recent developments for areal models, and we give an overview of the stochastic partial differential equation (SPDE) approach and some of the ways it can be extended beyond the assumptions of isotropy and separability. In particular, we describe how slight changes to the SPDE approach leads to straight‐forward approaches for nonstationary spatial models and nonseparable space–time models.Triple arrays from difference setsNilson, TomasCameron, Peter J.http://hdl.handle.net/10023/157752019-01-06T03:19:28Z2017-11-01T00:00:00ZThis paper addresses the question whether triple arrays can be constructed from Youden squares developed from difference sets. We prove that if the difference set is abelian, then having −1 as multiplier is both a necessary and sufficient condition for the construction to work. Using this, we are able to give a new infinite family of triple arrays. We also give an alternative and more direct version of the construction, leaving out the intermediate step via Youden squares. This is used when we analyse the case of non-abelian difference sets, for which we prove a sufficient condition for giving triple arrays. We do a computer search for such non-abelian difference sets, but have not found any examples satisfying the given condition.
2017-11-01T00:00:00ZNilson, TomasCameron, Peter J.This paper addresses the question whether triple arrays can be constructed from Youden squares developed from difference sets. We prove that if the difference set is abelian, then having −1 as multiplier is both a necessary and sufficient condition for the construction to work. Using this, we are able to give a new infinite family of triple arrays. We also give an alternative and more direct version of the construction, leaving out the intermediate step via Youden squares. This is used when we analyse the case of non-abelian difference sets, for which we prove a sufficient condition for giving triple arrays. We do a computer search for such non-abelian difference sets, but have not found any examples satisfying the given condition.On the Π^0_γ-completeness and Σ^0_γ-completeness of multifractal decomposition setsOlsen, Lars Ole Ronnowhttp://hdl.handle.net/10023/157622019-01-06T03:33:32Z2018-01-01T00:00:00ZThe purpose of this paper twofold. Firstly, we establish Π^0_γ-completeness and Σ^0_γ-completeness of several different classes of multifractal decomposition sets of arbitrary Borel measures (satisfying a mild non-degeneracy condition and two mild “smoothness” conditions). Secondly, we apply these results to study the Π^0_γ-completeness and Σ^0_γ-completeness of several multifractal decomposition sets of self-similar measures (satisfying a mild separation condition). For example, a corollary of our results shows if μ is a self-similar measure satisfying the strong separation condition and is not equal to the normalized Hausdorff measure on its support, then the classical multifractal decomposition sets of μ defined by {x ε ℝd | lim r ↘ 0 [log μ(B(x,r))/log r = α]} are Π^0_3-complete provided they are non-empty.
2018-01-01T00:00:00ZOlsen, Lars Ole RonnowThe purpose of this paper twofold. Firstly, we establish Π^0_γ-completeness and Σ^0_γ-completeness of several different classes of multifractal decomposition sets of arbitrary Borel measures (satisfying a mild non-degeneracy condition and two mild “smoothness” conditions). Secondly, we apply these results to study the Π^0_γ-completeness and Σ^0_γ-completeness of several multifractal decomposition sets of self-similar measures (satisfying a mild separation condition). For example, a corollary of our results shows if μ is a self-similar measure satisfying the strong separation condition and is not equal to the normalized Hausdorff measure on its support, then the classical multifractal decomposition sets of μ defined by {x ε ℝd | lim r ↘ 0 [log μ(B(x,r))/log r = α]} are Π^0_3-complete provided they are non-empty.Long-time analytic approximation of large stochastic oscillators : simulation, analysis and inferenceMinas, GiorgosRand, David A.http://hdl.handle.net/10023/157612019-01-06T03:37:53Z2017-07-24T00:00:00ZIn order to analyse large complex stochastic dynamical models such as those studied in systems biology there is currently a great need for both analytical tools and also algorithms for accurate and fast simulation and estimation. We present a new stochastic approximation of biological oscillators that addresses these needs. Our method, called phase-corrected LNA (pcLNA) overcomes the main limitations of the standard Linear Noise Approximation (LNA) to remain uniformly accurate for long times, still maintaining the speed and analytically tractability of the LNA. As part of this, we develop analytical expressions for key probability distributions and associated quantities, such as the Fisher Information Matrix and Kullback-Leibler divergence and we introduce a new approach to system-global sensitivity analysis. We also present algorithms for statistical inference and for long-term simulation of oscillating systems that are shown to be as accurate but much faster than leaping algorithms and algorithms for integration of diffusion equations. Stochastic versions of published models of the circadian clock and NF-κB system are used to illustrate our results.
This research was funded by the BBSRC Grant BB/K003097/1 (Systems Biology Analysis of Biological Timers and Inflammation). DAR was also supported by funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 305564. BBSRC web site: www.bbsrc.ac.uk Seventh Framework Programme (FP7) website: cordis.europa.eu/fp7/home_en.html.
2017-07-24T00:00:00ZMinas, GiorgosRand, David A.In order to analyse large complex stochastic dynamical models such as those studied in systems biology there is currently a great need for both analytical tools and also algorithms for accurate and fast simulation and estimation. We present a new stochastic approximation of biological oscillators that addresses these needs. Our method, called phase-corrected LNA (pcLNA) overcomes the main limitations of the standard Linear Noise Approximation (LNA) to remain uniformly accurate for long times, still maintaining the speed and analytically tractability of the LNA. As part of this, we develop analytical expressions for key probability distributions and associated quantities, such as the Fisher Information Matrix and Kullback-Leibler divergence and we introduce a new approach to system-global sensitivity analysis. We also present algorithms for statistical inference and for long-term simulation of oscillating systems that are shown to be as accurate but much faster than leaping algorithms and algorithms for integration of diffusion equations. Stochastic versions of published models of the circadian clock and NF-κB system are used to illustrate our results.Adaptive multivariate global testingMinas, GiorgosAston, John A DStallard, Nigelhttp://hdl.handle.net/10023/157602019-01-06T03:37:52Z2014-06-01T00:00:00ZWe present a methodology for dealing with recent challenges in testing global hypotheses using multivariate observations. The proposed tests target situations, often arising in emerging applications of neuroimaging, where the sample size n is relatively small compared with the observations' dimension K. We employ adaptive designs allowing for sequential modifications of the test statistics adapting to accumulated data. The adaptations are optimal in the sense of maximizing the predictive power of the test at each interim analysis while still controlling the Type I error. Optimality is obtained by a general result applicable to typical adaptive design settings. Further, we prove that the potentially high-dimensional design space of the tests can be reduced to a low-dimensional projection space enabling us to perform simpler power analysis studies, including comparisons to alternative tests. We illustrate the substantial improvement in efficiency that the proposed tests can make over standard tests, especially in the case of n smaller or slightly larger than K. The methods are also studied empirically using both simulated data and data from an EEG study, where the use of prior knowledge substantially increases the power of the test. Supplementary materials for this article are available online.
2014-06-01T00:00:00ZMinas, GiorgosAston, John A DStallard, NigelWe present a methodology for dealing with recent challenges in testing global hypotheses using multivariate observations. The proposed tests target situations, often arising in emerging applications of neuroimaging, where the sample size n is relatively small compared with the observations' dimension K. We employ adaptive designs allowing for sequential modifications of the test statistics adapting to accumulated data. The adaptations are optimal in the sense of maximizing the predictive power of the test at each interim analysis while still controlling the Type I error. Optimality is obtained by a general result applicable to typical adaptive design settings. Further, we prove that the potentially high-dimensional design space of the tests can be reduced to a low-dimensional projection space enabling us to perform simpler power analysis studies, including comparisons to alternative tests. We illustrate the substantial improvement in efficiency that the proposed tests can make over standard tests, especially in the case of n smaller or slightly larger than K. The methods are also studied empirically using both simulated data and data from an EEG study, where the use of prior knowledge substantially increases the power of the test. Supplementary materials for this article are available online.Inferring transcriptional logic from multiple dynamic experimentsMinas, GiorgosJenkins, Dafyd J.Rand, David A.Finkenstädt, Bärbelhttp://hdl.handle.net/10023/157582019-01-06T03:37:51Z2017-11-01T00:00:00ZMotivation: The availability of more data of dynamic gene expression under multiple experimental conditions provides new information that makes the key goal of identifying not only the transcriptional regulators of a gene but also the underlying logical structure attainable. Results: We propose a novel method for inferring transcriptional regulation using a simple, yet biologically interpretable, model to find the logic by which a set of candidate genes and their associated transcription factors (TFs) regulate the transcriptional process of a gene of interest. Our dynamic model links the mRNA transcription rate of the target gene to the activation states of the TFs assuming that these interactions are consistent across multiple experiments and over time. A trans-dimensional Markov Chain Monte Carlo (MCMC) algorithm is used to efficiently sample the regulatory logic under different combinations of parents and rank the estimated models by their posterior probabilities. We demonstrate and compare our methodology with other methods using simulation examples and apply it to a study of transcriptional regulation of selected target genes of Arabidopsis Thaliana from microarray time series data obtained under multiple biotic stresses. We show that our method is able to detect complex regulatory interactions that are consistent under multiple experimental conditions. Availability and implementation: Programs are written in MATLAB and Statistics Toolbox Release 2016b, The MathWorks, Inc., Natick, Massachusetts, United States and are available on GitHub https://github.com/giorgosminas/TRS and at http://www2.warwick.ac.uk/fac/sci/systemsbiology/research/software.
This work was supported by the Biotechnology and Biological Sciences Research Council [BB/F005806/1, BB/K003097/1], the Engineering and Physical Sciences Research Council [EP/C544587/1 to DAR] and the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 305564.
2017-11-01T00:00:00ZMinas, GiorgosJenkins, Dafyd J.Rand, David A.Finkenstädt, BärbelMotivation: The availability of more data of dynamic gene expression under multiple experimental conditions provides new information that makes the key goal of identifying not only the transcriptional regulators of a gene but also the underlying logical structure attainable. Results: We propose a novel method for inferring transcriptional regulation using a simple, yet biologically interpretable, model to find the logic by which a set of candidate genes and their associated transcription factors (TFs) regulate the transcriptional process of a gene of interest. Our dynamic model links the mRNA transcription rate of the target gene to the activation states of the TFs assuming that these interactions are consistent across multiple experiments and over time. A trans-dimensional Markov Chain Monte Carlo (MCMC) algorithm is used to efficiently sample the regulatory logic under different combinations of parents and rank the estimated models by their posterior probabilities. We demonstrate and compare our methodology with other methods using simulation examples and apply it to a study of transcriptional regulation of selected target genes of Arabidopsis Thaliana from microarray time series data obtained under multiple biotic stresses. We show that our method is able to detect complex regulatory interactions that are consistent under multiple experimental conditions. Availability and implementation: Programs are written in MATLAB and Statistics Toolbox Release 2016b, The MathWorks, Inc., Natick, Massachusetts, United States and are available on GitHub https://github.com/giorgosminas/TRS and at http://www2.warwick.ac.uk/fac/sci/systemsbiology/research/software.ReTrOS : a MATLAB toolbox for reconstructing transcriptional activity from gene and protein expression dataMinas, GiorgosMomiji, HiroshiJenkins, Dafyd JCosta, Maria JRand, David AFinkenstädt, Bärbelhttp://hdl.handle.net/10023/157592019-01-06T03:37:51Z2017-06-26T00:00:00ZBACKGROUND: Given the development of high-throughput experimental techniques, an increasing number of whole genome transcription profiling time series data sets, with good temporal resolution, are becoming available to researchers. The ReTrOS toolbox (Reconstructing Transcription Open Software) provides MATLAB-based implementations of two related methods, namely ReTrOS-Smooth and ReTrOS-Switch, for reconstructing the temporal transcriptional activity profile of a gene from given mRNA expression time series or protein reporter time series. The methods are based on fitting a differential equation model incorporating the processes of transcription, translation and degradation. RESULTS: The toolbox provides a framework for model fitting along with statistical analyses of the model with a graphical interface and model visualisation. We highlight several applications of the toolbox, including the reconstruction of the temporal cascade of transcriptional activity inferred from mRNA expression data and protein reporter data in the core circadian clock in Arabidopsis thaliana, and how such reconstructed transcription profiles can be used to study the effects of different cell lines and conditions. CONCLUSIONS: The ReTrOS toolbox allows users to analyse gene and/or protein expression time series where, with appropriate formulation of prior information about a minimum of kinetic parameters, in particular rates of degradation, users are able to infer timings of changes in transcriptional activity. Data from any organism and obtained from a range of technologies can be used as input due to the flexible and generic nature of the model and implementation. The output from this software provides a useful analysis of time series data and can be incorporated into further modelling approaches or in hypothesis generation.
This work was supported through providing funds by the Biotechnology and Biological Sciences Research Council [BB/F005806/1, BB/F005237/1]; and the Engineering and Physical Sciences Research Council [EP/C544587/1 to DAR].
2017-06-26T00:00:00ZMinas, GiorgosMomiji, HiroshiJenkins, Dafyd JCosta, Maria JRand, David AFinkenstädt, BärbelBACKGROUND: Given the development of high-throughput experimental techniques, an increasing number of whole genome transcription profiling time series data sets, with good temporal resolution, are becoming available to researchers. The ReTrOS toolbox (Reconstructing Transcription Open Software) provides MATLAB-based implementations of two related methods, namely ReTrOS-Smooth and ReTrOS-Switch, for reconstructing the temporal transcriptional activity profile of a gene from given mRNA expression time series or protein reporter time series. The methods are based on fitting a differential equation model incorporating the processes of transcription, translation and degradation. RESULTS: The toolbox provides a framework for model fitting along with statistical analyses of the model with a graphical interface and model visualisation. We highlight several applications of the toolbox, including the reconstruction of the temporal cascade of transcriptional activity inferred from mRNA expression data and protein reporter data in the core circadian clock in Arabidopsis thaliana, and how such reconstructed transcription profiles can be used to study the effects of different cell lines and conditions. CONCLUSIONS: The ReTrOS toolbox allows users to analyse gene and/or protein expression time series where, with appropriate formulation of prior information about a minimum of kinetic parameters, in particular rates of degradation, users are able to infer timings of changes in transcriptional activity. Data from any organism and obtained from a range of technologies can be used as input due to the flexible and generic nature of the model and implementation. The output from this software provides a useful analysis of time series data and can be incorporated into further modelling approaches or in hypothesis generation.Analytic analysis of auxetic metamaterials through analogy with rigid link systemsRayneau-Kirkhope, DanielZhang, ChengzhaoTheran, Louis SimonDias, Marcelohttp://hdl.handle.net/10023/157462019-01-06T03:33:40Z2018-02-21T00:00:00ZIn recent years many structural motifs have been designed with the aim of creating auxetic metamaterials. One area of particular interest in this subject is the creation of auxetic material properties through elastic instability. Such metamaterials switch from conventional behaviour to an auxetic response for loads greater than some threshold value. This paper develops a novel methodology in the analysis of auxetic metamaterials which exhibit elastic instability through analogy with rigid link lattice systems. The results of our analytic approach are confirmed by finite element simulations for both the onset of elastic instability and post-buckling behaviour including Poisson’s ratio. The method gives insight into the relationships between mechanisms within lattices and their mechanical behaviour; as such, it has the potential to allow existing knowledge of rigid link lattices with auxetic paths to be used in the design of future buckling induced auxetic metamaterials.
D.R.-K. acknowledges funding support from Academy of Finland and Aalto Science Institute. C.Z. acknowledges funding from Aalto Science Institute. L.T. acknowledges funding from Aalto Science Institute thematic program ‘Challenges in large geometric structures and big data’.
2018-02-21T00:00:00ZRayneau-Kirkhope, DanielZhang, ChengzhaoTheran, Louis SimonDias, MarceloIn recent years many structural motifs have been designed with the aim of creating auxetic metamaterials. One area of particular interest in this subject is the creation of auxetic material properties through elastic instability. Such metamaterials switch from conventional behaviour to an auxetic response for loads greater than some threshold value. This paper develops a novel methodology in the analysis of auxetic metamaterials which exhibit elastic instability through analogy with rigid link lattice systems. The results of our analytic approach are confirmed by finite element simulations for both the onset of elastic instability and post-buckling behaviour including Poisson’s ratio. The method gives insight into the relationships between mechanisms within lattices and their mechanical behaviour; as such, it has the potential to allow existing knowledge of rigid link lattices with auxetic paths to be used in the design of future buckling induced auxetic metamaterials.The broadband excitation of 3D Alfvén resonances in a MHD waveguideElsden, T.Wright, A. N.http://hdl.handle.net/10023/157312019-01-06T03:34:17Z2018-01-01T00:00:00ZThis paper considers the resonant coupling of fast and Alfvén magnetohydrodynamic (MHD) waves. We perform numerical simulations of the time-dependent excitation of Alfvén resonances in a dipole magnetic field, with nonuniform density providing a 3-D equilibrium. Wright and Elsden (2016) showed that in such a system where the poloidal and toroidal Alfvén eigenfrequencies are different, the resonance can have an intermediate polarization, between poloidal and toroidal. We extend this work by driving the system with a broadband rather than monochromatic source. Further, we investigate the effect of azimuthal inhomogeneity on the resonance path. It is found that when exposed to a broadband driver, the dominant frequencies are the fast waveguide eigenfrequencies, which act as the drivers of Alfvén resonances. We demonstrate how resonances can still form efficiently with significant amplitudes, even when forced by the medium to have a far from toroidal polarization. Indeed, larger-amplitude resonances can be generated with an intermediate polarization, rather than purely toroidal, as a result of larger gradients in the magnetic pressure formed by the azimuthal inhomogeneity. Importantly, the resonance structure is shown to be independent of the different forms of driving, meaning their locations and orientations may be used to infer properties of the equilibrium. However, the amplitude of the FLRs are sensitive to the spatial structure and frequency spectrum of the magnetopause driving. These results have implications for the structure of field line resonances (FLRs) in Earth's magnetosphere, although the focus of this paper is on the underlying physics involved.
T. Elsden and A. N. Wright were funded by The Leverhulme Trust through Research Grant RPG-2016-071. A. N. Wright was also funded by STFC through Consolidated Grant ST/N000609/1. Data from simulation results are available on Figshare: https://figshare.com/authors/Tom_Elsden/4743264.
2018-01-01T00:00:00ZElsden, T.Wright, A. N.This paper considers the resonant coupling of fast and Alfvén magnetohydrodynamic (MHD) waves. We perform numerical simulations of the time-dependent excitation of Alfvén resonances in a dipole magnetic field, with nonuniform density providing a 3-D equilibrium. Wright and Elsden (2016) showed that in such a system where the poloidal and toroidal Alfvén eigenfrequencies are different, the resonance can have an intermediate polarization, between poloidal and toroidal. We extend this work by driving the system with a broadband rather than monochromatic source. Further, we investigate the effect of azimuthal inhomogeneity on the resonance path. It is found that when exposed to a broadband driver, the dominant frequencies are the fast waveguide eigenfrequencies, which act as the drivers of Alfvén resonances. We demonstrate how resonances can still form efficiently with significant amplitudes, even when forced by the medium to have a far from toroidal polarization. Indeed, larger-amplitude resonances can be generated with an intermediate polarization, rather than purely toroidal, as a result of larger gradients in the magnetic pressure formed by the azimuthal inhomogeneity. Importantly, the resonance structure is shown to be independent of the different forms of driving, meaning their locations and orientations may be used to infer properties of the equilibrium. However, the amplitude of the FLRs are sensitive to the spatial structure and frequency spectrum of the magnetopause driving. These results have implications for the structure of field line resonances (FLRs) in Earth's magnetosphere, although the focus of this paper is on the underlying physics involved.Numerical modelling of ultra low frequency waves in Earth's magnetosphereElsden, Tomhttp://hdl.handle.net/10023/156632018-07-27T23:15:17Z2016-06-24T00:00:00ZUltra Low Frequency (ULF) waves are a ubiquitous feature of Earth's outer atmosphere, known as the magnetosphere, having been observed on the ground for almost two centuries, and in space over the last 50 years. These waves represent small oscillations in Earth's magnetic field, most often as a response to the external influence of the solar wind. They are important for the transfer of energy throughout the magnetosphere and for coupling different regions together. In this thesis, various features of these oscillations are considered. A detailed background on the history and previous study of ULF waves relevant to our work is given in the introductory chapter. In the following chapters, we predominantly use numerical methods to model ULF waves, which are carefully developed and thoroughly tested. We consider the application of these methods to reports on ground and spaced based observations, which allows a more in depth study of the data. In one case, the simulation results provide evidence for an alternative explanation of the data to the original report, which displays the power of theoretical modelling. An analytical model is also constructed, which is tested on simulation data, to identify the incidence and reflection of a class of ULF wave in the flank magnetosphere. This technique is developed with the aim of future applications to satellite data. Further to this, we develop models both in Cartesian and dipole geometries to investigate some of the theoretical aspects of the coupling between various waves modes. New light is shed on the coupling of compressional (fast) and transverse (Alfvén) magnetohydrodynamic (MHD) wave modes in a 3D dipole geometry. Overall, this thesis aims to develop useful numerical models, which can be used to aid in the interpretation of ULF wave observations, as well as probing new aspects of the existing wave theory.
2016-06-24T00:00:00ZElsden, TomUltra Low Frequency (ULF) waves are a ubiquitous feature of Earth's outer atmosphere, known as the magnetosphere, having been observed on the ground for almost two centuries, and in space over the last 50 years. These waves represent small oscillations in Earth's magnetic field, most often as a response to the external influence of the solar wind. They are important for the transfer of energy throughout the magnetosphere and for coupling different regions together. In this thesis, various features of these oscillations are considered. A detailed background on the history and previous study of ULF waves relevant to our work is given in the introductory chapter. In the following chapters, we predominantly use numerical methods to model ULF waves, which are carefully developed and thoroughly tested. We consider the application of these methods to reports on ground and spaced based observations, which allows a more in depth study of the data. In one case, the simulation results provide evidence for an alternative explanation of the data to the original report, which displays the power of theoretical modelling. An analytical model is also constructed, which is tested on simulation data, to identify the incidence and reflection of a class of ULF wave in the flank magnetosphere. This technique is developed with the aim of future applications to satellite data. Further to this, we develop models both in Cartesian and dipole geometries to investigate some of the theoretical aspects of the coupling between various waves modes. New light is shed on the coupling of compressional (fast) and transverse (Alfvén) magnetohydrodynamic (MHD) wave modes in a 3D dipole geometry. Overall, this thesis aims to develop useful numerical models, which can be used to aid in the interpretation of ULF wave observations, as well as probing new aspects of the existing wave theory.Eruptions and jets in the SunLee, Eon Juihttp://hdl.handle.net/10023/156482018-07-23T23:15:15Z2017-06-23T00:00:00ZMagnetic flux emergence is a fundamental process in the Sun, during which magnetic fields emerge from the solar interior to the surface, to build up active regions and give onset to spectacular dynamic phenomena, such as eruptions and jets. In this thesis, we performed 3D, resistive MHD simulations to study the emergence and the associated magnetic activity of a quadrupolar region in the Sun. Our aim behind the setup of this initial condition (i.e. a quadrupolar region) was to study a magnetic field configuration, which has not been studied in detail before, although it has been repeatedly observed in the Sun and it has been shown that it can host intense magnetic activity (e.g. in the form of jets, flares and eruptions).
The results of our experiments showed that the internal dynamics of such regions leads to the onset of eruptions in the form of twisted magnetic flux tubes (flux ropes). These eruptions are recurrent but they cannot escape the outermost field of the emerging flux (envelope field). They remain confined within the envelope field, as the downward tension of the outermost field lines overwhelms the upward Lorentz force of the erupting field. When we add an ambient magnetic field in the solar atmosphere, external reconnection between the emerging and the ambient field triggers the emission of (standard) reconnection jets. The external reconnection also releases the tension of the ambient field lines and, thus, the eruptions move in an ejective way towards the outer space. Namely, the confined eruptions become ejective eruptions, which escape from the numerical domain. These ejective eruptions drive a newly observed class of jets, the so called "blowout" jets. Our experiments reproduce some of the main observed characteristics of the "blowout" jets. We showed that "blowout" jets emit hot and cool plasma into the outer solar atmosphere simultaneously, and they undergo untwisting motion due to the relaxation of twist during their ejection. We found that the untwisting motion of the "blowout" jets is associated with the propagation of torsional Alfvén waves. Finally, we performed a parametric study to explore the effect of the ambient field strength on the onset and dynamics of the eruptive events. We found that one of the main effects is that the stronger ambient field suppresses the vertical expansion of the magnetic envelope of the quadrupolar region due to the higher magnetic pressure above it. This result has an effect on the emission of jets, which are emitted due to reconnection between the two fields. When the ambient field is relatively weak, it is pushed away from the strong emerging field and reconnection between them is not so persistent. On the other hand, when the ambient field is relatively strong, we find that more jets are ejected due to more efficient and more frequent reconnection between the two flux systems. As a consequence, we find that more mass and flux is being transferred into the solar corona by the reconnection jets. Also, we find that there are more eruptions when the ambient field is stronger. The study of the total energy flux carried by the jets showed that it is sufficient to provide the energy required to accelerate the high speed solar wind. This indicates that the "blowout" jets may play an important role in driving the solar wind.
2017-06-23T00:00:00ZLee, Eon JuiMagnetic flux emergence is a fundamental process in the Sun, during which magnetic fields emerge from the solar interior to the surface, to build up active regions and give onset to spectacular dynamic phenomena, such as eruptions and jets. In this thesis, we performed 3D, resistive MHD simulations to study the emergence and the associated magnetic activity of a quadrupolar region in the Sun. Our aim behind the setup of this initial condition (i.e. a quadrupolar region) was to study a magnetic field configuration, which has not been studied in detail before, although it has been repeatedly observed in the Sun and it has been shown that it can host intense magnetic activity (e.g. in the form of jets, flares and eruptions).
The results of our experiments showed that the internal dynamics of such regions leads to the onset of eruptions in the form of twisted magnetic flux tubes (flux ropes). These eruptions are recurrent but they cannot escape the outermost field of the emerging flux (envelope field). They remain confined within the envelope field, as the downward tension of the outermost field lines overwhelms the upward Lorentz force of the erupting field. When we add an ambient magnetic field in the solar atmosphere, external reconnection between the emerging and the ambient field triggers the emission of (standard) reconnection jets. The external reconnection also releases the tension of the ambient field lines and, thus, the eruptions move in an ejective way towards the outer space. Namely, the confined eruptions become ejective eruptions, which escape from the numerical domain. These ejective eruptions drive a newly observed class of jets, the so called "blowout" jets. Our experiments reproduce some of the main observed characteristics of the "blowout" jets. We showed that "blowout" jets emit hot and cool plasma into the outer solar atmosphere simultaneously, and they undergo untwisting motion due to the relaxation of twist during their ejection. We found that the untwisting motion of the "blowout" jets is associated with the propagation of torsional Alfvén waves. Finally, we performed a parametric study to explore the effect of the ambient field strength on the onset and dynamics of the eruptive events. We found that one of the main effects is that the stronger ambient field suppresses the vertical expansion of the magnetic envelope of the quadrupolar region due to the higher magnetic pressure above it. This result has an effect on the emission of jets, which are emitted due to reconnection between the two fields. When the ambient field is relatively weak, it is pushed away from the strong emerging field and reconnection between them is not so persistent. On the other hand, when the ambient field is relatively strong, we find that more jets are ejected due to more efficient and more frequent reconnection between the two flux systems. As a consequence, we find that more mass and flux is being transferred into the solar corona by the reconnection jets. Also, we find that there are more eruptions when the ambient field is stronger. The study of the total energy flux carried by the jets showed that it is sufficient to provide the energy required to accelerate the high speed solar wind. This indicates that the "blowout" jets may play an important role in driving the solar wind.On plausible counterexamples to Lehnert's conjectureBennett, Danielhttp://hdl.handle.net/10023/156312018-07-23T23:16:12Z2018-01-01T00:00:00ZA group whose co-word problem is a context free language is called co𝐶𝐹 . Lehnert's conjecture states that a group 𝐺 is co𝐶𝐹 if and only if 𝐺 embeds as a finitely generated subgroup of R. Thompson's group V . In this thesis we explore a class of groups, Faug, proposed by Berns-Zieze, Fry, Gillings, Hoganson, and Mathews to contain potential counterexamples to Lehnert's conjecture. We create infinite and finite presentations for such groups and go on to prove that a certain subclass of 𝓕𝑎𝑢𝑔 consists of groups that do embed into 𝑉.
By Anisimov a group has regular word problem if and only if it is finite. It is also known
that a group 𝐺 is finite if and only if there exists an embedding of 𝐺 into 𝑉 such that
its natural action on 𝕮₂:= {0, 1}[super]𝜔 is free on the whole space. We show that the class of
groups with a context free word problem, the class of 𝐶𝐹 groups, is precisely the class of finitely generated demonstrable groups for 𝑉 . A demonstrable group for V is a group 𝐺 which is isomorphic to a subgroup in 𝑉 whose natural action on 𝕮₂ acts freely on an open subset. Thus our result extends the correspondence between language theoretic properties of groups and dynamical properties of subgroups of V . Additionally, our result also shows that the final condition of the four known closure properties of the class of co𝐶𝐹 groups also holds for the set of finitely generated subgroups of 𝑉.
2018-01-01T00:00:00ZBennett, DanielA group whose co-word problem is a context free language is called co𝐶𝐹 . Lehnert's conjecture states that a group 𝐺 is co𝐶𝐹 if and only if 𝐺 embeds as a finitely generated subgroup of R. Thompson's group V . In this thesis we explore a class of groups, Faug, proposed by Berns-Zieze, Fry, Gillings, Hoganson, and Mathews to contain potential counterexamples to Lehnert's conjecture. We create infinite and finite presentations for such groups and go on to prove that a certain subclass of 𝓕𝑎𝑢𝑔 consists of groups that do embed into 𝑉.
By Anisimov a group has regular word problem if and only if it is finite. It is also known
that a group 𝐺 is finite if and only if there exists an embedding of 𝐺 into 𝑉 such that
its natural action on 𝕮₂:= {0, 1}[super]𝜔 is free on the whole space. We show that the class of
groups with a context free word problem, the class of 𝐶𝐹 groups, is precisely the class of finitely generated demonstrable groups for 𝑉 . A demonstrable group for V is a group 𝐺 which is isomorphic to a subgroup in 𝑉 whose natural action on 𝕮₂ acts freely on an open subset. Thus our result extends the correspondence between language theoretic properties of groups and dynamical properties of subgroups of V . Additionally, our result also shows that the final condition of the four known closure properties of the class of co𝐶𝐹 groups also holds for the set of finitely generated subgroups of 𝑉.Incorporating animal movement into circular plot and point transect surveys of wildlife abundancePrieto González, Rocíohttp://hdl.handle.net/10023/156122018-08-29T14:47:51Z2018-01-01T00:00:00ZEstimating wildlife abundance is fundamental for its effective management and conservation.
A range of methods exist: total counts, plot sampling, distance sampling and
capture-recapture based approaches. Methods have assumptions and their failure can
lead to substantial bias. Current research in the field is focused not on establishing new
methods but in extending existing methods to deal with their assumptions' violation.
This thesis focus on incorporating animal movement into circular plot sampling (CPS)
and point transect sampling (PTS), where a key assumption is that animals do not move
while within detection range, i.e., the survey is a snapshot in time. While targeting this
goal, we found some unexpected bias in PTS when animals were still and model selection
was used to choose among different candidate models for the detection function (the
model describing how detectability changes with observer-animal distance). Using a simulation
study, we found that, although PTS estimators are asymptotically unbiased, for
the recommended sample sizes the bias depended on the form of the true detection function.
We then extended the simulation study to include animal movement, and found this
led to further bias in CPS and PTS. We present novel methods that incorporate animal
movement with constant speed into estimates of abundance. First, in CPS, we present
an analytic expression to correct for the bias given linear movement. When movement
is de ned by a diffusion process, a simulation based approach, modelling the probability
of animal presence in the circular plot, results in less than 3% bias in the abundance
estimates. For PTS we introduce an estimator composed of two linked submodels: the
movement (animals moving linearly) and the detection model. The performance of the
proposed method is assessed via simulation. Despite being biased, the new estimator
yields improved results compared to ignoring animal movement using conventional PTS.
2018-01-01T00:00:00ZPrieto González, RocíoEstimating wildlife abundance is fundamental for its effective management and conservation.
A range of methods exist: total counts, plot sampling, distance sampling and
capture-recapture based approaches. Methods have assumptions and their failure can
lead to substantial bias. Current research in the field is focused not on establishing new
methods but in extending existing methods to deal with their assumptions' violation.
This thesis focus on incorporating animal movement into circular plot sampling (CPS)
and point transect sampling (PTS), where a key assumption is that animals do not move
while within detection range, i.e., the survey is a snapshot in time. While targeting this
goal, we found some unexpected bias in PTS when animals were still and model selection
was used to choose among different candidate models for the detection function (the
model describing how detectability changes with observer-animal distance). Using a simulation
study, we found that, although PTS estimators are asymptotically unbiased, for
the recommended sample sizes the bias depended on the form of the true detection function.
We then extended the simulation study to include animal movement, and found this
led to further bias in CPS and PTS. We present novel methods that incorporate animal
movement with constant speed into estimates of abundance. First, in CPS, we present
an analytic expression to correct for the bias given linear movement. When movement
is de ned by a diffusion process, a simulation based approach, modelling the probability
of animal presence in the circular plot, results in less than 3% bias in the abundance
estimates. For PTS we introduce an estimator composed of two linked submodels: the
movement (animals moving linearly) and the detection model. The performance of the
proposed method is assessed via simulation. Despite being biased, the new estimator
yields improved results compared to ignoring animal movement using conventional PTS.A continuous-time formulation for spatial capture-recapture modelsDistiller, Greghttp://hdl.handle.net/10023/155962019-01-18T11:47:30Z2017-01-01T00:00:00ZSpatial capture-recapture (SCR) models are relatively new but have become the
standard approach used to estimate animal density from capture-recapture data. It
has in the past been impractical to obtain sufficient data for analysis on species that
are very di cult to capture such as elusive carnivores that occur at low density and
range very widely. Advances in technology have led to alternative ways to virtually
\capture" individuals without having to physically hold them. Some examples of
these new non-invasive sampling methods include scat or hair collection for genetic
analysis, acoustic detection and camera trapping.
In traditional capture-recapture (CR) and SCR studies populations are sampled
at discrete points in time leading to clear and well de ned occasions whereas the
new detector types mentioned above sample populations continuously in time. Researchers
with data collected continuously currently need to de ne an appropriate
occasion and aggregate their data accordingly thereby imposing an artificial construct
on their data for analytical convenience.
This research develops a continuous-time (CT) framework for SCR models by
treating detections as a temporal non homogeneous Poisson process (NHPP) and
replacing the usual SCR detection function with a continuous detection hazard function.
The general CT likelihood is rst developed for data from passive (also called
\proximity") detectors like camera traps that do not physically hold individuals. The
likelihood is then modi ed to produce a likelihood for single-catch traps (traps that
are taken out of action by capturing an animal) that has proven di cult to develop
with a discrete-occasion approach.
The lack of a suitable single-catch trap likelihood has led to researchers using
a discrete-time (DT) multi-catch trap estimator to analyse single-catch trap data.
Previous work has found the DT multi-catch estimator to be robust despite the fact
that it is known to be based on the wrong model for single-catch traps (it assumes
that the traps continue operating after catching an individual). Simulation studies in
this work con rm that the multi-catch estimator is robust for estimating density when
density is constant or does not vary much in space. However, there are scenarios with
non-constant density surfaces when the multi-catch estimator is not able to correctly
identify regions of high density. Furthermore, the multi-catch estimator is known
to be negatively biased for the intercept parameter of SCR detection functions and
there may be interest in the detection function in its own right. On the other hand
the CT single-catch estimator is unbiased or nearly so for all parameters of interest
including those in the detection function and those in the model for density.
When one assumes that the detection hazard is constant through time there is
no impact of ignoring capture times and using only the detection frequencies. This
is of course a special case and in reality detection hazards will tend to vary in time.
However when one assumes that the effects of time and distance in the time-varying
hazard are independent, then similarly there is no information in the capture times
about density and detection function parameters. The work here uses a detection
hazard that assumes independence between time and distance. Different forms for
the detection hazard are explored with the most flexible choice being that of a cyclic
regression spline.
Extensive simulation studies suggest as expected that a DT proximity estimator is
unbiased for the estimation of density even when the detection hazard varies though
time. However there are indirect benefits of incorporating capture times because
doing so will lead to a better fitting detection component of the model, and this can
prevent unexplained variation being erroneously attributed to the wrong covariate.
The analysis of two real datasets supports this assertion because the models with the
best fitting detection hazard have different effects to the other models. In addition,
modelling the detection process in continuous-time leads to a more parsimonious
approach compared to using DT models when the detection hazard varies in time.
The underlying process is occurring in continuous-time and so using CT models
allows inferences to be drawn abo ut the underlying process, for example the timevarying
detection hazard can be viewed as a proxy for animal activity. The CT
formulation is able to model the underlying detection hazard accurately and provides
a formal modelling framework to explore different hypotheses about activity patterns.
There is scope to integrate the CT models developed here with models for space usage
and landscape connectivity to explore these processes on a finer temporal scale.
SCR models are experiencing a rapid growth in both application and method
development. The data generating process occurs in CT and hence a CT modelling
approach is a natural t and opens up several opportunities that are not possible
with a DT formulation. The work here makes a contribution by developing and
exploring the utility of such a CT SCR formulation.
2017-01-01T00:00:00ZDistiller, GregSpatial capture-recapture (SCR) models are relatively new but have become the
standard approach used to estimate animal density from capture-recapture data. It
has in the past been impractical to obtain sufficient data for analysis on species that
are very di cult to capture such as elusive carnivores that occur at low density and
range very widely. Advances in technology have led to alternative ways to virtually
\capture" individuals without having to physically hold them. Some examples of
these new non-invasive sampling methods include scat or hair collection for genetic
analysis, acoustic detection and camera trapping.
In traditional capture-recapture (CR) and SCR studies populations are sampled
at discrete points in time leading to clear and well de ned occasions whereas the
new detector types mentioned above sample populations continuously in time. Researchers
with data collected continuously currently need to de ne an appropriate
occasion and aggregate their data accordingly thereby imposing an artificial construct
on their data for analytical convenience.
This research develops a continuous-time (CT) framework for SCR models by
treating detections as a temporal non homogeneous Poisson process (NHPP) and
replacing the usual SCR detection function with a continuous detection hazard function.
The general CT likelihood is rst developed for data from passive (also called
\proximity") detectors like camera traps that do not physically hold individuals. The
likelihood is then modi ed to produce a likelihood for single-catch traps (traps that
are taken out of action by capturing an animal) that has proven di cult to develop
with a discrete-occasion approach.
The lack of a suitable single-catch trap likelihood has led to researchers using
a discrete-time (DT) multi-catch trap estimator to analyse single-catch trap data.
Previous work has found the DT multi-catch estimator to be robust despite the fact
that it is known to be based on the wrong model for single-catch traps (it assumes
that the traps continue operating after catching an individual). Simulation studies in
this work con rm that the multi-catch estimator is robust for estimating density when
density is constant or does not vary much in space. However, there are scenarios with
non-constant density surfaces when the multi-catch estimator is not able to correctly
identify regions of high density. Furthermore, the multi-catch estimator is known
to be negatively biased for the intercept parameter of SCR detection functions and
there may be interest in the detection function in its own right. On the other hand
the CT single-catch estimator is unbiased or nearly so for all parameters of interest
including those in the detection function and those in the model for density.
When one assumes that the detection hazard is constant through time there is
no impact of ignoring capture times and using only the detection frequencies. This
is of course a special case and in reality detection hazards will tend to vary in time.
However when one assumes that the effects of time and distance in the time-varying
hazard are independent, then similarly there is no information in the capture times
about density and detection function parameters. The work here uses a detection
hazard that assumes independence between time and distance. Different forms for
the detection hazard are explored with the most flexible choice being that of a cyclic
regression spline.
Extensive simulation studies suggest as expected that a DT proximity estimator is
unbiased for the estimation of density even when the detection hazard varies though
time. However there are indirect benefits of incorporating capture times because
doing so will lead to a better fitting detection component of the model, and this can
prevent unexplained variation being erroneously attributed to the wrong covariate.
The analysis of two real datasets supports this assertion because the models with the
best fitting detection hazard have different effects to the other models. In addition,
modelling the detection process in continuous-time leads to a more parsimonious
approach compared to using DT models when the detection hazard varies in time.
The underlying process is occurring in continuous-time and so using CT models
allows inferences to be drawn abo ut the underlying process, for example the timevarying
detection hazard can be viewed as a proxy for animal activity. The CT
formulation is able to model the underlying detection hazard accurately and provides
a formal modelling framework to explore different hypotheses about activity patterns.
There is scope to integrate the CT models developed here with models for space usage
and landscape connectivity to explore these processes on a finer temporal scale.
SCR models are experiencing a rapid growth in both application and method
development. The data generating process occurs in CT and hence a CT modelling
approach is a natural t and opens up several opportunities that are not possible
with a DT formulation. The work here makes a contribution by developing and
exploring the utility of such a CT SCR formulation.Equilibrium states, pressure and escape for multimodal maps with holesDemers, Mark F.Todd, Mikehttp://hdl.handle.net/10023/152142019-01-06T02:35:00Z2017-09-01T00:00:00ZFor a class of non-uniformly hyperbolic interval maps, we study rates of escape with respect to conformal measures associated with a family of geometric potentials. We establish the existence of physically relevant conditionally invariant measures and equilibrium states and prove a relation between the rate of escape and pressure with respect to these potentials. As a consequence, we obtain a Bowen formula: we express the Hausdor dimension of the set of points which never exit through the hole in terms of the relevant pressure function. Finally, we obtain an expression for the derivative of the escape rate in the zero-hole limit.
MD was partially supported by NSF grants DMS 1101572 and DMS 1362420. MT was partially supported by NSF grants DMS 0606343 and DMS 0908093.
2017-09-01T00:00:00ZDemers, Mark F.Todd, MikeFor a class of non-uniformly hyperbolic interval maps, we study rates of escape with respect to conformal measures associated with a family of geometric potentials. We establish the existence of physically relevant conditionally invariant measures and equilibrium states and prove a relation between the rate of escape and pressure with respect to these potentials. As a consequence, we obtain a Bowen formula: we express the Hausdor dimension of the set of points which never exit through the hole in terms of the relevant pressure function. Finally, we obtain an expression for the derivative of the escape rate in the zero-hole limit.Nonlinear partial differential equations on fractalsHu, Jiaxinhttp://hdl.handle.net/10023/151802018-07-10T23:19:13Z2001-01-01T00:00:00ZThe study of nonlinear partial differential equations on fractals is a burgeoning inter-disciplinary topic, allowing dynamic properties on fractals to be investigated. In this thesis we will investigate nonlinear PDEs of three basic types on bounded and unbounded fractals. We first review the definition of post-critically finite (p.c.f.) self-similar fractals with regular harmonic structure. A Dirichlet form exists on such a fractal; thus we may define a weak version of the Laplacian. The Sobolev-type inequality, established on p.c.f. self-similar fractals satisfying the separation condition, plays a crucial role in the analysis of PDEs on p.c.f. self-similar fractals. We use the classical approach to study the linear eigenvalue problem on p.c.f. self-similar fractals, which depends on the Sobolev-type inequality. Fundamental solutions such as Green's function, wave propagator and heat kernel are then explicitly expressed in terms of eigenvalues and eigenfunctions. The main aim of the thesis is to study nonlinear PDEs on fractals. We begin with nonlinear elliptic equations on p.c.f. self-similar fractals. We prove the existence of non-trivial solutions to elliptic equations with zero Dirichlet boundary conditions using the mountain pass theorem and the saddle point theorem. For nonlinear wave equations on p.c.f. self-similar fractals, we show the existence of global solutions for appropriate initial and boundary data. We also examine blow up at finite time which may occur for certain initial data. Finally, we consider nonlinear diffusion equations on p.c.f. self-similar fractals and unbounded fractals. Using the upper-lower solution technique, we prove the global existence of solutions of the nonlinear diffusion equation with initial value and boundary conditions on p.c.f. self-similar fractals. For unbounded fractals, starting with a heat kernel satisfying certain assumptions, we prove that the diffusion equation with a nonlinear term of the form uᵖ possesses a global solution if the initial data is small and p > 1 + ds/2, while solutions blow up if p ≤ 1 + ds/2 even for small initial data, where dg is the spectral dimension of the fractal. We investigate smoothness and Holder continuity of solutions when they exist.
2001-01-01T00:00:00ZHu, JiaxinThe study of nonlinear partial differential equations on fractals is a burgeoning inter-disciplinary topic, allowing dynamic properties on fractals to be investigated. In this thesis we will investigate nonlinear PDEs of three basic types on bounded and unbounded fractals. We first review the definition of post-critically finite (p.c.f.) self-similar fractals with regular harmonic structure. A Dirichlet form exists on such a fractal; thus we may define a weak version of the Laplacian. The Sobolev-type inequality, established on p.c.f. self-similar fractals satisfying the separation condition, plays a crucial role in the analysis of PDEs on p.c.f. self-similar fractals. We use the classical approach to study the linear eigenvalue problem on p.c.f. self-similar fractals, which depends on the Sobolev-type inequality. Fundamental solutions such as Green's function, wave propagator and heat kernel are then explicitly expressed in terms of eigenvalues and eigenfunctions. The main aim of the thesis is to study nonlinear PDEs on fractals. We begin with nonlinear elliptic equations on p.c.f. self-similar fractals. We prove the existence of non-trivial solutions to elliptic equations with zero Dirichlet boundary conditions using the mountain pass theorem and the saddle point theorem. For nonlinear wave equations on p.c.f. self-similar fractals, we show the existence of global solutions for appropriate initial and boundary data. We also examine blow up at finite time which may occur for certain initial data. Finally, we consider nonlinear diffusion equations on p.c.f. self-similar fractals and unbounded fractals. Using the upper-lower solution technique, we prove the global existence of solutions of the nonlinear diffusion equation with initial value and boundary conditions on p.c.f. self-similar fractals. For unbounded fractals, starting with a heat kernel satisfying certain assumptions, we prove that the diffusion equation with a nonlinear term of the form uᵖ possesses a global solution if the initial data is small and p > 1 + ds/2, while solutions blow up if p ≤ 1 + ds/2 even for small initial data, where dg is the spectral dimension of the fractal. We investigate smoothness and Holder continuity of solutions when they exist.In situ generation of transverse magnetohydrodynamic waves from colliding flows in the solar coronaAntolin, PatrickPagano, PaoloDe Moortel, InekeNakariakov, Valery M.http://hdl.handle.net/10023/151782019-01-18T12:30:07Z2018-07-09T00:00:00ZTransverse magnetohydrodynamic (MHD) waves permeate the solar atmosphere and are a candidate for coronal heating. However, the origin of these waves is still unclear. In this Letter, we analyze coordinated observations from Hinode/Solar Optical Telescope (SOT) and Interface Region Imaging Spectrograph (IRIS) of a prominence/coronal rain loop-like structure at the limb of the Sun. Cool and dense downflows and upflows are observed along the structure. A collision between a downward and an upward flow with an estimated energy flux of 107–108 erg cm−2 s−1 is observed to generate oscillatory transverse perturbations of the strands with an estimated ≈40 km s−1 total amplitude, and a short-lived brightening event with the plasma temperature increasing to at least 105 K. We interpret this response as sausage and kink transverse MHD waves based on 2D MHD simulations of plasma flow collision. The lengths, density, and velocity differences between the colliding clumps and the strength of the magnetic field are major parameters defining the response to the collision. The presence of asymmetry between the clumps (angle of impact surface and/or offset of flowing axis) is crucial for generating a kink mode. Using the observed values, we successfully reproduce the observed transverse perturbations and brightening, and show adiabatic heating to coronal temperatures. The numerical modeling indicates that the plasma β in this loop-like structure is confined between 0.09 and 0.36. These results suggest that such collisions from counter-streaming flows can be a source of in situ transverse MHD waves, and that for cool and dense prominence conditions such waves could have significant amplitudes.
This research has received funding from the UK Science and Technology Facilities Council (Consolidated Grant ST/K000950/1) and the European Union Horizon 2020 Research and Innovation Programme (grant agreement No. 647214). V.M.N. acknowledges the support of the BK21 plus program through the National Research Foundation funded by the Ministry of Education of Korea.
2018-07-09T00:00:00ZAntolin, PatrickPagano, PaoloDe Moortel, InekeNakariakov, Valery M.Transverse magnetohydrodynamic (MHD) waves permeate the solar atmosphere and are a candidate for coronal heating. However, the origin of these waves is still unclear. In this Letter, we analyze coordinated observations from Hinode/Solar Optical Telescope (SOT) and Interface Region Imaging Spectrograph (IRIS) of a prominence/coronal rain loop-like structure at the limb of the Sun. Cool and dense downflows and upflows are observed along the structure. A collision between a downward and an upward flow with an estimated energy flux of 107–108 erg cm−2 s−1 is observed to generate oscillatory transverse perturbations of the strands with an estimated ≈40 km s−1 total amplitude, and a short-lived brightening event with the plasma temperature increasing to at least 105 K. We interpret this response as sausage and kink transverse MHD waves based on 2D MHD simulations of plasma flow collision. The lengths, density, and velocity differences between the colliding clumps and the strength of the magnetic field are major parameters defining the response to the collision. The presence of asymmetry between the clumps (angle of impact surface and/or offset of flowing axis) is crucial for generating a kink mode. Using the observed values, we successfully reproduce the observed transverse perturbations and brightening, and show adiabatic heating to coronal temperatures. The numerical modeling indicates that the plasma β in this loop-like structure is confined between 0.09 and 0.36. These results suggest that such collisions from counter-streaming flows can be a source of in situ transverse MHD waves, and that for cool and dense prominence conditions such waves could have significant amplitudes.Adaptive distance samplingPollard, Johnhttp://hdl.handle.net/10023/151762018-07-10T23:19:12Z2002-01-01T00:00:00ZWe investigate mechanisms to improve efficiency for line and point transect surveys of clustered populations by combining the distance methods with adaptive sampling. In adaptive sampling, survey effort is increased when areas of high animal density are located, thereby increasing the number of observations. We begin by building on existing adaptive sampling techniques, to create both point and line transect adaptive estimators, these are then extended to allow the inclusion of covariates in the detection function estimator. However, the methods are limited, as the total effort required cannot be forecast at the start of a survey, and so a new fixed total effort adaptive approach is developed. A key difference in the new method is that it does not require the calculation of the inclusion probabilities typically used by existing adaptive estimators. The fixed effort method is primarily aimed at line transect sampling, but point transect derivations are also provided. We evaluate the new methodology by computer simulation, and report on surveys of harbour porpoise in the Gulf of Maine, in which the approach was compared with conventional line transect sampling. Line transect simulation results for a clustered population showed up to a 6% improvement in the adaptive density variance estimate over the conventional, whilst when there was no clustering the adaptive estimate was 1% less efficient than the conventional. For the harbour porpoise survey, the adaptive density estimate cvs showed improvements of 8% for individual porpoise density and 14% for school density over the conventional estimates. The primary benefit of the fixed effort method is the potential to improve survey coverage, allowing a survey to complete within a fixed time and effort; an important feature if expensive survey resources are involved, such as an aircraft, crew and observers.
2002-01-01T00:00:00ZPollard, JohnWe investigate mechanisms to improve efficiency for line and point transect surveys of clustered populations by combining the distance methods with adaptive sampling. In adaptive sampling, survey effort is increased when areas of high animal density are located, thereby increasing the number of observations. We begin by building on existing adaptive sampling techniques, to create both point and line transect adaptive estimators, these are then extended to allow the inclusion of covariates in the detection function estimator. However, the methods are limited, as the total effort required cannot be forecast at the start of a survey, and so a new fixed total effort adaptive approach is developed. A key difference in the new method is that it does not require the calculation of the inclusion probabilities typically used by existing adaptive estimators. The fixed effort method is primarily aimed at line transect sampling, but point transect derivations are also provided. We evaluate the new methodology by computer simulation, and report on surveys of harbour porpoise in the Gulf of Maine, in which the approach was compared with conventional line transect sampling. Line transect simulation results for a clustered population showed up to a 6% improvement in the adaptive density variance estimate over the conventional, whilst when there was no clustering the adaptive estimate was 1% less efficient than the conventional. For the harbour porpoise survey, the adaptive density estimate cvs showed improvements of 8% for individual porpoise density and 14% for school density over the conventional estimates. The primary benefit of the fixed effort method is the potential to improve survey coverage, allowing a survey to complete within a fixed time and effort; an important feature if expensive survey resources are involved, such as an aircraft, crew and observers.Contractive Markov systemsWerner, Ivanhttp://hdl.handle.net/10023/151732018-07-26T09:23:32Z2004-01-01T00:00:00ZWe introduce a theory of contractive Markov systems (CMS) which provides a unifying framework in so-called "fractal" geometry. It extends the known theory of iterated function systems (IFS) with place dependent probabilities [1][8] in a way that it also covers graph directed constructions of "fractal" sets [18]. Such systems naturally extend finite Markov chains and inherit some of their properties. In Chapter 1, we consider iterations of a Markov system and show that they preserve the essential structure of it. In Chapter 2, we show that the Markov operator defined by such a system has a unique invariant probability measure in the irreducible case and an attractive probability measure in the aperiodic case if the restrictions of the probability functions on their vertex sets are Dini-continuous and bounded away from zero, and the system satisfies a condition of a contractiveness on average. This generalizes a result from [1]. Furthermore, we show that the rate of convergence to the stationary state is exponential in the aperiodic case with constant probabilities and a compact state space. In Chapter 3, we construct a coding map for a contractive Markov system. In Chapter 4, we calculate Kolmogorov-Sinai entropy of the generalized Markov shift. In Chapter 5, we prove an ergodic theorem for Markov chains associated with the contractive Markov systems. It generalizes the ergodic theorem of Elton [8].
2004-01-01T00:00:00ZWerner, IvanWe introduce a theory of contractive Markov systems (CMS) which provides a unifying framework in so-called "fractal" geometry. It extends the known theory of iterated function systems (IFS) with place dependent probabilities [1][8] in a way that it also covers graph directed constructions of "fractal" sets [18]. Such systems naturally extend finite Markov chains and inherit some of their properties. In Chapter 1, we consider iterations of a Markov system and show that they preserve the essential structure of it. In Chapter 2, we show that the Markov operator defined by such a system has a unique invariant probability measure in the irreducible case and an attractive probability measure in the aperiodic case if the restrictions of the probability functions on their vertex sets are Dini-continuous and bounded away from zero, and the system satisfies a condition of a contractiveness on average. This generalizes a result from [1]. Furthermore, we show that the rate of convergence to the stationary state is exponential in the aperiodic case with constant probabilities and a compact state space. In Chapter 3, we construct a coding map for a contractive Markov system. In Chapter 4, we calculate Kolmogorov-Sinai entropy of the generalized Markov shift. In Chapter 5, we prove an ergodic theorem for Markov chains associated with the contractive Markov systems. It generalizes the ergodic theorem of Elton [8].Finding "small' matrices P,Q such that PDQ = SWainwright, Robert J.http://hdl.handle.net/10023/151712018-07-10T23:17:54Z2002-01-01T00:00:00ZGiven an integer matrix A, there is a unique matrix S of a particular form, called the Smith Normal Form, and non-unique unimodular matrices P and Q such that PAQ = S. It is often the case that these matrices P and Q will be used for further calculation, and as such it is desirable to find P and Q with small entries. In this thesis we address the problem of finding such P and Q with small entries, in particular in the case where A is a diagonal matrix, which arises as a final step in many published algorithms. Heuristic algorithms are developed which appear to do well in practice and some theory is developed to explain this behaviour. We also give an account of the implementation of an alternative algorithm which bypasses this intermediary diagonal form. The basic theoretical development of this is work by Storjohan.
2002-01-01T00:00:00ZWainwright, Robert J.Given an integer matrix A, there is a unique matrix S of a particular form, called the Smith Normal Form, and non-unique unimodular matrices P and Q such that PAQ = S. It is often the case that these matrices P and Q will be used for further calculation, and as such it is desirable to find P and Q with small entries. In this thesis we address the problem of finding such P and Q with small entries, in particular in the case where A is a diagonal matrix, which arises as a final step in many published algorithms. Heuristic algorithms are developed which appear to do well in practice and some theory is developed to explain this behaviour. We also give an account of the implementation of an alternative algorithm which bypasses this intermediary diagonal form. The basic theoretical development of this is work by Storjohan.Automatic S-acts and inverse semigroup presentationsDombi, Erzsebet Ritahttp://hdl.handle.net/10023/151232018-09-27T08:46:00Z2005-01-01T00:00:00ZTo provide a general framework for the theory of automatic groups and semigroups, we introduce the notion of an automatic semigroup act. This notion gives rise to a variety of definitions for automaticity depending on the set chosen as a semigroup act. Namely, we obtain the notions of automaticity, Schutzenberger automaticity, R- and L-class automaticity, etc. We discuss the basic properties of automatic semigroup acts. We show that if S is a semigroup with local right identities, then automaticty of a semigroup act is independent of the choice of both the generators of S and the generators of the semigroup act. We also discuss the equality problem of automatic semigroup acts. To give a geometric approach, we associate a directed labelled graph to each S-act and introduce the notion of the fellow traveller property in the associated graph. We verify that if S is a regular semigroup with finitely many idempotents, then Schutzenberger automaticity is characterized by the fellow traveller property of the Schutzenberger graph. We also verify that a Schutzenberger automatic regular semigroup with finitely many idempotents is finitely presented. We end Chapter 3 by proving that an inverse free product of Schutzenberger automatic inverse semigroups is Schutzenberger automatic. In Chapter 4, we first introduce the notion of finite generation and finite presentability with respect to a semigroup action. With the help of these concepts we give a necessary and sufficient condition for a semidirect product of a semilattice by a group to be finitely generated and finitely presented as an inverse semigroup. We end Chapter 4 by giving a necessary and sufficient condition for the semidirect product of a semilattice by a group to be Schutzenberger automatic. Chapter 5 is devoted to the study of HNN extensions of inverse semigroups from finite generation and finite presentability point of view. Namely, we give necessary and sufficient conditions for finite presentability of Gilbert's and Yamamura's HNN extension of inverse semigroups. The majority of the results contained in Chapter 5 are the result of a joint work with N.D. Gilbert and N. Ruskuc.
2005-01-01T00:00:00ZDombi, Erzsebet RitaTo provide a general framework for the theory of automatic groups and semigroups, we introduce the notion of an automatic semigroup act. This notion gives rise to a variety of definitions for automaticity depending on the set chosen as a semigroup act. Namely, we obtain the notions of automaticity, Schutzenberger automaticity, R- and L-class automaticity, etc. We discuss the basic properties of automatic semigroup acts. We show that if S is a semigroup with local right identities, then automaticty of a semigroup act is independent of the choice of both the generators of S and the generators of the semigroup act. We also discuss the equality problem of automatic semigroup acts. To give a geometric approach, we associate a directed labelled graph to each S-act and introduce the notion of the fellow traveller property in the associated graph. We verify that if S is a regular semigroup with finitely many idempotents, then Schutzenberger automaticity is characterized by the fellow traveller property of the Schutzenberger graph. We also verify that a Schutzenberger automatic regular semigroup with finitely many idempotents is finitely presented. We end Chapter 3 by proving that an inverse free product of Schutzenberger automatic inverse semigroups is Schutzenberger automatic. In Chapter 4, we first introduce the notion of finite generation and finite presentability with respect to a semigroup action. With the help of these concepts we give a necessary and sufficient condition for a semidirect product of a semilattice by a group to be finitely generated and finitely presented as an inverse semigroup. We end Chapter 4 by giving a necessary and sufficient condition for the semidirect product of a semilattice by a group to be Schutzenberger automatic. Chapter 5 is devoted to the study of HNN extensions of inverse semigroups from finite generation and finite presentability point of view. Namely, we give necessary and sufficient conditions for finite presentability of Gilbert's and Yamamura's HNN extension of inverse semigroups. The majority of the results contained in Chapter 5 are the result of a joint work with N.D. Gilbert and N. Ruskuc.Automatic semigroups : constructions and subsemigroupsDescalco, L.http://hdl.handle.net/10023/151222018-09-27T08:46:56Z2002-01-01T00:00:00ZIn this thesis we start by considering conditions under which some standard semigroup constructions preserve automaticity. We first consider Rees matrix semigroups over a semigroup, which we call the base, and work on the following questions: (i) If the base is automatic is the Rees matrix semigroup automatic? (ii) If the Rees matrix semigroup is automatic must the base be automatic as well? We also consider similar questions for Bruck-Reilly extensions of monoids and wreath products of semigroups. Then we consider subsemigroups of free products of semigroups and we study conditions that guarantee them to be automatic. Finally we obtain a description of the subsemigroups of the bicyclic monoid that allow us to study some of their properties, which include finite generation, automaticity and finite presentability.
2002-01-01T00:00:00ZDescalco, L.In this thesis we start by considering conditions under which some standard semigroup constructions preserve automaticity. We first consider Rees matrix semigroups over a semigroup, which we call the base, and work on the following questions: (i) If the base is automatic is the Rees matrix semigroup automatic? (ii) If the Rees matrix semigroup is automatic must the base be automatic as well? We also consider similar questions for Bruck-Reilly extensions of monoids and wreath products of semigroups. Then we consider subsemigroups of free products of semigroups and we study conditions that guarantee them to be automatic. Finally we obtain a description of the subsemigroups of the bicyclic monoid that allow us to study some of their properties, which include finite generation, automaticity and finite presentability.Presentations for subsemigroups of groupsCain, Alan Jameshttp://hdl.handle.net/10023/151192018-07-09T23:18:34Z2005-01-01T00:00:00ZThis thesis studies subsemigroups of groups from three perspectives: automatic structures, ordinary semigroup presentations, and Malcev presentaions. [A Malcev presentation is a presentation of a special type for a semigroup that can be embedded into a group. A group-embeddable semigroup is Malcev coherent if all of its finitely generated subsemigroups admit finite Malcev presentations.] The theory of synchronous and asynchronous automatic structures for semigroups is expounded, particularly for group-embeddable semigroups. In particular, automatic semigroups embeddable into groups are shown to inherit many of the pleasant geometric properties of automatic groups. It is proved that group- embeddable automatic semigroups admit finite Malcev presentations, and such presentations can be found effectively. An algorithm is exhibited to test whether an automatic semigroup is a free semigroup. Cancellativity of automatic semigroups is proved to be undecidable. Study is made of several classes of groups: virtually free groups; groups that satisfy semigroup laws (in particular [virtually] nilpotent and [virtually] abelian groups); polycyclic groups; free and direct products of certain groups; and one-relator groups. For each of these classes, the question of Malcev coherence is considered, together with the problems of whether finitely generated subsemigroups are finitely presented or automatic. This study yields closure and containment results regarding the class of Malcev coherent groups. The property of having a finite Malcev presentation is shown to be preserved under finite Rees index extensions and subsemigroups. Other concepts of index are also studied.
2005-01-01T00:00:00ZCain, Alan JamesThis thesis studies subsemigroups of groups from three perspectives: automatic structures, ordinary semigroup presentations, and Malcev presentaions. [A Malcev presentation is a presentation of a special type for a semigroup that can be embedded into a group. A group-embeddable semigroup is Malcev coherent if all of its finitely generated subsemigroups admit finite Malcev presentations.] The theory of synchronous and asynchronous automatic structures for semigroups is expounded, particularly for group-embeddable semigroups. In particular, automatic semigroups embeddable into groups are shown to inherit many of the pleasant geometric properties of automatic groups. It is proved that group- embeddable automatic semigroups admit finite Malcev presentations, and such presentations can be found effectively. An algorithm is exhibited to test whether an automatic semigroup is a free semigroup. Cancellativity of automatic semigroups is proved to be undecidable. Study is made of several classes of groups: virtually free groups; groups that satisfy semigroup laws (in particular [virtually] nilpotent and [virtually] abelian groups); polycyclic groups; free and direct products of certain groups; and one-relator groups. For each of these classes, the question of Malcev coherence is considered, together with the problems of whether finitely generated subsemigroups are finitely presented or automatic. This study yields closure and containment results regarding the class of Malcev coherent groups. The property of having a finite Malcev presentation is shown to be preserved under finite Rees index extensions and subsemigroups. Other concepts of index are also studied.Finiteness conditions of wreath products of semigroups and related properties of diagonal actsThomson, Michael R.http://hdl.handle.net/10023/151172018-09-27T08:48:45Z2001-01-01T00:00:00ZThe purpose of this thesis is to consider finite generation, finite presentability and related properties of restricted wreath products of semigroups. We show that the wreath product Awr B of two monoids is finitely generated if and only if A and B are finitely generated and the action by right multiplication on B of the group of units of B has only finitely many orbits. Also we show that the wreath product AwrB of two non-trivial monoids is finitely presented if and only if A is finitely presented and B is finite. The situation is more complicated in the case of the wreath product SₑwrT of two semigroups with respect to an idempotent e ϵ S. We give a complete characterization for finite generation in the case where T is finite. This result depends on the properties of the diagonal action of S on S x S. We also prove that if this action is not finitely generated, then SₑwrT (with S infinite and T finite) is finitely presented if and only if S x S is finitely presented and T is the direct product of a monoid and a left zero semigroup. In the case where T is infinite, we prove that S must be a monoid in order for SwrT to be finitely generated. We show that the finiteness properties of periodicity and local finiteness are preserved under the wreath product construction. We conclude the thesis with a systematic investigation into the properties of diagonal acts of semigroups, and make some interesting connections between diagonal acts and power semigroups.
2001-01-01T00:00:00ZThomson, Michael R.The purpose of this thesis is to consider finite generation, finite presentability and related properties of restricted wreath products of semigroups. We show that the wreath product Awr B of two monoids is finitely generated if and only if A and B are finitely generated and the action by right multiplication on B of the group of units of B has only finitely many orbits. Also we show that the wreath product AwrB of two non-trivial monoids is finitely presented if and only if A is finitely presented and B is finite. The situation is more complicated in the case of the wreath product SₑwrT of two semigroups with respect to an idempotent e ϵ S. We give a complete characterization for finite generation in the case where T is finite. This result depends on the properties of the diagonal action of S on S x S. We also prove that if this action is not finitely generated, then SₑwrT (with S infinite and T finite) is finitely presented if and only if S x S is finitely presented and T is the direct product of a monoid and a left zero semigroup. In the case where T is infinite, we prove that S must be a monoid in order for SwrT to be finitely generated. We show that the finiteness properties of periodicity and local finiteness are preserved under the wreath product construction. We conclude the thesis with a systematic investigation into the properties of diagonal acts of semigroups, and make some interesting connections between diagonal acts and power semigroups.Todd-Coxeter methods for inverse monoidsCutting, Andrewhttp://hdl.handle.net/10023/150522018-07-26T09:28:12Z2001-01-01T00:00:00ZLet P be the inverse monoid presentation (X|U) for the inverse monoid M, let π be the set of generators for a right congruence on M and let u Є M. Using the work of J. Stephen [15], the current work demonstrates a coset enumeration technique for the R-class Rᵤ similar to the coset enumeration algorithm developed by J. A. Todd and H. S. M. Coxeter for groups. Furthermore it is demonstrated how to test whether Rᵤ = Rᵥ, for u, v Є M and so a technique for enumerating inverse monoids is described. This technique is generalised to enumerate the H-classes of M. The algorithms have been implemented in GAP 3.4.4 [25], and have been used to analyse some examples given in Chapter 6. The thesis concludes by a related discussion of normal forms and automaticity of free inverse semigroups.
2001-01-01T00:00:00ZCutting, AndrewLet P be the inverse monoid presentation (X|U) for the inverse monoid M, let π be the set of generators for a right congruence on M and let u Є M. Using the work of J. Stephen [15], the current work demonstrates a coset enumeration technique for the R-class Rᵤ similar to the coset enumeration algorithm developed by J. A. Todd and H. S. M. Coxeter for groups. Furthermore it is demonstrated how to test whether Rᵤ = Rᵥ, for u, v Є M and so a technique for enumerating inverse monoids is described. This technique is generalised to enumerate the H-classes of M. The algorithms have been implemented in GAP 3.4.4 [25], and have been used to analyse some examples given in Chapter 6. The thesis concludes by a related discussion of normal forms and automaticity of free inverse semigroups.Fibonacci length and efficiency in group presentationsCampbell, Peter P.http://hdl.handle.net/10023/150482018-07-06T23:18:17Z2003-01-01T00:00:00ZIn this thesis we shall consider two topics that are contained in combinatorial group theory and concern properties of finitely presented groups. The first problem we examine is that of calculating the Fibonacci length of certain families of finitely presented groups. In pursuing this we come across ideas and unsolved problems from number theory. We mainly concentrate on finding the Fibonacci length of powers of dihedral groups, certain Fibonacci groups and a family of metacyclic groups. The second problem we investigate in this thesis is finding if the group PGL(2, p), for p a prime, is efficient on a minimal generating set. We find various presentations that define PGL(2,p) or C₂ x PSL(2,p) and direct products of these groups. As in the previous sections we come across number theoretic problems. We also have occasion to use results from tensor theory and homological algebra in order to obtain our results.
2003-01-01T00:00:00ZCampbell, Peter P.In this thesis we shall consider two topics that are contained in combinatorial group theory and concern properties of finitely presented groups. The first problem we examine is that of calculating the Fibonacci length of certain families of finitely presented groups. In pursuing this we come across ideas and unsolved problems from number theory. We mainly concentrate on finding the Fibonacci length of powers of dihedral groups, certain Fibonacci groups and a family of metacyclic groups. The second problem we investigate in this thesis is finding if the group PGL(2, p), for p a prime, is efficient on a minimal generating set. We find various presentations that define PGL(2,p) or C₂ x PSL(2,p) and direct products of these groups. As in the previous sections we come across number theoretic problems. We also have occasion to use results from tensor theory and homological algebra in order to obtain our results.Automated theorem proving for mathematics : real analysis in PVSGottliebsen, Hannehttp://hdl.handle.net/10023/150462018-07-06T23:18:13Z2002-01-01T00:00:00ZComputer Algebra Systems (CASs), such as Maple and Mathematica, are now widely used in both industry and education. In many areas of mathematics they perform well. However, many well-established methods in mathematics, such as definite integration via the fundamental theorem of calculus, rely on analytic side conditions which CASs in general do not support. This thesis presents our work with automatic, formal mathematics using the theorem prover PVS. Based on an existing real analysis library for PVS, we have implemented transcendental functions such as exp, cos, sin, tan and their inverses, and we have provided strategies to prove that a function is continuous at a given point. In general, this is undecidable, but using certain restrictions we can still provide proofs for a large collection of functions. Similarly, we can prove that a function has a limit at a point. We illustrate how the extended library may be used with Maple to provide correct results where Maple's are incorrect. We present a case study of definite integration in the CASs axiom. Maple, Mathematica and Matlab. The case study clearly shows that apart from axiom the systems do not fully check the necessary conditions for the definite integral to exist, thus giving results varying from plain incorrect to correct, even if the latter is difficult to detect without manipulating the result. The extension and correction of the PVS library consists of around 1000 theorems proven by around 18000 PVS proof commands. We also have a test suite of 88 lemmas for the automatic checks for continuity and existence of limits. Thus we have devised and tested automatic computational logic support for the use of formal mathematics in applications, particularly computer algebra.
2002-01-01T00:00:00ZGottliebsen, HanneComputer Algebra Systems (CASs), such as Maple and Mathematica, are now widely used in both industry and education. In many areas of mathematics they perform well. However, many well-established methods in mathematics, such as definite integration via the fundamental theorem of calculus, rely on analytic side conditions which CASs in general do not support. This thesis presents our work with automatic, formal mathematics using the theorem prover PVS. Based on an existing real analysis library for PVS, we have implemented transcendental functions such as exp, cos, sin, tan and their inverses, and we have provided strategies to prove that a function is continuous at a given point. In general, this is undecidable, but using certain restrictions we can still provide proofs for a large collection of functions. Similarly, we can prove that a function has a limit at a point. We illustrate how the extended library may be used with Maple to provide correct results where Maple's are incorrect. We present a case study of definite integration in the CASs axiom. Maple, Mathematica and Matlab. The case study clearly shows that apart from axiom the systems do not fully check the necessary conditions for the definite integral to exist, thus giving results varying from plain incorrect to correct, even if the latter is difficult to detect without manipulating the result. The extension and correction of the PVS library consists of around 1000 theorems proven by around 18000 PVS proof commands. We also have a test suite of 88 lemmas for the automatic checks for continuity and existence of limits. Thus we have devised and tested automatic computational logic support for the use of formal mathematics in applications, particularly computer algebra.Statistical modelling of individual animal movement : an overview of key methods and a discussion of practical challengesPatterson, Toby A.Parton, AlisonLangrock, RolandBlackwell, Paul G.Thomas, LenKing, Ruthhttp://hdl.handle.net/10023/148772019-01-20T01:35:36Z2017-10-01T00:00:00ZWith the influx of complex and detailed tracking data gathered from electronic tracking devices, the analysis of animal movement data has recently emerged as a cottage industry among biostatisticians. New approaches of ever greater complexity are continue to be added to the literature. In this paper, we review what we believe to be some of the most popular and most useful classes of statistical models used to analyse individual animal movement data. Specifically, we consider discrete-time hidden Markov models, more general state-space models and diffusion processes. We argue that these models should be core components in the toolbox for quantitative researchers working on stochastic modelling of individual animal movement. The paper concludes by offering some general observations on the direction of statistical analysis of animal movement. There is a trend in movement ecology towards what are arguably overly complex modelling approaches which are inaccessible to ecologists, unwieldy with large data sets or not based on mainstream statistical practice. Additionally, some analysis methods developed within the ecological community ignore fundamental properties of movement data, potentially leading to misleading conclusions about animal movement. Corresponding approaches, e.g. based on Lévy walk-type models, continue to be popular despite having been largely discredited. We contend that there is a need for an appropriate balance between the extremes of either being overly complex or being overly simplistic, whereby the discipline relies on models of intermediate complexity that are usable by general ecologists, but grounded in well-developed statistical practice and efficient to fit to large data sets.
2017-10-01T00:00:00ZPatterson, Toby A.Parton, AlisonLangrock, RolandBlackwell, Paul G.Thomas, LenKing, RuthWith the influx of complex and detailed tracking data gathered from electronic tracking devices, the analysis of animal movement data has recently emerged as a cottage industry among biostatisticians. New approaches of ever greater complexity are continue to be added to the literature. In this paper, we review what we believe to be some of the most popular and most useful classes of statistical models used to analyse individual animal movement data. Specifically, we consider discrete-time hidden Markov models, more general state-space models and diffusion processes. We argue that these models should be core components in the toolbox for quantitative researchers working on stochastic modelling of individual animal movement. The paper concludes by offering some general observations on the direction of statistical analysis of animal movement. There is a trend in movement ecology towards what are arguably overly complex modelling approaches which are inaccessible to ecologists, unwieldy with large data sets or not based on mainstream statistical practice. Additionally, some analysis methods developed within the ecological community ignore fundamental properties of movement data, potentially leading to misleading conclusions about animal movement. Corresponding approaches, e.g. based on Lévy walk-type models, continue to be popular despite having been largely discredited. We contend that there is a need for an appropriate balance between the extremes of either being overly complex or being overly simplistic, whereby the discipline relies on models of intermediate complexity that are usable by general ecologists, but grounded in well-developed statistical practice and efficient to fit to large data sets.Flux rope formation due to shearing and zipper reconnectionThrelfall, J.Hood, A. W.Priest, E. R.http://hdl.handle.net/10023/148622019-01-06T03:37:10Z2018-06-01T00:00:00ZZipper reconnection has been proposed as a mechanism for creating most of the twist in the flux tubes that are present {prior to} eruptive flares and coronal mass ejections. We have conducted a first numerical experiment on this new regime of reconnection, where two initially untwisted parallel flux tubes are sheared and reconnected to form a large flux rope. We describe the properties of this experiment, including the linkage of magnetic flux between concentrated flux sources at the base of the simulation, the twist of the newly formed flux rope and the conversion of mutual magnetic helicity in the sheared pre-reconnection state into the self-helicity of the newly formed flux rope.
The authors acknowledge the financial support of STFC through the Consolidated grant, ST/N000609/1, to the University of St Andrews. These systems are operated on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The equipment was funded by BIS National E-infrastructure capital grants (ST/K00042X/1 and ST/K001590/1), STFC capital grants (ST/K00087X/1, ST/H008861/1 and ST/H00887X/1) and DiRAC Operations grants (ST/K003267/1 and ST/K00333X/1). The research data supporting this publication can be accessed at http://dx.doi.org/10.17630/dea50f38-7aff-48c3-b090-c689d02dd305.
2018-06-01T00:00:00ZThrelfall, J.Hood, A. W.Priest, E. R.Zipper reconnection has been proposed as a mechanism for creating most of the twist in the flux tubes that are present {prior to} eruptive flares and coronal mass ejections. We have conducted a first numerical experiment on this new regime of reconnection, where two initially untwisted parallel flux tubes are sheared and reconnected to form a large flux rope. We describe the properties of this experiment, including the linkage of magnetic flux between concentrated flux sources at the base of the simulation, the twist of the newly formed flux rope and the conversion of mutual magnetic helicity in the sheared pre-reconnection state into the self-helicity of the newly formed flux rope.Commutativity and free products in Thompson's group VBieniecka, Ewahttp://hdl.handle.net/10023/146522018-11-15T11:34:20Z2018-06-26T00:00:00ZWe broaden the theory of dynamical interpretation, investigate the property of commutativity and explore the subject of subgroups forming free products in Thompson's group V.
We expand Brin's terminology for a revealing pair to an any tree pair. We use it to analyse the dynamical behaviour of an arbitrary tree pair which cannot occur in a revealing pair. Hence, we design a series of algorithms generating Brin's revealing pair from any tree pair, by successively eliminating the undesirable structures. To detect patterns and transitioning between tree pairs, we introduce a new combinatorial object called the chains graph. A newly defined, unique and symmetrical type of a tree pair, called a balanced tree pair, stems from the use of the chains graphs.
The main theorem of Bleak et al. in "Centralizers in the R. Thompson's Group V_n" states the necessary structure of the centraliser of an element of V. We provide a converse to this theorem, by proving that each of the predicted structures is realisable. Hence we obtain a complete classification of centralisers in V. We give an explicit construction of an element of V with prescribed centraliser. The underlying concept is to embed a Cayley graph of a finite group into the flow graph (introduced in Bleak et al.) of the desired element. To reflect the symmetry, we present the resulting element in terms of a balanced tree pair.
The group V is conjectured to be a universal coCF group, which generates interest in studying its subgroups. We develop a better understanding of embeddings into V by providing a necessary and sufficient dynamical condition for two subgroups (not both torsion) to form a free product in V. For this, we use the properties, explored in Bleak and Salazar-Díaz "Free Products in Thompson's Group V", of sets of so--called important points, and the Ping-Pong action induced on them.
2018-06-26T00:00:00ZBieniecka, EwaWe broaden the theory of dynamical interpretation, investigate the property of commutativity and explore the subject of subgroups forming free products in Thompson's group V.
We expand Brin's terminology for a revealing pair to an any tree pair. We use it to analyse the dynamical behaviour of an arbitrary tree pair which cannot occur in a revealing pair. Hence, we design a series of algorithms generating Brin's revealing pair from any tree pair, by successively eliminating the undesirable structures. To detect patterns and transitioning between tree pairs, we introduce a new combinatorial object called the chains graph. A newly defined, unique and symmetrical type of a tree pair, called a balanced tree pair, stems from the use of the chains graphs.
The main theorem of Bleak et al. in "Centralizers in the R. Thompson's Group V_n" states the necessary structure of the centraliser of an element of V. We provide a converse to this theorem, by proving that each of the predicted structures is realisable. Hence we obtain a complete classification of centralisers in V. We give an explicit construction of an element of V with prescribed centraliser. The underlying concept is to embed a Cayley graph of a finite group into the flow graph (introduced in Bleak et al.) of the desired element. To reflect the symmetry, we present the resulting element in terms of a balanced tree pair.
The group V is conjectured to be a universal coCF group, which generates interest in studying its subgroups. We develop a better understanding of embeddings into V by providing a necessary and sufficient dynamical condition for two subgroups (not both torsion) to form a free product in V. For this, we use the properties, explored in Bleak and Salazar-Díaz "Free Products in Thompson's Group V", of sets of so--called important points, and the Ping-Pong action induced on them.The dynamic topology of the solar corona : mapping the Sun’s three dimensional magnetic skeletonWilliams, Benjamin Matthewhttp://hdl.handle.net/10023/146372018-06-28T09:38:17Z2018-06-26T00:00:00ZObservations of the surface of the Sun reveal multi-scaled, mixed magnetic features
that carpet the entire solar surface. Not surprisingly, the global magnetic fields
extrapolated from these observations are highly complex. This thesis explores the
topology of the Sun’s global coronal magnetic fields. The magnetic skeleton of a
magnetic field provides us with a way of examining the magnetic field and
quantifying its complexity.
Using specialised codes to find the magnetic skeletons which were written during the
course of this work, we first examine potential field extrapolations of the global solar
coronal magnetic field determined from observed synoptic magnetograms from the
Heliospheric Magnetic Imager on the Solar Dynamics Observatory. The resolution of
the PFSS models is found to be very important for discovering the true nature of the
global magnetic skeleton. By increasing the maximum number of harmonics used in
the potential field extrapolations and, therefore, the grid resolution, 60 times more
null points may be found in the coronal magnetic field. These high resolution fields
also have a large global separator network which connects the coronal magnetic field
over large distances and involves between 40 % and 60 % of all the null points in the
solar atmosphere. This global separator network exists at both solar minimum and
solar maximum and has separators that reach high into the solar atmosphere
(> 1R☉) even though they connect null points close to the solar surface.
These potential field extrapolations are then compared with magnetohydrostatic
(MHS) extrapolations of the coronal magnetic field which also provide us with
information about the plasma in the corona. With a small component of electric
current density in the direction perpendicular to the radial direction, these MHS fields
are found to have a plasma beta and pressure typical of the corona. As this small
component of electric current density grows, the heliospheric current sheet is warped
significantly and the magnetic field, plasma beta and pressure become unphysical.
Torsional spine reconnection is also studied local to a single null point. First using a
dynamical relaxation of a spiral null point under non-resistive magnetohydrodynamics
(MHD) to a MHS equilibrium is form in which a current layer has built up around
the spine lines. Then the reconnection under resistive MHD in this current sheet is
studied. The current about the spine lines is dissipated and the magnetic energy is
mainly converted into heat directly as the field lines untwist about the spine line.
2018-06-26T00:00:00ZWilliams, Benjamin MatthewObservations of the surface of the Sun reveal multi-scaled, mixed magnetic features
that carpet the entire solar surface. Not surprisingly, the global magnetic fields
extrapolated from these observations are highly complex. This thesis explores the
topology of the Sun’s global coronal magnetic fields. The magnetic skeleton of a
magnetic field provides us with a way of examining the magnetic field and
quantifying its complexity.
Using specialised codes to find the magnetic skeletons which were written during the
course of this work, we first examine potential field extrapolations of the global solar
coronal magnetic field determined from observed synoptic magnetograms from the
Heliospheric Magnetic Imager on the Solar Dynamics Observatory. The resolution of
the PFSS models is found to be very important for discovering the true nature of the
global magnetic skeleton. By increasing the maximum number of harmonics used in
the potential field extrapolations and, therefore, the grid resolution, 60 times more
null points may be found in the coronal magnetic field. These high resolution fields
also have a large global separator network which connects the coronal magnetic field
over large distances and involves between 40 % and 60 % of all the null points in the
solar atmosphere. This global separator network exists at both solar minimum and
solar maximum and has separators that reach high into the solar atmosphere
(> 1R☉) even though they connect null points close to the solar surface.
These potential field extrapolations are then compared with magnetohydrostatic
(MHS) extrapolations of the coronal magnetic field which also provide us with
information about the plasma in the corona. With a small component of electric
current density in the direction perpendicular to the radial direction, these MHS fields
are found to have a plasma beta and pressure typical of the corona. As this small
component of electric current density grows, the heliospheric current sheet is warped
significantly and the magnetic field, plasma beta and pressure become unphysical.
Torsional spine reconnection is also studied local to a single null point. First using a
dynamical relaxation of a spiral null point under non-resistive magnetohydrodynamics
(MHD) to a MHS equilibrium is form in which a current layer has built up around
the spine lines. Then the reconnection under resistive MHD in this current sheet is
studied. The current about the spine lines is dissipated and the magnetic energy is
mainly converted into heat directly as the field lines untwist about the spine line.Modelling chromospheric evaporation in response to coronal heatingJohnston, Craig Davidhttp://hdl.handle.net/10023/146302018-06-28T09:50:48Z2018-06-26T00:00:00ZThis thesis presents a new computationally efficient method for modelling the response of the solar corona to the release of energy. During impulsive heating events, the coronal temperature increases which leads to a downward heat flux into the transition region (TR). The plasma is unable to radiate this excess conductive heating and so the gas pressure increases locally. The resulting pressure gradient drives an upflow of dense material, creating an increase in the coronal density. This density increase is often called chromospheric evaporation. A process which is highly sensitive to the TR resolution in numerical simulations. If the resolution is not adequate, then the downward heat flux jumps over the TR and deposits the heat in the chromosphere, where it is radiated away. The outcome is that with an under-resolved TR, major errors occur in simulating the coronal density evolution. We address this problem by treating the lower transition region as a discontinuity that responds to changing coronal conditions through the imposition of a jump condition that is derived from an integrated form of energy conservation. In this thesis, it is shown that this method permits fast and accurate numerical solutions in both one-dimensional and multi-dimensional simulations. By modelling the TR with this appropriate jump condition, we remove the influence of poor numerical resolution and obtain the correct evaporative response to coronal heating, even when using resolutions that are compatible with multi-dimensional magnetohydrodynamic simulations.
2018-06-26T00:00:00ZJohnston, Craig DavidThis thesis presents a new computationally efficient method for modelling the response of the solar corona to the release of energy. During impulsive heating events, the coronal temperature increases which leads to a downward heat flux into the transition region (TR). The plasma is unable to radiate this excess conductive heating and so the gas pressure increases locally. The resulting pressure gradient drives an upflow of dense material, creating an increase in the coronal density. This density increase is often called chromospheric evaporation. A process which is highly sensitive to the TR resolution in numerical simulations. If the resolution is not adequate, then the downward heat flux jumps over the TR and deposits the heat in the chromosphere, where it is radiated away. The outcome is that with an under-resolved TR, major errors occur in simulating the coronal density evolution. We address this problem by treating the lower transition region as a discontinuity that responds to changing coronal conditions through the imposition of a jump condition that is derived from an integrated form of energy conservation. In this thesis, it is shown that this method permits fast and accurate numerical solutions in both one-dimensional and multi-dimensional simulations. By modelling the TR with this appropriate jump condition, we remove the influence of poor numerical resolution and obtain the correct evaporative response to coronal heating, even when using resolutions that are compatible with multi-dimensional magnetohydrodynamic simulations.Origin and ion charge state evolution of solar wind transients during 4 - 7 August 2011Rodkin, D.Goryaev, F.Pagano, P.Gibb, G.Slemzin, V.Shugay, Y.Veselovsky, I.Mackay, D. H.http://hdl.handle.net/10023/144322019-01-06T03:18:36Z2017-07-01T00:00:00ZWe present a study of the complex event consisting of several solar wind transients detected by the Advanced Composition Explorer (ACE) on 4 - 7 August 2011, which caused a geomagnetic storm with Dst=-110 nT. The supposed coronal sources, three flares and coronal mass ejections (CMEs), occurred on 2 - 4 August 2011 in active region (AR) 11261. To investigate the solar origin and formation of these transients, we study the kinematic and thermodynamic properties of the expanding coronal structures using the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV images and differential emission measure (DEM) diagnostics. The Helioseismic and Magnetic Imager (HMI) magnetic field maps were used as the input data for the 3D magnetohydrodynamic (MHD) model to describe the flux rope ejection (Pagano, Mackay, and Poedts, 2013b). We characterize the early phase of the flux rope ejection in the corona, where the usual three-component CME structure formed. The fluxrope was ejected with a speed of about 200 km s-1 to the height of 0.25 R⊙. The kinematics of the modeled CME front agrees well with the Solar Terrestrial Relations Observatory (STEREO) EUV measurements. Using the results of the plasma diagnostics and MHD modeling, we calculate the ion charge ratios of carbon and oxygen as well as the mean charge state of iron ions of the 2 August 2011 CME, taking into account the processes of heating, cooling, expansion, ionization, and recombination of the moving plasma in the corona up to the frozen-in region. We estimate a probable heating rate of the CME plasma in the low corona by matching the calculated ion composition parameters of the CME with those measured in situ for the solar wind transients. We also consider the similarities and discrepancies between the results of the MHD simulation and the observations.
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 647214). The computational work for this article was carried out on the joint STFC and SFC (SRIF) funded clusters at the University of St Andrews (Scotland, UK). The work is partially supported by RFBR grants 17-02-00787, 14-02-00945 and the P7 Program of the Russian Academy of Sciences.
2017-07-01T00:00:00ZRodkin, D.Goryaev, F.Pagano, P.Gibb, G.Slemzin, V.Shugay, Y.Veselovsky, I.Mackay, D. H.We present a study of the complex event consisting of several solar wind transients detected by the Advanced Composition Explorer (ACE) on 4 - 7 August 2011, which caused a geomagnetic storm with Dst=-110 nT. The supposed coronal sources, three flares and coronal mass ejections (CMEs), occurred on 2 - 4 August 2011 in active region (AR) 11261. To investigate the solar origin and formation of these transients, we study the kinematic and thermodynamic properties of the expanding coronal structures using the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV images and differential emission measure (DEM) diagnostics. The Helioseismic and Magnetic Imager (HMI) magnetic field maps were used as the input data for the 3D magnetohydrodynamic (MHD) model to describe the flux rope ejection (Pagano, Mackay, and Poedts, 2013b). We characterize the early phase of the flux rope ejection in the corona, where the usual three-component CME structure formed. The fluxrope was ejected with a speed of about 200 km s-1 to the height of 0.25 R⊙. The kinematics of the modeled CME front agrees well with the Solar Terrestrial Relations Observatory (STEREO) EUV measurements. Using the results of the plasma diagnostics and MHD modeling, we calculate the ion charge ratios of carbon and oxygen as well as the mean charge state of iron ions of the 2 August 2011 CME, taking into account the processes of heating, cooling, expansion, ionization, and recombination of the moving plasma in the corona up to the frozen-in region. We estimate a probable heating rate of the CME plasma in the low corona by matching the calculated ion composition parameters of the CME with those measured in situ for the solar wind transients. We also consider the similarities and discrepancies between the results of the MHD simulation and the observations.Computational modelling of cancer development and growth : modelling at multiple scales and multiscale modellingSzymanska, ZuzannaCytowski, MaciejMitchell, ElaineMacnamara, Cicely K.Chaplain, Mark A. J.http://hdl.handle.net/10023/143642019-01-06T03:16:53Z2018-05-01T00:00:00ZIn this paper, we present two mathematical models related to different aspects and scales of cancer growth. The first model is a stochastic spatiotemporal model of both a synthetic gene regulatory network (the example of a three-gene repressilator is given) and an actual gene regulatory network, the NF- κB pathway. The second model is a force-based individual-based model of the development of a solid avascular tumour with specific application to tumour cords, i.e. a mass of cancer cells growing around a central blood vessel. In each case, we compare our computational simulation results with experimental data. In the final discussion section, we outline how to take the work forward through the development of a multiscale model focussed at the cell level. This would incorporate key intracellular signalling pathways associated with cancer within each cell (e.g. p53–Mdm2, NF- κB) and through the use of high-performance computing be capable of simulating up to 109 cells, i.e. the tissue scale. In this way, mathematical models at multiple scales would be combined to formulate a multiscale computational model.
MAJC and CKM gratefully acknowledge support of EPSRC grant no. EP/N014642/1 (EPSRC Centre for Multiscale Soft Tissue Mechanics – With Application to Heart & Cancer).
2018-05-01T00:00:00ZSzymanska, ZuzannaCytowski, MaciejMitchell, ElaineMacnamara, Cicely K.Chaplain, Mark A. J.In this paper, we present two mathematical models related to different aspects and scales of cancer growth. The first model is a stochastic spatiotemporal model of both a synthetic gene regulatory network (the example of a three-gene repressilator is given) and an actual gene regulatory network, the NF- κB pathway. The second model is a force-based individual-based model of the development of a solid avascular tumour with specific application to tumour cords, i.e. a mass of cancer cells growing around a central blood vessel. In each case, we compare our computational simulation results with experimental data. In the final discussion section, we outline how to take the work forward through the development of a multiscale model focussed at the cell level. This would incorporate key intracellular signalling pathways associated with cancer within each cell (e.g. p53–Mdm2, NF- κB) and through the use of high-performance computing be capable of simulating up to 109 cells, i.e. the tissue scale. In this way, mathematical models at multiple scales would be combined to formulate a multiscale computational model.Photometry of star clustersEvans, Thomas Harry Hope Lloydhttp://hdl.handle.net/10023/142832018-06-19T23:17:59Z1968-01-01T00:00:00ZThe suitability of the Cassegrain Schmidt telescopes at St. Andrews University Observatory for the measurement of stellar magnitudes and colours by in-focus multicolour photography has been examined. A major requirement is that the photographic plate should coincide with the focal surface. Thermal effects in the Scott Lang Telescope and optical and mechanical problems in the James Gregory Telescope cause difficulty in, attaining this. These difficulties have been overcome in the case of the Scott Lang Telescope but no certain method for focussing the James Gregory Telescope was found. The photometric field limited by field error, is approximately one degree in diameter in each case. Colour equations between the instrumental and standard B, V systems depend on magnitude and, in the case of the Scott Lang Telescope, on exposure time as well. The methods used to measure UBV magnitudes and colours with the Radcliffe 74-inch reflector are described and the accuracy of the results discussed. Magnitudes and colours of stars brighter than V - 15.5 in the open cluster IC 2581 have been measured, together with MK spectral types for a few of the brighter stars. The interstellar absorption provides a criterion for the recognition of cluster members. A discrepancy between the shape of the cluster main sequence and that of the zero age main sequence is attributed to an error in the derivation of the standard zero age main sequence. The cluster is found to be at a distance of 2500 parsecs and may form part of the Carinae complex. The positions of the brightest stars in the colour magnitude diagram are discussed in the light of modern theories of stellar evolution and an age of approximately 10 million years is deduced. The colour magnitude diagram of the open cluster NGC 6383 has been obtained for stars brighter than V - 18.1; the limiting magnitudes in B and U are 19.7 and 17.9, respectively. MK spectral types have permitted the cluster membership of several bright B stars to be established; some stars of later type are non-members. The observation of this cluster are more complete than for most young clusters studied to date, but the poorness of the cluster and the unfavourable distribution of interstellar absorption with distance make it impossible to be certain of the membership of stars fainter than V = 13. The lack of stars fainter than V = 12.8 on the zero age main sequence indicates a contraction age of 5 million years. The distance is 1300 parsecs, like those of other young groups in the vicinity. The dense dust clouds which divide the Milky Way in Scorpius are immediately beyond this. Several faint variable stars may be of the T Tauri type.
1968-01-01T00:00:00ZEvans, Thomas Harry Hope LloydThe suitability of the Cassegrain Schmidt telescopes at St. Andrews University Observatory for the measurement of stellar magnitudes and colours by in-focus multicolour photography has been examined. A major requirement is that the photographic plate should coincide with the focal surface. Thermal effects in the Scott Lang Telescope and optical and mechanical problems in the James Gregory Telescope cause difficulty in, attaining this. These difficulties have been overcome in the case of the Scott Lang Telescope but no certain method for focussing the James Gregory Telescope was found. The photometric field limited by field error, is approximately one degree in diameter in each case. Colour equations between the instrumental and standard B, V systems depend on magnitude and, in the case of the Scott Lang Telescope, on exposure time as well. The methods used to measure UBV magnitudes and colours with the Radcliffe 74-inch reflector are described and the accuracy of the results discussed. Magnitudes and colours of stars brighter than V - 15.5 in the open cluster IC 2581 have been measured, together with MK spectral types for a few of the brighter stars. The interstellar absorption provides a criterion for the recognition of cluster members. A discrepancy between the shape of the cluster main sequence and that of the zero age main sequence is attributed to an error in the derivation of the standard zero age main sequence. The cluster is found to be at a distance of 2500 parsecs and may form part of the Carinae complex. The positions of the brightest stars in the colour magnitude diagram are discussed in the light of modern theories of stellar evolution and an age of approximately 10 million years is deduced. The colour magnitude diagram of the open cluster NGC 6383 has been obtained for stars brighter than V - 18.1; the limiting magnitudes in B and U are 19.7 and 17.9, respectively. MK spectral types have permitted the cluster membership of several bright B stars to be established; some stars of later type are non-members. The observation of this cluster are more complete than for most young clusters studied to date, but the poorness of the cluster and the unfavourable distribution of interstellar absorption with distance make it impossible to be certain of the membership of stars fainter than V = 13. The lack of stars fainter than V = 12.8 on the zero age main sequence indicates a contraction age of 5 million years. The distance is 1300 parsecs, like those of other young groups in the vicinity. The dense dust clouds which divide the Milky Way in Scorpius are immediately beyond this. Several faint variable stars may be of the T Tauri type.Solar intense magnetic fieldsWebb, Andrew Roberthttp://hdl.handle.net/10023/142772018-06-19T23:17:54Z1980-01-01T00:00:00ZThe nature of motions in intense magnetic fields is investigated. For a flux tube in a uniform atmosphere a dispersion relation is derived for the modes of vibration and analytic approximations are obtained for a slender tube. In a stratified atmosphere an expansion procedure is used to derive an equation for the vertical velocity perturbation. The behaviour of motions within the flux tube is shown to depend upon a transition frequency 𝜔[sub]v such that vertically propagating waves are possible only for frequencies greater than 𝜔[sub]v. Also, the nature of convective instability in a slender magnetic flux tube is explored. A sufficient condition for stability is derived for the case of an arbitrary temperature profile in the external medium. For a tube of infinite depth, with a uniform-temperature gradient inside the tube equal to that in the exterior, a necessary and sufficient condition for convective stability to occur inside the tube is derived. Under the assumptions of the model, intense flux tubes are convectively stable if sufficiently shallow (with depths 1 - 2 x 10³ km or less). Tubes that extend deeper into the convection zone are potentially (convectively) unstable, but may be stabilised for sufficiently strong magnetic fields. Radiative damping of waves is important in the upper photosphere and the effect of radiative relaxation on the propagation of waves in an intense flux tube is examined both for a uniform and stratified atmosphere. The cut-off frequency is generalized to include the effects of radiative relaxation. The phase-shift between velocity oscillations at two different levels and the phase difference between temperature and velocity perturbations are derived and compared with the available observations. Finally, the consequences of the observed steady downflow are discussed.
1980-01-01T00:00:00ZWebb, Andrew RobertThe nature of motions in intense magnetic fields is investigated. For a flux tube in a uniform atmosphere a dispersion relation is derived for the modes of vibration and analytic approximations are obtained for a slender tube. In a stratified atmosphere an expansion procedure is used to derive an equation for the vertical velocity perturbation. The behaviour of motions within the flux tube is shown to depend upon a transition frequency 𝜔[sub]v such that vertically propagating waves are possible only for frequencies greater than 𝜔[sub]v. Also, the nature of convective instability in a slender magnetic flux tube is explored. A sufficient condition for stability is derived for the case of an arbitrary temperature profile in the external medium. For a tube of infinite depth, with a uniform-temperature gradient inside the tube equal to that in the exterior, a necessary and sufficient condition for convective stability to occur inside the tube is derived. Under the assumptions of the model, intense flux tubes are convectively stable if sufficiently shallow (with depths 1 - 2 x 10³ km or less). Tubes that extend deeper into the convection zone are potentially (convectively) unstable, but may be stabilised for sufficiently strong magnetic fields. Radiative damping of waves is important in the upper photosphere and the effect of radiative relaxation on the propagation of waves in an intense flux tube is examined both for a uniform and stratified atmosphere. The cut-off frequency is generalized to include the effects of radiative relaxation. The phase-shift between velocity oscillations at two different levels and the phase difference between temperature and velocity perturbations are derived and compared with the available observations. Finally, the consequences of the observed steady downflow are discussed.Nonlinear stability of flows over rigid and flexible boundariesThomas, Michael Dhttp://hdl.handle.net/10023/142732018-06-19T23:17:42Z1990-01-01T00:00:00ZThis work assesses the importance of nonlinearity in the stability of flows over compliant and rigid walls, and comprises three main parts. The first part considers inviscid flow with a free surface over a flexible boundary. The dispersion relation is obtained, and the conditions for linear instability investigated. The linear dispersion relation is then used to show that the conditions for nonlinear three-wave resonance are often met. In some circumstances, the resonance may be of 'explosive' sort, involving waves of opposite energy sign; but non-explosive resonant configurations are most common. Next, the wave- amplitude evolution equations for three-wave resonance are derived, firstly by a 'direct' approach, and then via a variational (averaged Lagrangian) method. Results agree with those of Case & Chiu (1977) for capillary-gravity waves, and Craik & Adam (1979), for three-layer fluid flow, on taking the appropriate limits. We also consider a nonlinear model for the flexible boundary. In the second part, stability of Blasius flow over a compliant surface is studied. This extension of rigid-wall work of Craik (1971) and Hendriks (appendix to Usher & Craik 1975) determines the quadratic interaction coefficients of three-wave resonance, and complements the linear analysis of Carpenter & Garrad (1985, 1986) and others. First, the linear eigenvalue spectrum is investigated for various values of the wall parameters. Then, resonant triads are located and the quadratic interaction coefficients determined numerically. By way of introduction some rigid-wall results are also presented, extending those of Hendriks.
1990-01-01T00:00:00ZThomas, Michael DThis work assesses the importance of nonlinearity in the stability of flows over compliant and rigid walls, and comprises three main parts. The first part considers inviscid flow with a free surface over a flexible boundary. The dispersion relation is obtained, and the conditions for linear instability investigated. The linear dispersion relation is then used to show that the conditions for nonlinear three-wave resonance are often met. In some circumstances, the resonance may be of 'explosive' sort, involving waves of opposite energy sign; but non-explosive resonant configurations are most common. Next, the wave- amplitude evolution equations for three-wave resonance are derived, firstly by a 'direct' approach, and then via a variational (averaged Lagrangian) method. Results agree with those of Case & Chiu (1977) for capillary-gravity waves, and Craik & Adam (1979), for three-layer fluid flow, on taking the appropriate limits. We also consider a nonlinear model for the flexible boundary. In the second part, stability of Blasius flow over a compliant surface is studied. This extension of rigid-wall work of Craik (1971) and Hendriks (appendix to Usher & Craik 1975) determines the quadratic interaction coefficients of three-wave resonance, and complements the linear analysis of Carpenter & Garrad (1985, 1986) and others. First, the linear eigenvalue spectrum is investigated for various values of the wall parameters. Then, resonant triads are located and the quadratic interaction coefficients determined numerically. By way of introduction some rigid-wall results are also presented, extending those of Hendriks.Time dependent heating of the solar coronaWalsh, Robert Williamhttp://hdl.handle.net/10023/142672018-06-19T23:17:33Z1996-01-01T00:00:00ZThe problem of how the Sun's corona is heated is of central importance in Solar Physics research. In this thesis, a model is constructed of a typical coronal magnetic loop in order to investigate the response of coronal plasma to a time-dependent heating source. It is not the aim of the research to study in detail a particular heating mechanism but rather to understand the important features arising from time-dependent heating in general. A time-varying energy input into the coronal loop is required because it is likely that none of the suggested theoretical heating methods can provide a constant supply of heat to the corona. The magnetic field is taken to be strong enough that the loop dynamics reduce to a one-dimensional problem along the field. In addition, it is assumed that the radiative timescale in the corona is much longer than the sound travel time and thus, the plasma evolves isobarically. The thermal equilibria profiles along the coronal loop are then investigated for a simplified form of the optically thin radiation. Initially, a heating function that displays a regular, sinusoidal variation in time is introduced and it is found that there is a critical heating frequency above which a hot coronal loop solution can be maintained and below which the plasma temperature cools to chromospheric values. Pulse heating and the deposition of random-sized energy quanta in a loop are also investigated. An evaluation of the isobaric assumption to the corona is presented by allowing sound waves to propagate back and forth along the loop. It is found that the system can exhibit isobaric-like behaviour provided the acoustic timescale is short enough. Possible extensions of the developed loop model are discussed as well as the implications of time-dependent heating upon observations from the SOHO satellite.
1996-01-01T00:00:00ZWalsh, Robert WilliamThe problem of how the Sun's corona is heated is of central importance in Solar Physics research. In this thesis, a model is constructed of a typical coronal magnetic loop in order to investigate the response of coronal plasma to a time-dependent heating source. It is not the aim of the research to study in detail a particular heating mechanism but rather to understand the important features arising from time-dependent heating in general. A time-varying energy input into the coronal loop is required because it is likely that none of the suggested theoretical heating methods can provide a constant supply of heat to the corona. The magnetic field is taken to be strong enough that the loop dynamics reduce to a one-dimensional problem along the field. In addition, it is assumed that the radiative timescale in the corona is much longer than the sound travel time and thus, the plasma evolves isobarically. The thermal equilibria profiles along the coronal loop are then investigated for a simplified form of the optically thin radiation. Initially, a heating function that displays a regular, sinusoidal variation in time is introduced and it is found that there is a critical heating frequency above which a hot coronal loop solution can be maintained and below which the plasma temperature cools to chromospheric values. Pulse heating and the deposition of random-sized energy quanta in a loop are also investigated. An evaluation of the isobaric assumption to the corona is presented by allowing sound waves to propagate back and forth along the loop. It is found that the system can exhibit isobaric-like behaviour provided the acoustic timescale is short enough. Possible extensions of the developed loop model are discussed as well as the implications of time-dependent heating upon observations from the SOHO satellite.Time-dependent MHD wave coupling in non-uniform mediaMann, Ian R.http://hdl.handle.net/10023/142642018-06-19T23:17:43Z1996-01-01T00:00:00ZThis thesis studies the time dependent evolution of MHD waves in cold, fully compressible non-uniform plasmas. We used a 1-D box model (e.g., Southwood (1974)) to study wave mode coupling, and concentrate upon developing an understanding of the underlying physics that governs waves in the Earth's magnetosphere. We begin by discussing the form of the (often singular) governing eigenmodes of the system, and subsequently use these eigenmodes as a basis with which to construct the solution to a variety of initial value problems. We consider a detailed analysis of both the widths and the internal length scales developed by cavity mode driven held line resonances (FLRs), and compare our results to observations presented in the literature. We find that (especially asymptotically in time) the coupled waves derive their dominant characteristics from the form of undriven decoupled toroidal Alfvén eigenmodes. Ideal numerical solutions show that fine spatial scales are developed across the background magnetic field, and we demonstrate that this is accurately estimated as the decoupled phase mixing length
L[sub]p[sub]h = 2π/𝜔ⁱ[sub]A = d 𝜔[sub]A/dx
We also discuss the likely ionospheric and kinetic modifications to our theory. Later, we consider the evolution of poloidal Alfvén waves having large azimuthal wavenumber (𝜆). We find that the 𝜆 → ∞ decoupled poloidal Alfvén wave evaluation (Dungey, 1967) is modified for finite 𝜆 lambda, approaching decoupled toroidal field line oscillations for large t. We define a poloidal lifetime 𝛵, when toroidal and poloidal displacements become equal, and demonstrate that this is when the phase mixing length is equal to 2pi/lambda. We examine numerically the poloidal Alfvén wave evolution for 𝜆 ≫ k[sub]z, and k[sub]≳ lambda, when k[sub]x(x,t = 0) ≪ lambda or k[sub]z. We interpret the lambda ≪ kz results (applicable to the Earth's magnetosphere) in the context of poloidal Alfvén wave observations, and compare our study to the numerical analysis of Ding et al. (1995). We conclude the thesis by undertaking an asymptotic derivation of the large 𝜆 solutions by using the method of multiple time scales. We find our analytic solutions are in excellent agreement with those determined numerically. A central result of the thesis is the importance and dominance of the phase mixing length for time dependent solutions, irrespective of the value of 𝜆.
1996-01-01T00:00:00ZMann, Ian R.This thesis studies the time dependent evolution of MHD waves in cold, fully compressible non-uniform plasmas. We used a 1-D box model (e.g., Southwood (1974)) to study wave mode coupling, and concentrate upon developing an understanding of the underlying physics that governs waves in the Earth's magnetosphere. We begin by discussing the form of the (often singular) governing eigenmodes of the system, and subsequently use these eigenmodes as a basis with which to construct the solution to a variety of initial value problems. We consider a detailed analysis of both the widths and the internal length scales developed by cavity mode driven held line resonances (FLRs), and compare our results to observations presented in the literature. We find that (especially asymptotically in time) the coupled waves derive their dominant characteristics from the form of undriven decoupled toroidal Alfvén eigenmodes. Ideal numerical solutions show that fine spatial scales are developed across the background magnetic field, and we demonstrate that this is accurately estimated as the decoupled phase mixing length
L[sub]p[sub]h = 2π/𝜔ⁱ[sub]A = d 𝜔[sub]A/dx
We also discuss the likely ionospheric and kinetic modifications to our theory. Later, we consider the evolution of poloidal Alfvén waves having large azimuthal wavenumber (𝜆). We find that the 𝜆 → ∞ decoupled poloidal Alfvén wave evaluation (Dungey, 1967) is modified for finite 𝜆 lambda, approaching decoupled toroidal field line oscillations for large t. We define a poloidal lifetime 𝛵, when toroidal and poloidal displacements become equal, and demonstrate that this is when the phase mixing length is equal to 2pi/lambda. We examine numerically the poloidal Alfvén wave evolution for 𝜆 ≫ k[sub]z, and k[sub]≳ lambda, when k[sub]x(x,t = 0) ≪ lambda or k[sub]z. We interpret the lambda ≪ kz results (applicable to the Earth's magnetosphere) in the context of poloidal Alfvén wave observations, and compare our study to the numerical analysis of Ding et al. (1995). We conclude the thesis by undertaking an asymptotic derivation of the large 𝜆 solutions by using the method of multiple time scales. We find our analytic solutions are in excellent agreement with those determined numerically. A central result of the thesis is the importance and dominance of the phase mixing length for time dependent solutions, irrespective of the value of 𝜆.Magnetic neutral points and nonuniform reconnectionStrachan, N. R.http://hdl.handle.net/10023/142502018-06-19T23:17:17Z1994-01-01T00:00:00ZEver since the first recorded observation of a solar flare in September 1859, it has been a key question - for physics as a whole and for astrophsics in particular - to ask what mechanism lies behind the sudden, violent release of energy from the sun. It has become increasingly apparent that the complex structure of the solar magnetic field lies at the heart of the answer. The process of magnetic reconnection has, over the years, become the accepted explanation by which magnetic energy can be released on both large and small scales in astrophysical and laboratory plasmas. The results of reconnection can be seen, for instance, in star formation, solar flares and the earth's aurorae; indeed the 1859 flare was followed by exceptional auroral activity. The mechanism of magnetic reconnection was first postulated by Giovanelli (1947) as a way of releasing the magnetic energy stored in the Sun. He, and later Dungey (1953), realised that the behaviour of the plasma in the vicinity of a magnetic neutral or null point, where the field disappears, is quite different from other regions of space. In this thesis the nature of magnetic neutral points and their role in the process of reconnection is investigated. Firstly, a general classification of magnetic neutral points is presented. The chapter includes equilibrium and steady-state solutions for two-dimensional magnetic neutral points. The differences in the field behaviour close to each type of neutral point are explained and criteria for the existence of steady-state solutions and equilibria involving pressure balance are presented. In the last section, a self-similar solution for a collapsed X-point is explored. The X-point necessarily becomes cusp-like in nature if shearing is applied in the ignorable direction. Two reconnection models are considered. The first is an extension of the Priest-Lee model (1990). It incorporates large pressure gradients in the inflow corresponding to the Forbes-Priest Almost-Uniform Model. The investigation includes both analytical and numerical solutions and a study of the separatrix jet. In the numerical study, current spikes are found at the end of the current sheets and a much increased reconnection rate is found analytically in the extreme flux file-up limit. The second reconnection model presented is also based on the Priest-Lee configuration. A uniform field is imposed on the basic structure producing a cusp-point with a non-zero field strength as the neutral point is approached from above. This results in the removal of the singularity in the flow above the separatrix. A non-singular solution is found analytically for a double-cusp. A much larger reconnection rate is found and a numerical solution is presented.
1994-01-01T00:00:00ZStrachan, N. R.Ever since the first recorded observation of a solar flare in September 1859, it has been a key question - for physics as a whole and for astrophsics in particular - to ask what mechanism lies behind the sudden, violent release of energy from the sun. It has become increasingly apparent that the complex structure of the solar magnetic field lies at the heart of the answer. The process of magnetic reconnection has, over the years, become the accepted explanation by which magnetic energy can be released on both large and small scales in astrophysical and laboratory plasmas. The results of reconnection can be seen, for instance, in star formation, solar flares and the earth's aurorae; indeed the 1859 flare was followed by exceptional auroral activity. The mechanism of magnetic reconnection was first postulated by Giovanelli (1947) as a way of releasing the magnetic energy stored in the Sun. He, and later Dungey (1953), realised that the behaviour of the plasma in the vicinity of a magnetic neutral or null point, where the field disappears, is quite different from other regions of space. In this thesis the nature of magnetic neutral points and their role in the process of reconnection is investigated. Firstly, a general classification of magnetic neutral points is presented. The chapter includes equilibrium and steady-state solutions for two-dimensional magnetic neutral points. The differences in the field behaviour close to each type of neutral point are explained and criteria for the existence of steady-state solutions and equilibria involving pressure balance are presented. In the last section, a self-similar solution for a collapsed X-point is explored. The X-point necessarily becomes cusp-like in nature if shearing is applied in the ignorable direction. Two reconnection models are considered. The first is an extension of the Priest-Lee model (1990). It incorporates large pressure gradients in the inflow corresponding to the Forbes-Priest Almost-Uniform Model. The investigation includes both analytical and numerical solutions and a study of the separatrix jet. In the numerical study, current spikes are found at the end of the current sheets and a much increased reconnection rate is found analytically in the extreme flux file-up limit. The second reconnection model presented is also based on the Priest-Lee configuration. A uniform field is imposed on the basic structure producing a cusp-point with a non-zero field strength as the neutral point is approached from above. This results in the removal of the singularity in the flow above the separatrix. A non-singular solution is found analytically for a double-cusp. A much larger reconnection rate is found and a numerical solution is presented.Aspects of the MHD stability of coronal and laboratory plasmasClifford, Leo J.http://hdl.handle.net/10023/142482018-06-19T23:16:39Z1993-01-01T00:00:00ZThe magnetohydrodynamic (MHD) model is a simple mathematical model that treats a plasma as a perfectly conducting fluid acted upon by magnetic and pressure-driven forces. Many instabilities in plasmas can be predicted using this model. In this Thesis, aspects of the linear stability of solar and laboratory plasmas are studied using the MHD model. Firstly, we investigate the thermal instability of coronal plasmas with line-tied magnetic fields and with anisotropical heat conduction, using an analytical analysis which concentrates on isobaric perturbations, and a time-dependent numerical code. We find that including perpendicular thermal conduction means that condensations are restricted to a narrow layer around the region where the local isobaric growth rate is largest and that, while the growth rate of the thermal mode is largely unaffected by perpendicular thermal conduction, this may be an important factor in determining the lengthscale for the width of condensations. Secondly, the effect of a finitely conducting wall on the linear stability of Spheromak and Reversed Field Finch equilibria is investigated. We find growth rates for the modes that are present because of the finite resistivity of the wall, which grow proportionally to the "long" time constant of the wall. Finally, we apply a tractable method, derived by De Bruyne (1990), for investigating the stability of 2-D line-tied magnetic fields, to cylindrically symmetric spheromak equilibria. The method involves the solution of two sets of ordinary differential equations, integrated along the field lines, which give necessary and sufficient conditions for stability. The role of plasma pressure and of the width of the entrance region are investigated.
1993-01-01T00:00:00ZClifford, Leo J.The magnetohydrodynamic (MHD) model is a simple mathematical model that treats a plasma as a perfectly conducting fluid acted upon by magnetic and pressure-driven forces. Many instabilities in plasmas can be predicted using this model. In this Thesis, aspects of the linear stability of solar and laboratory plasmas are studied using the MHD model. Firstly, we investigate the thermal instability of coronal plasmas with line-tied magnetic fields and with anisotropical heat conduction, using an analytical analysis which concentrates on isobaric perturbations, and a time-dependent numerical code. We find that including perpendicular thermal conduction means that condensations are restricted to a narrow layer around the region where the local isobaric growth rate is largest and that, while the growth rate of the thermal mode is largely unaffected by perpendicular thermal conduction, this may be an important factor in determining the lengthscale for the width of condensations. Secondly, the effect of a finitely conducting wall on the linear stability of Spheromak and Reversed Field Finch equilibria is investigated. We find growth rates for the modes that are present because of the finite resistivity of the wall, which grow proportionally to the "long" time constant of the wall. Finally, we apply a tractable method, derived by De Bruyne (1990), for investigating the stability of 2-D line-tied magnetic fields, to cylindrically symmetric spheromak equilibria. The method involves the solution of two sets of ordinary differential equations, integrated along the field lines, which give necessary and sufficient conditions for stability. The role of plasma pressure and of the width of the entrance region are investigated.Ducted magnetoacoustic waves in the solar coronaSmith, Jason M.http://hdl.handle.net/10023/142462018-06-19T23:17:27Z1997-01-01T00:00:00ZThis thesis investigates the ducting of magnetoacoustic waves in coronal structures. The propagation of waves in current sheets and coronal loops has been examined in order to understand wave ducting in structured plasmas, and to provide an explanation of the observed oscillatory behaviour in the solar corona. Firstly a comprehensive review of the observations of loops and oscillations in the corona is given. An investigation into how the curvature of the loop alters the ducting of magnetoacoustic waves is then presented by studying the effect of the length, width and the density enhancement of the loop and also the frequency of oscillation. The effect of the curvature is to generate wave leakage from the loop. The guiding of magnetoacoustic waves by a current sheet is also considered. An investigation into the type of modes which may propagate and the time scales of oscillation is performed. Impulsively generated waves exhibit similar temporal signatures to observations of X-ray and radio emission. Periods of oscillation for all the ducted wave models are in good agreement with reported observations. The effect of a random boundary motion on a magnetospheric cavity is examined through numerical simulations. A broadband driving spectrum excites the quasi-monochromatic fast modes whose frequencies lie within the driving spectrum. These fast modes couple to an Alfvén mode if the frequency lies within the Alfvén continuum. The position of the resonant field lines and the Alfvén mode eigenfunction may be accurately calculated by assuming a periodic boundary motion. To conclude the work in this thesis the three-dimensional magnetic topologies surrounding neutral points are studied. The local linear magnetic structure about the null is found to depend only on a 3 X 3 matrix containing four parameters. The type of topology is dependent upon the nature of the eigenvalues and eigenvectors of this matrix.
1997-01-01T00:00:00ZSmith, Jason M.This thesis investigates the ducting of magnetoacoustic waves in coronal structures. The propagation of waves in current sheets and coronal loops has been examined in order to understand wave ducting in structured plasmas, and to provide an explanation of the observed oscillatory behaviour in the solar corona. Firstly a comprehensive review of the observations of loops and oscillations in the corona is given. An investigation into how the curvature of the loop alters the ducting of magnetoacoustic waves is then presented by studying the effect of the length, width and the density enhancement of the loop and also the frequency of oscillation. The effect of the curvature is to generate wave leakage from the loop. The guiding of magnetoacoustic waves by a current sheet is also considered. An investigation into the type of modes which may propagate and the time scales of oscillation is performed. Impulsively generated waves exhibit similar temporal signatures to observations of X-ray and radio emission. Periods of oscillation for all the ducted wave models are in good agreement with reported observations. The effect of a random boundary motion on a magnetospheric cavity is examined through numerical simulations. A broadband driving spectrum excites the quasi-monochromatic fast modes whose frequencies lie within the driving spectrum. These fast modes couple to an Alfvén mode if the frequency lies within the Alfvén continuum. The position of the resonant field lines and the Alfvén mode eigenfunction may be accurately calculated by assuming a periodic boundary motion. To conclude the work in this thesis the three-dimensional magnetic topologies surrounding neutral points are studied. The local linear magnetic structure about the null is found to depend only on a 3 X 3 matrix containing four parameters. The type of topology is dependent upon the nature of the eigenvalues and eigenvectors of this matrix.Inverse polarity prominence equilibriaSchönfelder, Apollonia Maria Oktaviahttp://hdl.handle.net/10023/142432018-06-19T23:17:18Z1995-01-01T00:00:00ZIt has been supposed since the middle of this century that it is the global magnetic field surrounding a quiescent prominence that provides the force to prevent its collapse due to the sun’s gravitational field. Many theoretical models, assuming that the prominence plasma is supported in a dip in the magnetic field lines associated by the magnetic tension force, have since been put forward. The aim of this thesis is to propose further models of quiescent prominences to widen our understanding and knowledge of these remarkable features.
A short overview over the magnetohydrodynamic equations used to describe solar prominences, or most of the solar phenomena for that matter, are discussed in chapter 2, and a short summary of prominence observations and attempts to model them is given in chapter 3.
A brief description of the numerical code used in chapters 5 and 7 is given in chapter 4.
Observations of Kim (1990) and Leroy (1985) have found that most large quiescent prominences are of inverse polarity type for which the magnetic field passes through the prominence in the opposite direction to that expected from the photospheric magnetic field. Many theoretical models have been proposed, but failed. Hence, in chapter 5 we investigate first – without the inclusion of a prominence sheet – when an inverse polarity magnetic field must have the correct topology for an inverse polarity configuration before the formation of the prominence itself. Only very recently, the first basic successful model of an I-type polarity prominence was proposed by Low (1993). In chapter 6 we examine this model and investigate current sheets more complicated and realistic than the one used by Low. These analytical models deal with the force-free solution, which is matched onto an external, unsheared, potential coronal magnetic field. These solutions are mathematically interesting and allow an investigation of different profiles of the current intensity of the magnetic field vector and of the mass density in the sheet. The prominence properties predicted by these models have been examined and have been found to match the observational values. The mathematics of current sheets in general is also briefly discussed.
Chapter 7 deals with numerical solutions of inverse polarity prominences embedded in a force-free magnetic flux tube, matched onto an unsheared potential coronal field. Unfortunately the solutions gained are quite sensitive to the boundary conditions imposed on them through the numerical box, showing a loss of convergence and a tendency for the solution to blow up.
Finally, a short summary as well as possible future work is given in chapter 8.
1995-01-01T00:00:00ZSchönfelder, Apollonia Maria OktaviaIt has been supposed since the middle of this century that it is the global magnetic field surrounding a quiescent prominence that provides the force to prevent its collapse due to the sun’s gravitational field. Many theoretical models, assuming that the prominence plasma is supported in a dip in the magnetic field lines associated by the magnetic tension force, have since been put forward. The aim of this thesis is to propose further models of quiescent prominences to widen our understanding and knowledge of these remarkable features.
A short overview over the magnetohydrodynamic equations used to describe solar prominences, or most of the solar phenomena for that matter, are discussed in chapter 2, and a short summary of prominence observations and attempts to model them is given in chapter 3.
A brief description of the numerical code used in chapters 5 and 7 is given in chapter 4.
Observations of Kim (1990) and Leroy (1985) have found that most large quiescent prominences are of inverse polarity type for which the magnetic field passes through the prominence in the opposite direction to that expected from the photospheric magnetic field. Many theoretical models have been proposed, but failed. Hence, in chapter 5 we investigate first – without the inclusion of a prominence sheet – when an inverse polarity magnetic field must have the correct topology for an inverse polarity configuration before the formation of the prominence itself. Only very recently, the first basic successful model of an I-type polarity prominence was proposed by Low (1993). In chapter 6 we examine this model and investigate current sheets more complicated and realistic than the one used by Low. These analytical models deal with the force-free solution, which is matched onto an external, unsheared, potential coronal magnetic field. These solutions are mathematically interesting and allow an investigation of different profiles of the current intensity of the magnetic field vector and of the mass density in the sheet. The prominence properties predicted by these models have been examined and have been found to match the observational values. The mathematics of current sheets in general is also briefly discussed.
Chapter 7 deals with numerical solutions of inverse polarity prominences embedded in a force-free magnetic flux tube, matched onto an unsheared potential coronal field. Unfortunately the solutions gained are quite sensitive to the boundary conditions imposed on them through the numerical box, showing a loss of convergence and a tendency for the solution to blow up.
Finally, a short summary as well as possible future work is given in chapter 8.The magnetohydrostatic equilibrium of quiescent solar prominencesRidgway, Christopherhttp://hdl.handle.net/10023/142392018-06-19T23:17:21Z1992-01-01T00:00:00ZSince the mid 1900's it has been supposed that the global magnetic field surrounding a quiescent prominence provides the force required to prevent its collapse under the influence of the Sun's gravitational field. Many theoretical models of this magnetic field have been produced in which it is assumed that the prominence plasma is supported in a dip in the field lines by the associated magnetic tension force. It is the aim of this thesis to propose further models of the magnetic field in order to extend our knowledge and understanding of prominences. In doing so we present three distinct models. The first is an extension of the twisted flux tube model for prominences proposed by Priest et al. (1989). Here we present analytical solutions to the magnetohydrostatic equilibrium equation within the tube using the so- called generating function method in which we select two distinct functional forms of the longitudinal field component. Unlike the solutions found by Priest et al., we allow for large deviations of the field from cylindrical symmetry. The prominence is represented by a finite vertical sheet of mass and current and we show that it is possible for such a sheet to be in static equilibrium everywhere along its vertical extent. Next we consider the model of van Ballegooijen and Martens in which photospheric motions drive a reconnection process leading to the formation of a helical magnetic structure capable of supporting dense prominence plasma in the low points of the helical windings. Under the assumption of cylindrical symmetry we analyse two methods of solving the magnetohydrostatic equilibrium equation in which the positions of the field line footpoints at the photosphere are imposed. Using a combination of analytical and numerical techniques, we study the quasi-static evolution of the model as the height of the helical axis increases. Unlike the numerical analysis of van Ballegooijen and Martens we are able to produce inverse polarity configurations without the problem of singular field components at the helical axis. Lastly we present an analysis of the interaction of a finite, vertical sheet of mass and current (representing a prominence) with an external constant-current force-free field. We formalise two distinct boundary-value problems in which the distribution of the normal magnetic field component along the photosphere is imposed along with the distribution of either the normal magnetic field component across the prominence or the prominence surface current. In both cases we demonstrate for particular boundary conditions that it is possible for equilibrium solutions to exist of both normal and inverse polarity in which dense material is supported everywhere along the prominence sheet. In particular we are, for the first time, able to produce an inverse polarity equilibrium configuration in which the field components are locally bounded and closed field lines exist above the prominence sheet while an X-type neutral point lies below it.
1992-01-01T00:00:00ZRidgway, ChristopherSince the mid 1900's it has been supposed that the global magnetic field surrounding a quiescent prominence provides the force required to prevent its collapse under the influence of the Sun's gravitational field. Many theoretical models of this magnetic field have been produced in which it is assumed that the prominence plasma is supported in a dip in the field lines by the associated magnetic tension force. It is the aim of this thesis to propose further models of the magnetic field in order to extend our knowledge and understanding of prominences. In doing so we present three distinct models. The first is an extension of the twisted flux tube model for prominences proposed by Priest et al. (1989). Here we present analytical solutions to the magnetohydrostatic equilibrium equation within the tube using the so- called generating function method in which we select two distinct functional forms of the longitudinal field component. Unlike the solutions found by Priest et al., we allow for large deviations of the field from cylindrical symmetry. The prominence is represented by a finite vertical sheet of mass and current and we show that it is possible for such a sheet to be in static equilibrium everywhere along its vertical extent. Next we consider the model of van Ballegooijen and Martens in which photospheric motions drive a reconnection process leading to the formation of a helical magnetic structure capable of supporting dense prominence plasma in the low points of the helical windings. Under the assumption of cylindrical symmetry we analyse two methods of solving the magnetohydrostatic equilibrium equation in which the positions of the field line footpoints at the photosphere are imposed. Using a combination of analytical and numerical techniques, we study the quasi-static evolution of the model as the height of the helical axis increases. Unlike the numerical analysis of van Ballegooijen and Martens we are able to produce inverse polarity configurations without the problem of singular field components at the helical axis. Lastly we present an analysis of the interaction of a finite, vertical sheet of mass and current (representing a prominence) with an external constant-current force-free field. We formalise two distinct boundary-value problems in which the distribution of the normal magnetic field component along the photosphere is imposed along with the distribution of either the normal magnetic field component across the prominence or the prominence surface current. In both cases we demonstrate for particular boundary conditions that it is possible for equilibrium solutions to exist of both normal and inverse polarity in which dense material is supported everywhere along the prominence sheet. In particular we are, for the first time, able to produce an inverse polarity equilibrium configuration in which the field components are locally bounded and closed field lines exist above the prominence sheet while an X-type neutral point lies below it.Wave-particle dynamics in a hot inhomogenous fusion plasmaTaylor, Michael Anthonyhttp://hdl.handle.net/10023/142352018-06-19T23:17:23Z1996-01-01T00:00:00ZAn outstanding problem in the field of nuclear fusion research is the precise mechanism by which a hot, magnetically inhomogeneous plasma is heated when illuminated by a constant beam of small amplitude radio waves matched in frequency to harmonics of the ion Larmor frequency. An accurate model must include microscopic dynamics and inevitably a kinetic theory is required. Highly energetic ions (> 1MeV) born from fusion reactions or powered by gyroresonance have large Larmor radii (> 10cm) which are comparable in size to the wavelength of the incident radiation. In particular we will focus on fast magnetosonic waves. Exact full wave equations describing a thermal plasma in a weakly inhomogeneous field are presently at least fourth order integro-differential equations (Sauter, 1992). These are computationally taxing. Recently a method was proposed to reduce the problem to a second order integro-differential equation at the expense of information related to the propagation of mode-converted waves (Holt, 1992). We present here a generalisation of the theory to allow for arbitrary velocity-dependent equilibria while at the same time retaining a general functional form for the field profile. We consider the specific case of a bi-Maxwellian plasma immersed in a linearly inhomogenous magnetic field. We find that thermal anisotropy produces resonance localisation when the perpendicular ion temperature is greater than that parallel to the ambient field. A study of the symmetry properties of the conductivity tensor reveals that the Onsager reciprocal relations are obeyed only for an isotropic plasma in an inhomogeneous field. This is a generalisation of the result obtained by Nambu (1995). We present a generalisation of the reduction method to include effects due to changes in wave amplitude. We find that we are able to include the odd-order field derivatives responsible for energy conservation. Our numerical study of fundamental Helium-3 gyroresonance in a majority Deuterium plasma reveals that we have > 99.9% energy conservation in all cases. We show that locally-uniform theory can be very inaccurate (≃ 70% in one case presented in our recent paper, Cairns et al., 1995) particularly for higher energy ions whose non-locality is more extreme. We present a representative sample of results for minority heating and mode conversion heating schemes. We report the appearance of an unexpected cut-off on the low field side of the minority gyroresonance which may have important consequences for antennae presently placed on the outside of Tokamaks.
1996-01-01T00:00:00ZTaylor, Michael AnthonyAn outstanding problem in the field of nuclear fusion research is the precise mechanism by which a hot, magnetically inhomogeneous plasma is heated when illuminated by a constant beam of small amplitude radio waves matched in frequency to harmonics of the ion Larmor frequency. An accurate model must include microscopic dynamics and inevitably a kinetic theory is required. Highly energetic ions (> 1MeV) born from fusion reactions or powered by gyroresonance have large Larmor radii (> 10cm) which are comparable in size to the wavelength of the incident radiation. In particular we will focus on fast magnetosonic waves. Exact full wave equations describing a thermal plasma in a weakly inhomogeneous field are presently at least fourth order integro-differential equations (Sauter, 1992). These are computationally taxing. Recently a method was proposed to reduce the problem to a second order integro-differential equation at the expense of information related to the propagation of mode-converted waves (Holt, 1992). We present here a generalisation of the theory to allow for arbitrary velocity-dependent equilibria while at the same time retaining a general functional form for the field profile. We consider the specific case of a bi-Maxwellian plasma immersed in a linearly inhomogenous magnetic field. We find that thermal anisotropy produces resonance localisation when the perpendicular ion temperature is greater than that parallel to the ambient field. A study of the symmetry properties of the conductivity tensor reveals that the Onsager reciprocal relations are obeyed only for an isotropic plasma in an inhomogeneous field. This is a generalisation of the result obtained by Nambu (1995). We present a generalisation of the reduction method to include effects due to changes in wave amplitude. We find that we are able to include the odd-order field derivatives responsible for energy conservation. Our numerical study of fundamental Helium-3 gyroresonance in a majority Deuterium plasma reveals that we have > 99.9% energy conservation in all cases. We show that locally-uniform theory can be very inaccurate (≃ 70% in one case presented in our recent paper, Cairns et al., 1995) particularly for higher energy ions whose non-locality is more extreme. We present a representative sample of results for minority heating and mode conversion heating schemes. We report the appearance of an unexpected cut-off on the low field side of the minority gyroresonance which may have important consequences for antennae presently placed on the outside of Tokamaks.Models of X-ray bright points and concelling magnetic featuresParnell, Clare Elizabethhttp://hdl.handle.net/10023/142322018-06-26T15:35:59Z1995-01-01T00:00:00ZSmall brightenings called x-ray bright points (Golub et al, 1974) occur in the solar corona. They are observed with the soft x-ray telescope on Skylab to be approximately 22 Mm in diameter with a brighter inner core of width 4-7 Mm although with the Normal Incidence X-ray Telescope their dimensions are observed to be typically 6 Mm x 9 Mm. By comparison with magnetograms of the photosphere it has been noticed recently that there is a high correlation between the occurrence of x-ray bright points and the mutual reduction of flux between two opposite polarity magnetic fragments. These fragments are originally unconnected magnetically, but move towards each other and simultaneously lose equal amounts of flux (cancel): they are called cancelling magnetic features (Martin et al, 1984). The observations relating to these features were reviewed by Priest et al. (1994) who suggested that they naturally evolve through three phases: the pre-interaction, interaction and cancellation phases. From this evidence qualitative pictures of the magnetic field structure for an x-ray bright point and associated cancelling magnetic feature were established. The aim of this thesis has been to build on the ideas of Priest et al. (1994) to produce a detailed theoretical model of an x-ray bright point and a cancelling magnetic feature. The magnetic field structures are estimated, and the position and lifetime of the bright point are calculated, as is the total amount of energy released during the bright point. This work is also extended to study more complex cancelling configurations representing the main basic types of cancelling magnetic feature. The results of these models determine the factors that affect the lifetime and position of a bright point and indicate which types of cancelling magnetic features are most likely to produce bright points that are long-lived, lie directly above the cancellation site and occur simultaneously with the cancellation phase. The complex structure of a bright point cannot be explained from the above two-dimensional models: thus two recently observed bright points were studied to see if the above model could be extended into three dimensions to explain the structure seen in soft x-ray images. The available observational data was used and leads to reasonable explanations for the complex shapes of both bright points. Finally, a more realistic model for the overlying field was set up involving a model of the field above a supergranule cell field with fragments of finite width. The interaction of an ephemeral region within this field was then studied and led to five different scenarios. The results obtained reaffirmed those found in the previous simpler models and suggest where bright points may appear in a cell relative to the cancelling magnetic feature and for how long the bright points might last. Predictions for the lifetimes of cancelling magnetic features are also made, indicating when the cancelling magnetic feature occurs relative to the bright point.
1995-01-01T00:00:00ZParnell, Clare ElizabethSmall brightenings called x-ray bright points (Golub et al, 1974) occur in the solar corona. They are observed with the soft x-ray telescope on Skylab to be approximately 22 Mm in diameter with a brighter inner core of width 4-7 Mm although with the Normal Incidence X-ray Telescope their dimensions are observed to be typically 6 Mm x 9 Mm. By comparison with magnetograms of the photosphere it has been noticed recently that there is a high correlation between the occurrence of x-ray bright points and the mutual reduction of flux between two opposite polarity magnetic fragments. These fragments are originally unconnected magnetically, but move towards each other and simultaneously lose equal amounts of flux (cancel): they are called cancelling magnetic features (Martin et al, 1984). The observations relating to these features were reviewed by Priest et al. (1994) who suggested that they naturally evolve through three phases: the pre-interaction, interaction and cancellation phases. From this evidence qualitative pictures of the magnetic field structure for an x-ray bright point and associated cancelling magnetic feature were established. The aim of this thesis has been to build on the ideas of Priest et al. (1994) to produce a detailed theoretical model of an x-ray bright point and a cancelling magnetic feature. The magnetic field structures are estimated, and the position and lifetime of the bright point are calculated, as is the total amount of energy released during the bright point. This work is also extended to study more complex cancelling configurations representing the main basic types of cancelling magnetic feature. The results of these models determine the factors that affect the lifetime and position of a bright point and indicate which types of cancelling magnetic features are most likely to produce bright points that are long-lived, lie directly above the cancellation site and occur simultaneously with the cancellation phase. The complex structure of a bright point cannot be explained from the above two-dimensional models: thus two recently observed bright points were studied to see if the above model could be extended into three dimensions to explain the structure seen in soft x-ray images. The available observational data was used and leads to reasonable explanations for the complex shapes of both bright points. Finally, a more realistic model for the overlying field was set up involving a model of the field above a supergranule cell field with fragments of finite width. The interaction of an ephemeral region within this field was then studied and led to five different scenarios. The results obtained reaffirmed those found in the previous simpler models and suggest where bright points may appear in a cell relative to the cancelling magnetic feature and for how long the bright points might last. Predictions for the lifetimes of cancelling magnetic features are also made, indicating when the cancelling magnetic feature occurs relative to the bright point.Aspects of MHD wave propagation in solar atmospheric studiesMundie, Cheryl Annhttp://hdl.handle.net/10023/142272018-06-19T23:16:34Z1998-01-01T00:00:00ZThe theme of this thesis is ideal linear MHD wave propagation in structured media, using models relevant to structures in the solar atmosphere. We derive dispersion relations governing the ideal linear MHD modes for stationary states which are discretely structured in velocity and other plasma properties, in a direction transverse to the magnetic field, with field-aligned steady flow; the discrete structures considered are the single interface, uniform slab and uniform cylinder. This represents an extension of earlier models for the static case (Edwin 1984), by the inclusion of structured flows. The basic effects of flow are described, drawing on a discussion of the dispersion relations. The dispersion relations for the case of incompressible surface modes are examined in detail. We identify the qualitative effects of flow, including the onset of instability, by tracing the evolution of stable solutions and their propagation windows, as the relative flow is increased. Our analysis is presented in terms of a general formulation applicable to all three geometries (interface, slab and cylinder), revealing the combined role of dispersion and the ratio of densities in the two media. We go on to consider the relevance of the incompressible approximation to compressible surface modes, with particular reference to the static case of a single interface one side of which is field-free. We present and investigate analytical solutions for several special cases. The properties of the solutions obtained are compared with those for the equivalent incompressible case. Finally, we turn to the topic of global oscillations of quiescent prominences. A uniform slab model (Joarder 1993) yields, under conditions appropriate to the prominence-coronal inhomogeneity with the magnetic field threading the prominence being line-tied in the photosphere, modes which are analogous to the oscillations of a uniform string loaded with a point mass, and a formula approximating the period is given. We investigate the robustness of this formula for various plasma density profiles, assessing the applicability of the results from the uniform slab calculation to more realistic density profiles of the prominence-coronal inhomogeneity.
1998-01-01T00:00:00ZMundie, Cheryl AnnThe theme of this thesis is ideal linear MHD wave propagation in structured media, using models relevant to structures in the solar atmosphere. We derive dispersion relations governing the ideal linear MHD modes for stationary states which are discretely structured in velocity and other plasma properties, in a direction transverse to the magnetic field, with field-aligned steady flow; the discrete structures considered are the single interface, uniform slab and uniform cylinder. This represents an extension of earlier models for the static case (Edwin 1984), by the inclusion of structured flows. The basic effects of flow are described, drawing on a discussion of the dispersion relations. The dispersion relations for the case of incompressible surface modes are examined in detail. We identify the qualitative effects of flow, including the onset of instability, by tracing the evolution of stable solutions and their propagation windows, as the relative flow is increased. Our analysis is presented in terms of a general formulation applicable to all three geometries (interface, slab and cylinder), revealing the combined role of dispersion and the ratio of densities in the two media. We go on to consider the relevance of the incompressible approximation to compressible surface modes, with particular reference to the static case of a single interface one side of which is field-free. We present and investigate analytical solutions for several special cases. The properties of the solutions obtained are compared with those for the equivalent incompressible case. Finally, we turn to the topic of global oscillations of quiescent prominences. A uniform slab model (Joarder 1993) yields, under conditions appropriate to the prominence-coronal inhomogeneity with the magnetic field threading the prominence being line-tied in the photosphere, modes which are analogous to the oscillations of a uniform string loaded with a point mass, and a formula approximating the period is given. We investigate the robustness of this formula for various plasma density profiles, assessing the applicability of the results from the uniform slab calculation to more realistic density profiles of the prominence-coronal inhomogeneity.Magnetohydrodynamic surface waves in the solar atmosphereMiles, Alan J.http://hdl.handle.net/10023/142252018-06-19T23:16:22Z1991-01-01T00:00:00ZIn this thesis the nature of magnetoacoustic surface waves at a single magnetic interface is examined for the case of parallel propagation. Above the interface is an isothermal medium permeated by a horizontal magnetic field. The lower region is a field-free medium of different density to the magnetic atmosphere. We consider both the incompressible and compressible situations and the effect of including gravity. In each case a transcendental dispersion relation is solved numerically for a range of parameters and the resulting dispersion curves plotted. In the first chapter we provide a general introduction to the work, reviewing previous work in this area and considering applications of surface waves. In the second chapter we consider the existence of surface waves for the case when the media are incompressible either side of the interface. We consider the cases of both uniform and non-uniform distributions of densities and the effect of including gravity. We show that the f-mode exists in a restricted band of horizontal wavenumber. In the subsequent chapters we consider the effect of compressibility on surface waves. The media either side of the interface are taken to be isothermal. In the absence of gravity the interface may support one or two surface modes determined by the relative temperatures and magnetism of the two media. This case is studied in Chapter 3 where phase-speeds and penetration depths of the waves and the associated pressure perturbations are investigated for a variety of field strengths and sound speeds. In Chapters 4 and 5 we consider the effect of gravity on the compressible modes described in Chapter 3. In Chapter 4 an exact dispersion relation is obtained for the case of a constant Alfven speed, whilst in Chapter 5 the case of a uniform magnetic field is considered. In the absence of the magnetic field the transcendental dispersion relation may be reduced to a polynomial. This polynomial possesses two acceptable solutions, only one of which may exist at any given circumstance depending on the densities either side of the interface. If the gas density within the field exceeds that in the field-free medium, then the f-mode may propagate; otherwise, a magnetic surface gravity mode propagates. As in the incompressible case, the f-mode exists in a restricted band of horizontal wavenumber. An analytical form for the wave speed of the f-mode is obtained for small values of the Alfven speed. It is shown that the f-mode is related to the fast magnetoacoustic surface wave, merging into that mode at short wavelengths.
1991-01-01T00:00:00ZMiles, Alan J.In this thesis the nature of magnetoacoustic surface waves at a single magnetic interface is examined for the case of parallel propagation. Above the interface is an isothermal medium permeated by a horizontal magnetic field. The lower region is a field-free medium of different density to the magnetic atmosphere. We consider both the incompressible and compressible situations and the effect of including gravity. In each case a transcendental dispersion relation is solved numerically for a range of parameters and the resulting dispersion curves plotted. In the first chapter we provide a general introduction to the work, reviewing previous work in this area and considering applications of surface waves. In the second chapter we consider the existence of surface waves for the case when the media are incompressible either side of the interface. We consider the cases of both uniform and non-uniform distributions of densities and the effect of including gravity. We show that the f-mode exists in a restricted band of horizontal wavenumber. In the subsequent chapters we consider the effect of compressibility on surface waves. The media either side of the interface are taken to be isothermal. In the absence of gravity the interface may support one or two surface modes determined by the relative temperatures and magnetism of the two media. This case is studied in Chapter 3 where phase-speeds and penetration depths of the waves and the associated pressure perturbations are investigated for a variety of field strengths and sound speeds. In Chapters 4 and 5 we consider the effect of gravity on the compressible modes described in Chapter 3. In Chapter 4 an exact dispersion relation is obtained for the case of a constant Alfven speed, whilst in Chapter 5 the case of a uniform magnetic field is considered. In the absence of the magnetic field the transcendental dispersion relation may be reduced to a polynomial. This polynomial possesses two acceptable solutions, only one of which may exist at any given circumstance depending on the densities either side of the interface. If the gas density within the field exceeds that in the field-free medium, then the f-mode may propagate; otherwise, a magnetic surface gravity mode propagates. As in the incompressible case, the f-mode exists in a restricted band of horizontal wavenumber. An analytical form for the wave speed of the f-mode is obtained for small values of the Alfven speed. It is shown that the f-mode is related to the fast magnetoacoustic surface wave, merging into that mode at short wavelengths.Some aspects of solar flare and prominence theoryMilne, Alexander Mitchellhttp://hdl.handle.net/10023/142222018-06-19T23:16:28Z1980-01-01T00:00:00ZSolar flares and solar prominences are amongst the best known features of solar activity. Despite this familiarity, however, there are still significant gaps in our knowledge of these phenomena. In this thesis some theoretical aspects of these events are considered. We first consider solar prominences. We propose a model for the static equilibrium of quiescent prominences which will simultaneously explain the support mechanism for the dense prominence material and take account roughly of the required energy balance. This model contains two parameters, namely the coronal plasma beta and the horizontal shear angle 𝜙, that the magnetic fieldlines make with the prominence normal. We obtain limits on both these parameters which, when exceeded, imply that no equilibrium state is possible. The results obtained provide a possible explanation for several prominence features. For the remainder of the thesis we consider one aspect of the solar flare problem, namely the possibility of a trigger mechanism for the rapid release of energy in a flare. One candidate for this mechanism is the sudden release of energy stored in excess of potential by a force-free magnetic field which becomes unstable as a result of photospheric motions. For this reason we seek simple analytic solutions to the force-free field equations which may exhibit such an instability. An alternative trigger mechanism, which requires the presence of a current sheet, is given by the emerging flux model for solar flares. We thus develop a one-dimensional model for current sheets in general, where the conditions within the current sheet are given in terms of several non-dimensional parameters which describe the external conditions. These results are then applied to the emerging flux model.
1980-01-01T00:00:00ZMilne, Alexander MitchellSolar flares and solar prominences are amongst the best known features of solar activity. Despite this familiarity, however, there are still significant gaps in our knowledge of these phenomena. In this thesis some theoretical aspects of these events are considered. We first consider solar prominences. We propose a model for the static equilibrium of quiescent prominences which will simultaneously explain the support mechanism for the dense prominence material and take account roughly of the required energy balance. This model contains two parameters, namely the coronal plasma beta and the horizontal shear angle 𝜙, that the magnetic fieldlines make with the prominence normal. We obtain limits on both these parameters which, when exceeded, imply that no equilibrium state is possible. The results obtained provide a possible explanation for several prominence features. For the remainder of the thesis we consider one aspect of the solar flare problem, namely the possibility of a trigger mechanism for the rapid release of energy in a flare. One candidate for this mechanism is the sudden release of energy stored in excess of potential by a force-free magnetic field which becomes unstable as a result of photospheric motions. For this reason we seek simple analytic solutions to the force-free field equations which may exhibit such an instability. An alternative trigger mechanism, which requires the presence of a current sheet, is given by the emerging flux model for solar flares. We thus develop a one-dimensional model for current sheets in general, where the conditions within the current sheet are given in terms of several non-dimensional parameters which describe the external conditions. These results are then applied to the emerging flux model.Polynomial generated polygonsSoares, Benedict J.http://hdl.handle.net/10023/141982018-06-18T23:17:17Z1999-01-01T00:00:00ZA turtle geometric construction on the plane, called a polynomial generated polygon (PGP) and represented by 𝒫[sub]f,[sub]pᵐ, is generated from the sequence obtained from evaluating f(𝓍) ∈ ℤ [𝓍] over ℤ modulo pᵐ where p is a prime and m ∈ ℕ. Computational methods are developed to pre-calculate the symmetries exhibited by [sub]f,[sub]pᵐ for a given f and pᵐ.
These include procedures to find whether [sub]f,[sub]pᵐ is bounded or unbounded, the degree of rotational symmetry present, whether lines of reflectional symmetry can be observed, and in the case of 𝒫[sub]f,[sub]pᵐ unbounded, whether the PGP has a glide reflection.
Methods are also sought to find a suitable f and pᵐ to produce a desired 'feasible' shape in a PGP construction, and how the same shape might be generated modulo pᵐ⁺ᵏ if it cannot be produced modulo pᵐ.
1999-01-01T00:00:00ZSoares, Benedict J.A turtle geometric construction on the plane, called a polynomial generated polygon (PGP) and represented by 𝒫[sub]f,[sub]pᵐ, is generated from the sequence obtained from evaluating f(𝓍) ∈ ℤ [𝓍] over ℤ modulo pᵐ where p is a prime and m ∈ ℕ. Computational methods are developed to pre-calculate the symmetries exhibited by [sub]f,[sub]pᵐ for a given f and pᵐ.
These include procedures to find whether [sub]f,[sub]pᵐ is bounded or unbounded, the degree of rotational symmetry present, whether lines of reflectional symmetry can be observed, and in the case of 𝒫[sub]f,[sub]pᵐ unbounded, whether the PGP has a glide reflection.
Methods are also sought to find a suitable f and pᵐ to produce a desired 'feasible' shape in a PGP construction, and how the same shape might be generated modulo pᵐ⁺ᵏ if it cannot be produced modulo pᵐ.Nonlinear magnetic reconnectionColin, A. M.http://hdl.handle.net/10023/141952018-06-18T23:16:47Z1987-01-01T00:00:00ZIn many astrophysical problems magnetic reconnection plays a major role. Despite this reconnection theory remains incompletely understood, partly due to the strong non-linearity of the governing equations and the resulting difficulties in demonstrating analytical solutions. This thesis examines some fundamental aspects of reconnection theory: namely, the dynamics of driven and spontaneously reconnecting systems. We first consider the dynamics of a driven reconnecting system by numerically modelling a configuration consisting of two oppositely oriented flux systems with a variety of different boundary conditions and internal parameters. The results indicate that the rate of reconnection is chiefly dependent on the magnetic Reynolds number, but that the plasma flow is weakly dependent on this parameter, being more affected by the curvature of Incoming magnetic field. Scaling laws for the dimensions of the diffusion region are derived, and the existence of several reconnection regimes is shown. Using the same computer code we also simulate tearing modes in Cartesian geometry under different boundary conditions. By imposing a suitable perturbation a magnetic island is generated. The plasma flows show marked differences for the different boundary conditions implemented. Lastly, we examine some aspects of the coalescence instability. The usual flux function taken to represent a tearing node Island in the linear growth phase is shown to be erroneous, and we derive a correct expression. We show that under certain conditions there exists a threshold to coalescence that depends on the island wavenumbers and the associated perturbation.
1987-01-01T00:00:00ZColin, A. M.In many astrophysical problems magnetic reconnection plays a major role. Despite this reconnection theory remains incompletely understood, partly due to the strong non-linearity of the governing equations and the resulting difficulties in demonstrating analytical solutions. This thesis examines some fundamental aspects of reconnection theory: namely, the dynamics of driven and spontaneously reconnecting systems. We first consider the dynamics of a driven reconnecting system by numerically modelling a configuration consisting of two oppositely oriented flux systems with a variety of different boundary conditions and internal parameters. The results indicate that the rate of reconnection is chiefly dependent on the magnetic Reynolds number, but that the plasma flow is weakly dependent on this parameter, being more affected by the curvature of Incoming magnetic field. Scaling laws for the dimensions of the diffusion region are derived, and the existence of several reconnection regimes is shown. Using the same computer code we also simulate tearing modes in Cartesian geometry under different boundary conditions. By imposing a suitable perturbation a magnetic island is generated. The plasma flows show marked differences for the different boundary conditions implemented. Lastly, we examine some aspects of the coalescence instability. The usual flux function taken to represent a tearing node Island in the linear growth phase is shown to be erroneous, and we derive a correct expression. We show that under certain conditions there exists a threshold to coalescence that depends on the island wavenumbers and the associated perturbation.The nonlinear thermal evolution of coronal structuresMendoza Briceño, César Augustohttp://hdl.handle.net/10023/141932018-06-18T23:17:18Z1996-01-01T00:00:00ZThe thermal equilibrium and evolution of coronal structure is studied in this thesis. A symmetric and constant cross-sectional coronal loop is considered and, because of the strong magnetic field, the plasma is confined to move along the field lines, so that a one-dimensional problem can be assumed. We begin by giving a brief description of the Sun and corresponding phenomena. Then a discussion of the basic MHD equations is given. Here, it is assumed that the heating function is spatially dependent and the cooling function is due to an optically thin plasma. The thermal equilibrium of uniform-pressure coronal loop is investigated. The effects due to varying the values of the parameters involved in the governing equations are studied. It is found that there is a critical decay length of the heating below which a hot coronal loop does not exist. It is suggested that thermal non-equilibrium occurs, allowing the existence of catastrophic cooling. A study of the stability of the equilibrium up to the second order approximation is presented, and it is found that the response of the structure not only depends on the amplitude of the disturbance, but also on whether the disturbance increases or decreases the static temperature. The thermal evolution of the above structure is also investigated by assuming that the inertial terms are small. The previous results related to the critical heating decay length are verified. The numerical simulation shows that an initial hot plasma evolves to a new equilibrium which has a cool summit. This equilibrium is identified as a prominence-like solution. Further investigations are made in order to show how the structure can either evolve to a hot or a cool summit temperature depending on whether the initial conditions are above or below threshold values. The evolution of a disturbance increasing or decreasing an initial equilibrium temperature is followed numerically verifying the prediction made in the stability analysis. Furthermore, the effect of gravity is considered in the thermal equilibrium of loop. Similar results were found as studied for a constant-pressure loop. However, it was found that the critical values in which thermal non-equilibrium can occur is increased. A magnetic dip is also included in this model and the thermal equilibrium is studied. Finally, extensions of the present work is presented and some preliminary results are discussed.
1996-01-01T00:00:00ZMendoza Briceño, César AugustoThe thermal equilibrium and evolution of coronal structure is studied in this thesis. A symmetric and constant cross-sectional coronal loop is considered and, because of the strong magnetic field, the plasma is confined to move along the field lines, so that a one-dimensional problem can be assumed. We begin by giving a brief description of the Sun and corresponding phenomena. Then a discussion of the basic MHD equations is given. Here, it is assumed that the heating function is spatially dependent and the cooling function is due to an optically thin plasma. The thermal equilibrium of uniform-pressure coronal loop is investigated. The effects due to varying the values of the parameters involved in the governing equations are studied. It is found that there is a critical decay length of the heating below which a hot coronal loop does not exist. It is suggested that thermal non-equilibrium occurs, allowing the existence of catastrophic cooling. A study of the stability of the equilibrium up to the second order approximation is presented, and it is found that the response of the structure not only depends on the amplitude of the disturbance, but also on whether the disturbance increases or decreases the static temperature. The thermal evolution of the above structure is also investigated by assuming that the inertial terms are small. The previous results related to the critical heating decay length are verified. The numerical simulation shows that an initial hot plasma evolves to a new equilibrium which has a cool summit. This equilibrium is identified as a prominence-like solution. Further investigations are made in order to show how the structure can either evolve to a hot or a cool summit temperature depending on whether the initial conditions are above or below threshold values. The evolution of a disturbance increasing or decreasing an initial equilibrium temperature is followed numerically verifying the prediction made in the stability analysis. Furthermore, the effect of gravity is considered in the thermal equilibrium of loop. Similar results were found as studied for a constant-pressure loop. However, it was found that the critical values in which thermal non-equilibrium can occur is increased. A magnetic dip is also included in this model and the thermal equilibrium is studied. Finally, extensions of the present work is presented and some preliminary results are discussed.Alfvén waves in low-mass star-forming regionsMartin, Clare E.http://hdl.handle.net/10023/141902018-06-18T23:17:15Z1999-01-01T00:00:00ZLow-mass star-forming regions have a lifetime which is greater than their dynamical time and must therefore be, in an average sense, in mechanical equilibrium. The work presented here proposes that an equilibrium exists between the self-gravity, gas pressure, and the magnetic field and the waves it supports. Specifically the equilibrium in the direction perpendicular to the ordered magnetic field is given by the Lorentz force, while that parallel to the field is given by an Alfvén wave pressure force. The work detailed in this thesis models a low-mass star-forming region as a one-dimensional gas slab with a magnetic field lying perpendicular to the layer. Analytical, self-consistent models are formulated to study the equilibrium parallel to the background magnetic field. It is found that both short-wavelength (modelled using the WKB approximation) and large-amplitude, long-wavelength Alfvén waves can provide the necessary support parallel to the magnetic field, generating model cloud thicknesses that are consistent with the observations. The effect of damping by the linear process of ion-neutral friction is considered. It is found that the damping of the waves is not a necessary condition for the support of the cloud although it is an advantage. The possible sources of these waves are discussed. The Alfvén waves are also found to make an important contribution to the heating of a low-mass star-forming region. By modelling the dominant heating and cooling mechanisms in a molecular cloud, it is discovered that a cloud supported against its self-gravity by short-wavelength Alfvén waves will be hotter at its outer edge than in the central regions. These models successfully describe a low-mass star-forming region in equilibrium between its self-gravity, the gas pressure and an Alfvén wave pressure force. The question of the stability of such an equilibrium is considered, specifically that of an isothermal gas slab supported by short-wavelength Alfvén waves. The initial results suggest that the presence of a magnetic field and its associated Alfvén waves have a stabilising effect on the layer, and encourage further consideration of the role of Alfvén waves in low-mass star-forming regions.
1999-01-01T00:00:00ZMartin, Clare E.Low-mass star-forming regions have a lifetime which is greater than their dynamical time and must therefore be, in an average sense, in mechanical equilibrium. The work presented here proposes that an equilibrium exists between the self-gravity, gas pressure, and the magnetic field and the waves it supports. Specifically the equilibrium in the direction perpendicular to the ordered magnetic field is given by the Lorentz force, while that parallel to the field is given by an Alfvén wave pressure force. The work detailed in this thesis models a low-mass star-forming region as a one-dimensional gas slab with a magnetic field lying perpendicular to the layer. Analytical, self-consistent models are formulated to study the equilibrium parallel to the background magnetic field. It is found that both short-wavelength (modelled using the WKB approximation) and large-amplitude, long-wavelength Alfvén waves can provide the necessary support parallel to the magnetic field, generating model cloud thicknesses that are consistent with the observations. The effect of damping by the linear process of ion-neutral friction is considered. It is found that the damping of the waves is not a necessary condition for the support of the cloud although it is an advantage. The possible sources of these waves are discussed. The Alfvén waves are also found to make an important contribution to the heating of a low-mass star-forming region. By modelling the dominant heating and cooling mechanisms in a molecular cloud, it is discovered that a cloud supported against its self-gravity by short-wavelength Alfvén waves will be hotter at its outer edge than in the central regions. These models successfully describe a low-mass star-forming region in equilibrium between its self-gravity, the gas pressure and an Alfvén wave pressure force. The question of the stability of such an equilibrium is considered, specifically that of an isothermal gas slab supported by short-wavelength Alfvén waves. The initial results suggest that the presence of a magnetic field and its associated Alfvén waves have a stabilising effect on the layer, and encourage further consideration of the role of Alfvén waves in low-mass star-forming regions.Basic magnetic field configurations for solar filament channels and filamentsMackay, Duncan Hendryhttp://hdl.handle.net/10023/141882018-06-18T23:17:12Z1997-01-01T00:00:00ZThe three-dimensional magnetic structure of solar filament channels and filaments is considered. A simple analytical potential model of a filament channel is setup with line sources representing the overlying arcades and point sources the flux of the filament. A possible explanation of the distinct upper and lower bounds of a filament is given. A more detailed numerical force-free model with discrete flux sources is then developed and the effect of magnetic shear on the separatrix surface explored. Dextral channels are shown to exist for a wider range of negative values of the force-free alpha and sinistral channels for positive values of alpha. Potential models of a variety of coronal structures are then considered. The bending of a filament is modelled and a method of determining the horizontal component of a filament's magnetic field is proposed. Next, the observed opposite skew of arcades lying above switchbacks of polarity inversion lines is shown to be produced by a local flux imbalance at the corner of the switchback. Then, the magnetic structure of a particular filament in a filament channel is modelled using observations from a photospheric magnetogram. It is shown that dips in the filaments magnetic field could result from opposite polarity fragments lying below the filament. Finally, the formation of a specific filament channel and filament is modelled. The formation of the channel is shown to be due to the emergence of new flux in a sheared state. It is shown that convergence and reconnections between the new flux and old remnant flux is required for the filament to form. The field lines that represent the filament form a thin vertical sheet of flux. The changing angle of inclination of the sheet gives the appearance of twist. The method of formation is then generalised to other cases and it is shown that a hemispheric pattern consistent with the results of Martin et al. (1995) can be obtained.
1997-01-01T00:00:00ZMackay, Duncan HendryThe three-dimensional magnetic structure of solar filament channels and filaments is considered. A simple analytical potential model of a filament channel is setup with line sources representing the overlying arcades and point sources the flux of the filament. A possible explanation of the distinct upper and lower bounds of a filament is given. A more detailed numerical force-free model with discrete flux sources is then developed and the effect of magnetic shear on the separatrix surface explored. Dextral channels are shown to exist for a wider range of negative values of the force-free alpha and sinistral channels for positive values of alpha. Potential models of a variety of coronal structures are then considered. The bending of a filament is modelled and a method of determining the horizontal component of a filament's magnetic field is proposed. Next, the observed opposite skew of arcades lying above switchbacks of polarity inversion lines is shown to be produced by a local flux imbalance at the corner of the switchback. Then, the magnetic structure of a particular filament in a filament channel is modelled using observations from a photospheric magnetogram. It is shown that dips in the filaments magnetic field could result from opposite polarity fragments lying below the filament. Finally, the formation of a specific filament channel and filament is modelled. The formation of the channel is shown to be due to the emergence of new flux in a sheared state. It is shown that convergence and reconnections between the new flux and old remnant flux is required for the filament to form. The field lines that represent the filament form a thin vertical sheet of flux. The changing angle of inclination of the sheet gives the appearance of twist. The method of formation is then generalised to other cases and it is shown that a hemispheric pattern consistent with the results of Martin et al. (1995) can be obtained.Aspects of magnetic field theory in solar and laboratory plasmasLothian, Robert M.http://hdl.handle.net/10023/141832018-06-18T23:17:16Z1990-01-01T00:00:00ZUsing the Magnetohydrodynamic model, two problems in the behaviour of magnetic field structures are investigated. Firstly, the stability of tokamak equilibria to coupled tearing modes is calculated. Secondly, the equilibrium structure of a solar coronal loop is examined. The flux co-ordinate method is used to construct toroidal equilibria of the type found in large aspect ratio tokamaks. In such a field configuration, the analysis of tearing modes is complicated by the coupling of different poloidal fourier modes. The effect of coupling through elliptic shaping of plasma surfaces is calculated. For certain current profiles, this effect may cause instability. The response of coronal loops to twisting at their photospheric footpoints is investigated. Long loops are shown to have an essentially 1-D nature. This observation is used to develop a 1-D, line-tied model for such loops. This model is used to conduct an extensive survey of the non-linear twist regime, including the effects of enhanced fluid pressure. The possibility of non-equilibrium, which would provide energy for coronal heating and compact flares, is examined. When the physical variable of footpoint displacement is specified, no loss of equilibrium is found by twisting. Loss of equilibrium is found for high pressures, which we do not, however, expect to find in the corona.
1990-01-01T00:00:00ZLothian, Robert M.Using the Magnetohydrodynamic model, two problems in the behaviour of magnetic field structures are investigated. Firstly, the stability of tokamak equilibria to coupled tearing modes is calculated. Secondly, the equilibrium structure of a solar coronal loop is examined. The flux co-ordinate method is used to construct toroidal equilibria of the type found in large aspect ratio tokamaks. In such a field configuration, the analysis of tearing modes is complicated by the coupling of different poloidal fourier modes. The effect of coupling through elliptic shaping of plasma surfaces is calculated. For certain current profiles, this effect may cause instability. The response of coronal loops to twisting at their photospheric footpoints is investigated. Long loops are shown to have an essentially 1-D nature. This observation is used to develop a 1-D, line-tied model for such loops. This model is used to conduct an extensive survey of the non-linear twist regime, including the effects of enhanced fluid pressure. The possibility of non-equilibrium, which would provide energy for coronal heating and compact flares, is examined. When the physical variable of footpoint displacement is specified, no loss of equilibrium is found by twisting. Loss of equilibrium is found for high pressures, which we do not, however, expect to find in the corona.Exact solutions for axisymmetric and nonpolytropic astrophysical windsLima, João José de Faria Graça Afonsohttp://hdl.handle.net/10023/141802018-06-18T23:17:07Z1995-01-01T00:00:00ZAstrophysical outflows are common in a large variety of objects with very different length-scales. They can be almost spherical, as in the case of the solar wind, or show a high degree of anisotropy as in pre-main sequence stars, star-forming regions or even extragalactic objects. This work is aimed at finding exact solutions of the axisymmetric wind equations in which all variables depend not only on the distance to the central object but on latitude as well. The geometry of the stream/field-lines is taken as helicoidal and this seems to be a good approximation in some examples of collimated flows. From a simple hydrodynamic approach, a straightforward technique based on separation of the variables yields the most general solution of the wind equations under the above assumptions. The way the different variables depend on latitude is controlled by three anisotropy parameters which are related to typical ratios at the base of the atmosphere. The density needs to be higher at the equator than at the pole for the outflow to be able to accelerate. In these circumstances, the radial velocity always increases from equator to pole. Contrary to Parker's model of the solar wind, the solution does not pass through any critical point, since no polytropic law is assumed. However, the general behaviour is similar, with a high acceleration at the base and the velocity rapidly attaining an almost constant asymptotic value. The heating rate that sustains this rapid increase is mostly concentrated near the surface of the central object. The inclusion of the magnetic field in the analysis introduces two critical points: the Alfvenic point and an extra X -type point filtering the solution that gives a vanishing pressure at infinity. If the density anisotropy is too low the wind is unable to accelerate to large asymptotic values. The dependence of the angular velocity of the roots of the fieldlines with latitude reproduces well the observed rotation profile of photospheric magnetic features. The mass loss rate can be substantially increased if the structure of the outflow is highly anisotropic. Some applications to the solar wind are also discussed. In particular, recent results from ULYSSES (pointing out that solar speed increases with latitude while the density decreases from equator to the pole) are in good agreement with the general behaviour of the solutions presented in this work.
1995-01-01T00:00:00ZLima, João José de Faria Graça AfonsoAstrophysical outflows are common in a large variety of objects with very different length-scales. They can be almost spherical, as in the case of the solar wind, or show a high degree of anisotropy as in pre-main sequence stars, star-forming regions or even extragalactic objects. This work is aimed at finding exact solutions of the axisymmetric wind equations in which all variables depend not only on the distance to the central object but on latitude as well. The geometry of the stream/field-lines is taken as helicoidal and this seems to be a good approximation in some examples of collimated flows. From a simple hydrodynamic approach, a straightforward technique based on separation of the variables yields the most general solution of the wind equations under the above assumptions. The way the different variables depend on latitude is controlled by three anisotropy parameters which are related to typical ratios at the base of the atmosphere. The density needs to be higher at the equator than at the pole for the outflow to be able to accelerate. In these circumstances, the radial velocity always increases from equator to pole. Contrary to Parker's model of the solar wind, the solution does not pass through any critical point, since no polytropic law is assumed. However, the general behaviour is similar, with a high acceleration at the base and the velocity rapidly attaining an almost constant asymptotic value. The heating rate that sustains this rapid increase is mostly concentrated near the surface of the central object. The inclusion of the magnetic field in the analysis introduces two critical points: the Alfvenic point and an extra X -type point filtering the solution that gives a vanishing pressure at infinity. If the density anisotropy is too low the wind is unable to accelerate to large asymptotic values. The dependence of the angular velocity of the roots of the fieldlines with latitude reproduces well the observed rotation profile of photospheric magnetic features. The mass loss rate can be substantially increased if the structure of the outflow is highly anisotropic. Some applications to the solar wind are also discussed. In particular, recent results from ULYSSES (pointing out that solar speed increases with latitude while the density decreases from equator to the pole) are in good agreement with the general behaviour of the solutions presented in this work.Chromospheric effects on global solar oscillationsJohnston, Alanhttp://hdl.handle.net/10023/141732018-06-18T23:16:48Z1994-01-01T00:00:00ZA study has been made of the global solar oscillations known as p-modes. The Sun is represented by a plane-parallel stratified plasma. Solutions are found to the magnetohydrodynamic equations of motion in such a plasma, and normal mode frequencies are calculated by applying realistic boundary conditions to these solutions. The normal modes model solar p-modes. For a model consisting of an isothermal chromosphere with a uniform horizontal magnetic field, it is demonstrated that modes may form at all frequencies. Consideration is also given to the related problem of vertical propagation of fast magnetoacoustic waves in a uniform magnetic field. An investigation is carried out into the observed solar cycle variations in the frequencies of p-modes in the classical, low frequency range (1-5 mHz). A possible mechanism for the observed "turnover" effect is discussed. Through the use of a modified Bohr- Sommerfeld condition, the effect of a non-isothermal chromosphere is also considered, and a physical description of chromospheric effects on p-mode frequencies is given. The formation of modes above the acoustic cut-off frequency is investigated. The theoretically calcidated forms of frequency shift curves in this high frequency range agree well with observations. The special case of modes of degree zero is also briefly examined. A mathematical formulation for such modes is constructed, and frequency shifts are determined for a simple chromospheric model atmosphere.
1994-01-01T00:00:00ZJohnston, AlanA study has been made of the global solar oscillations known as p-modes. The Sun is represented by a plane-parallel stratified plasma. Solutions are found to the magnetohydrodynamic equations of motion in such a plasma, and normal mode frequencies are calculated by applying realistic boundary conditions to these solutions. The normal modes model solar p-modes. For a model consisting of an isothermal chromosphere with a uniform horizontal magnetic field, it is demonstrated that modes may form at all frequencies. Consideration is also given to the related problem of vertical propagation of fast magnetoacoustic waves in a uniform magnetic field. An investigation is carried out into the observed solar cycle variations in the frequencies of p-modes in the classical, low frequency range (1-5 mHz). A possible mechanism for the observed "turnover" effect is discussed. Through the use of a modified Bohr- Sommerfeld condition, the effect of a non-isothermal chromosphere is also considered, and a physical description of chromospheric effects on p-mode frequencies is given. The formation of modes above the acoustic cut-off frequency is investigated. The theoretically calcidated forms of frequency shift curves in this high frequency range agree well with observations. The special case of modes of degree zero is also briefly examined. A mathematical formulation for such modes is constructed, and frequency shifts are determined for a simple chromospheric model atmosphere.Theoretical modelling of global oscillations in solar prominencesJoarder, Parthasarathihttp://hdl.handle.net/10023/141692018-06-18T23:16:21Z1994-01-01T00:00:00ZThis thesis aims to provide a basic theoretical explanation for the oscillatory motions observed in solar quiescent prominences. The prominence is treated as a simple plasma slab embedded in a hotter and rarer uniform coronal plasma. Both the slab and its environment are permeated by a uniform magnetic field. The field lines are anchored at rigid walls placed on either side of the plasma slab and representing the photospheric line-tying effect. The magnetohydrodynamic modes of oscillation of the plasma slab are then examined for different orientations of the magnetic field with respect to the long axis of the slab. Particularly interesting in this study is the appearance of the 'string MHD' modes that are analogous to the fundamental vibrations of a mass- loaded stretched elastic string. Such modes appear whenever the magnetic field vector is inclined to the long axis of the slab, thus producing a magnetic field component in the direction transverse to the axis of the slab. Observationally, this inclination of the field is generally small. For realistic values of the angle of inclination of the magnetic field lines, the 'string Alfven' mode and an 'internal slow' mode yield periods in the range 1/2-2 hr. These modes may correspond to the observed long period (40-90 minutes) oscillations in quiescent prominences. Intermediate periodicities, in the range 8-20 min, may be associated with an 'internal Alfven' mode and a 'fast string' mode of the prominence slab. The observed short periodicities, in the range 2-5 min, may correspond to an 'internal fast' mode in prominences. Having thus established a foundation for the theoretical modelling of prominence oscillations in terms of the magnetohydrodynamic modes of oscillation of a non-gravitating plasma slab, we discuss several factors, such as the effects of gravitational stratification, the curvature of the magnetic field lines, and the fine-structures in a prominence, that may complicate a description of its oscillatory modes. Some preliminary investigations of simple magnetohydrostatic equilibrium models suggest that gravity and the curvature of the magnetic field lines play only a secondary role in determining the periods of the oscillatory modes in prominences, the basic structure of the modes being similar to that present in simple slab models.
1994-01-01T00:00:00ZJoarder, ParthasarathiThis thesis aims to provide a basic theoretical explanation for the oscillatory motions observed in solar quiescent prominences. The prominence is treated as a simple plasma slab embedded in a hotter and rarer uniform coronal plasma. Both the slab and its environment are permeated by a uniform magnetic field. The field lines are anchored at rigid walls placed on either side of the plasma slab and representing the photospheric line-tying effect. The magnetohydrodynamic modes of oscillation of the plasma slab are then examined for different orientations of the magnetic field with respect to the long axis of the slab. Particularly interesting in this study is the appearance of the 'string MHD' modes that are analogous to the fundamental vibrations of a mass- loaded stretched elastic string. Such modes appear whenever the magnetic field vector is inclined to the long axis of the slab, thus producing a magnetic field component in the direction transverse to the axis of the slab. Observationally, this inclination of the field is generally small. For realistic values of the angle of inclination of the magnetic field lines, the 'string Alfven' mode and an 'internal slow' mode yield periods in the range 1/2-2 hr. These modes may correspond to the observed long period (40-90 minutes) oscillations in quiescent prominences. Intermediate periodicities, in the range 8-20 min, may be associated with an 'internal Alfven' mode and a 'fast string' mode of the prominence slab. The observed short periodicities, in the range 2-5 min, may correspond to an 'internal fast' mode in prominences. Having thus established a foundation for the theoretical modelling of prominence oscillations in terms of the magnetohydrodynamic modes of oscillation of a non-gravitating plasma slab, we discuss several factors, such as the effects of gravitational stratification, the curvature of the magnetic field lines, and the fine-structures in a prominence, that may complicate a description of its oscillatory modes. Some preliminary investigations of simple magnetohydrostatic equilibrium models suggest that gravity and the curvature of the magnetic field lines play only a secondary role in determining the periods of the oscillatory modes in prominences, the basic structure of the modes being similar to that present in simple slab models.Magnetic surface effects on solar oscillationsJain, Rekhahttp://hdl.handle.net/10023/141532018-06-18T23:16:08Z1993-01-01T00:00:00ZThis thesis is concerned with the effects of magnetic atmospheres on solar oscillations. The behaviour of magnetohydrodynamic surface waves propagating on a single magnetic interface is discussed ignoring the effects of gravity. The effects of non-parallel propagation (where the wave vector is at an angle to the magnetic field direction) are considered. The effects of chromospheric magnetic fields on solar p- and f-modes in a stratified atmosphere are examined for three different models. In the first of these models, the chromosphere is assumed to be isothermal and permeated by a uniform and horizontal magnetic field. A dispersion relation for the p-modes trapped below such an atmosphere is derived. Asymptotic and numerical solutions for the p-modes are discussed in detail. An increase in chromospheric magnetic field strength leads to an increase in the frequency of the p-modes, whereas an increase in the chromospheric temperature leads to a decrease in the frequencies of these modes. Comparison with observational data suggests that both these effects may indeed take place. The second model is set up for magnetic fields which decrease with height in such a way that the Alfven speed remains constant. In addition to magnetic effects, the effects of non-parallel propagation on and f-modes are considered in the presence of such a non-uniform magnetic field. After deriving a very general dispersion relation, various asymptotic and numerical solutions have been obtained and the possible effects of magnetic fields and non-parallel propagation on these modes are examined. The presence of a horizontal non-uniform chromospheric field produces changes in the frequencies of the p- and f-modes, reducing the frequencies of p-modes and increasing the frequency of the f-mode. Besides depending upon magnetic field strength, frequencies also depend on both the mode's order n and its degree l. The effects of non-parallel propagation are found to be most significant for the f-mode and the low order p-modes. The magnetic structure of the chromosphere has been further generalised by combining the two models described above. In this three layer model, a dispersion relation is derived in a general manner and discussed in detail for the p-modes. The role of magnetoacoustic cut-off frequency is studied. Again, the results are qualitatively similar to those found from observation.
1993-01-01T00:00:00ZJain, RekhaThis thesis is concerned with the effects of magnetic atmospheres on solar oscillations. The behaviour of magnetohydrodynamic surface waves propagating on a single magnetic interface is discussed ignoring the effects of gravity. The effects of non-parallel propagation (where the wave vector is at an angle to the magnetic field direction) are considered. The effects of chromospheric magnetic fields on solar p- and f-modes in a stratified atmosphere are examined for three different models. In the first of these models, the chromosphere is assumed to be isothermal and permeated by a uniform and horizontal magnetic field. A dispersion relation for the p-modes trapped below such an atmosphere is derived. Asymptotic and numerical solutions for the p-modes are discussed in detail. An increase in chromospheric magnetic field strength leads to an increase in the frequency of the p-modes, whereas an increase in the chromospheric temperature leads to a decrease in the frequencies of these modes. Comparison with observational data suggests that both these effects may indeed take place. The second model is set up for magnetic fields which decrease with height in such a way that the Alfven speed remains constant. In addition to magnetic effects, the effects of non-parallel propagation on and f-modes are considered in the presence of such a non-uniform magnetic field. After deriving a very general dispersion relation, various asymptotic and numerical solutions have been obtained and the possible effects of magnetic fields and non-parallel propagation on these modes are examined. The presence of a horizontal non-uniform chromospheric field produces changes in the frequencies of the p- and f-modes, reducing the frequencies of p-modes and increasing the frequency of the f-mode. Besides depending upon magnetic field strength, frequencies also depend on both the mode's order n and its degree l. The effects of non-parallel propagation are found to be most significant for the f-mode and the low order p-modes. The magnetic structure of the chromosphere has been further generalised by combining the two models described above. In this three layer model, a dispersion relation is derived in a general manner and discussed in detail for the p-modes. The role of magnetoacoustic cut-off frequency is studied. Again, the results are qualitatively similar to those found from observation.Thermal instabilities in the solar coronaIreland, Richard C.http://hdl.handle.net/10023/141502018-06-18T23:15:57Z1995-01-01T00:00:00ZIn this thesis, several problems relating to thermal instabilities in the solar corona are examined. Chapter 1 gives a brief description of the Sun and corresponding events with particular attention focused on prominences, their formation and eruption. Various problems concerning thermal instabilities are then tackled in the later Chapters. In Chapter 2, the basic MHD equations are introduced and a physical description of the thermal instability mechanism given. The MHD equations are linearised in a uniform, infinite medium and the basic instability criteria obtained. Chapter 3 investigates the normal mode spectrum for the linearised MHD equations for a cylindrical equilibrium. This spectrum is examined for zero perpendicular thermal conduction, with both zero and non-zero scalar resistivity. Particular attention is paid to the continuous branches of this spectrum, or continuous spectra. For zero resistivity there are three types of continuous spectra present, namely the Alfven, slow and thermal continua. It is shown that when dissipation due to resistivity is included, the slow and Alfven continua are removed and the thermal continuum is shifted to a different position (where the shift is independent of the exact value of resistivity). The 'old' location of the thermal continuum is covered by a dense set of nearly singular discrete modes called a quasi-continuum, for equilibria with the thermal time scale much smaller than the Alfven time scale. This quasi-continuum is investigated numerically and the eigenfunctions are shown to have rapid spatial oscillating behaviour. These oscillations are confined to the most unstable part of the equilibrium based on the Field criterion and may be the cause of fine structure in prominences. In Chapter 4, the normal mode spectrum for the linearised MHD equations is examined for a plasma in a cylindrical equilibrium. The equations describing these normal modes are solved numerically using a finite element code. In the ideal case the Hain-Lust equation is expanded and a WKB solution obtained for large axial wave numbers. This is compared to the numerical solutions. In the non-ideal case, the ballooning equations describing localised modes are manipulated in an arcade geometry and a dispersion relation derived. It is illustrated that as the axial wave number k is increased, the fundamental thermal and Alfven modes can coalesce to form overstable magnetothermal modes. The ratio between the magnetic and thermal terms is varied and the existence of the magnetothermal modes examined. The corresponding growth rates are predicted by a WKB solution to the ballooning equations. The interaction of thermal and magnetic instabilities and the existence of these magnetothermal modes may be significant in the eruption of prominences into solar flares. Chapter 5 extends the work presented in Chapter 4 to include the effects of line-tying in a coronal arcade. The ballooning equations which were introduced in Chapter 4 are manipulated to give a dispersion relation. This relation is a quadratic in the square of the azimuthal wave number m if parallel thermal conduction is neglected and a cubic in m2 if parallel conduction is included. Rigid wall boundary conditions are applied to this dispersion relation. This dispersion relation is then solved numerically subject to these boundary conditions and the solutions plotted. Unfortunately the expression for the thermal continuum in line-tied arcades is required since the thermal continuum must play an important role in the proceedings. This calculation is left for future work. From the results obtained, it can be seen that the thermal instability can play a major part in prominence formation and destruction. The thermal instability may help create the prominence. Resistivity and perpendicular thermal conduction can cause of the observed fine scale structure. Finally, a neighbouring thermal instability may trigger a magnetic instability that causes the prominence to erupt.
1995-01-01T00:00:00ZIreland, Richard C.In this thesis, several problems relating to thermal instabilities in the solar corona are examined. Chapter 1 gives a brief description of the Sun and corresponding events with particular attention focused on prominences, their formation and eruption. Various problems concerning thermal instabilities are then tackled in the later Chapters. In Chapter 2, the basic MHD equations are introduced and a physical description of the thermal instability mechanism given. The MHD equations are linearised in a uniform, infinite medium and the basic instability criteria obtained. Chapter 3 investigates the normal mode spectrum for the linearised MHD equations for a cylindrical equilibrium. This spectrum is examined for zero perpendicular thermal conduction, with both zero and non-zero scalar resistivity. Particular attention is paid to the continuous branches of this spectrum, or continuous spectra. For zero resistivity there are three types of continuous spectra present, namely the Alfven, slow and thermal continua. It is shown that when dissipation due to resistivity is included, the slow and Alfven continua are removed and the thermal continuum is shifted to a different position (where the shift is independent of the exact value of resistivity). The 'old' location of the thermal continuum is covered by a dense set of nearly singular discrete modes called a quasi-continuum, for equilibria with the thermal time scale much smaller than the Alfven time scale. This quasi-continuum is investigated numerically and the eigenfunctions are shown to have rapid spatial oscillating behaviour. These oscillations are confined to the most unstable part of the equilibrium based on the Field criterion and may be the cause of fine structure in prominences. In Chapter 4, the normal mode spectrum for the linearised MHD equations is examined for a plasma in a cylindrical equilibrium. The equations describing these normal modes are solved numerically using a finite element code. In the ideal case the Hain-Lust equation is expanded and a WKB solution obtained for large axial wave numbers. This is compared to the numerical solutions. In the non-ideal case, the ballooning equations describing localised modes are manipulated in an arcade geometry and a dispersion relation derived. It is illustrated that as the axial wave number k is increased, the fundamental thermal and Alfven modes can coalesce to form overstable magnetothermal modes. The ratio between the magnetic and thermal terms is varied and the existence of the magnetothermal modes examined. The corresponding growth rates are predicted by a WKB solution to the ballooning equations. The interaction of thermal and magnetic instabilities and the existence of these magnetothermal modes may be significant in the eruption of prominences into solar flares. Chapter 5 extends the work presented in Chapter 4 to include the effects of line-tying in a coronal arcade. The ballooning equations which were introduced in Chapter 4 are manipulated to give a dispersion relation. This relation is a quadratic in the square of the azimuthal wave number m if parallel thermal conduction is neglected and a cubic in m2 if parallel conduction is included. Rigid wall boundary conditions are applied to this dispersion relation. This dispersion relation is then solved numerically subject to these boundary conditions and the solutions plotted. Unfortunately the expression for the thermal continuum in line-tied arcades is required since the thermal continuum must play an important role in the proceedings. This calculation is left for future work. From the results obtained, it can be seen that the thermal instability can play a major part in prominence formation and destruction. The thermal instability may help create the prominence. Resistivity and perpendicular thermal conduction can cause of the observed fine scale structure. Finally, a neighbouring thermal instability may trigger a magnetic instability that causes the prominence to erupt.Heating of turbulent solar and laboratory plasmasInverarity, Gordon W.http://hdl.handle.net/10023/141462018-06-18T23:15:32Z1995-01-01T00:00:00ZThe model of Heyvaerts and Priest (1992) for steady-state heating of the turbulent medium within a sheared solar coronal arcade structure is here developed. The energy input into the corona is calculated at the large scales of the model. At the smaller scales the effects of coronal turbulence are modelled in the form of an enhanced turbulent viscosity and magnetic diffusivity, which are related to the injected power density in the steady state. Matching the expressions for the injected and dissipated power enables the calculation of a heating power consistent with both boundary motions and turbulent effects with a minimum of arbitrary parameters - the price to be paid is that the inertial range spectrum must be prescribed and imposed at all scales. While it is capable of reproducing the observed levels of coronal heating (300 Wm⁻² 3x10⁵ erg cm⁻² s⁻ⁱ for the quiet Sun, 800 Wm⁻² (8 x 10⁵ erg cm⁻² s⁻ⁱ) for a coronal hole and 10⁴ Wm ⁻² (10⁷ erg cm⁻² s⁻ⁱ) for an active region (Withbroe and Noyes, 1977)), there are some mathematical and physical difficulties present. These are eliminated as far as is possible and it is found that the final results for heating levels differ little from the original model although there is a much greater consistency between the imposed and predicted energy power spectra. The modified approach is applied to the problems of photospheric motions twisting a coronal flux tube and of rapid motions injecting Alfven waves into an arcade. In the former case comparable levels of heating are obtained. For a driven and damped standing wave, however, desired levels of heating are only obtained when a global resonance occurs. Attempts are also made to find similar steady-state equilibria possessing flow for fusion experiments in order to apply the above procedure to investigate turbulence in laboratory plasmas. This has been hampered by the difficulty in finding simple appropriate equilibria with many scales present.
1995-01-01T00:00:00ZInverarity, Gordon W.The model of Heyvaerts and Priest (1992) for steady-state heating of the turbulent medium within a sheared solar coronal arcade structure is here developed. The energy input into the corona is calculated at the large scales of the model. At the smaller scales the effects of coronal turbulence are modelled in the form of an enhanced turbulent viscosity and magnetic diffusivity, which are related to the injected power density in the steady state. Matching the expressions for the injected and dissipated power enables the calculation of a heating power consistent with both boundary motions and turbulent effects with a minimum of arbitrary parameters - the price to be paid is that the inertial range spectrum must be prescribed and imposed at all scales. While it is capable of reproducing the observed levels of coronal heating (300 Wm⁻² 3x10⁵ erg cm⁻² s⁻ⁱ for the quiet Sun, 800 Wm⁻² (8 x 10⁵ erg cm⁻² s⁻ⁱ) for a coronal hole and 10⁴ Wm ⁻² (10⁷ erg cm⁻² s⁻ⁱ) for an active region (Withbroe and Noyes, 1977)), there are some mathematical and physical difficulties present. These are eliminated as far as is possible and it is found that the final results for heating levels differ little from the original model although there is a much greater consistency between the imposed and predicted energy power spectra. The modified approach is applied to the problems of photospheric motions twisting a coronal flux tube and of rapid motions injecting Alfven waves into an arcade. In the former case comparable levels of heating are obtained. For a driven and damped standing wave, however, desired levels of heating are only obtained when a global resonance occurs. Attempts are also made to find similar steady-state equilibria possessing flow for fusion experiments in order to apply the above procedure to investigate turbulence in laboratory plasmas. This has been hampered by the difficulty in finding simple appropriate equilibria with many scales present.Constraining the pass-band of future space-based coronagraphs for observations of solar eruptions in the FeXIV 530.3 nm “green line”Bemporad, AlessandroPagano, PaoloGiordano, SilvioFineschi, Silvanohttp://hdl.handle.net/10023/141392019-01-06T03:18:28Z2017-10-01T00:00:00ZObservations of the solar corona in the FeXIV 530.3 nm “green line” have been very important in the past, and are planned for future coronagraphs on-board forthcoming space missions such as PROBA-3 and Aditya. For these instruments, a very important parameter to be optimized is the spectral width of the band-pass filter to be centred over the “green line”. Focusing on solar eruptions, motions occurring along the line of sight will Doppler shift the line profiles producing an emission that will partially fall out of the narrower pass-band, while broader pass-band will provide observations with reduced spectral purity. To address these issues, we performed numerical (MHD) simulation of CME emission in the “green line” and produced synthetic images assuming 4 different widths of the pass-band (Δλ = 20 Å, 10 Å, 5 Å, and 2 Å). It turns out that, as expected, during solar eruptions a significant fraction of “green line” emission will be lost using narrower filters; on the other hand these images will have a higher spectral purity and will contain emission coming from parcels of plasma expanding only along the plane of the sky. This will provide a better definition of single filamentary features and will help isolating single slices of plasma through the eruption, thus reducing the problem of superposition of different features along the line of sight and helping physical interpretation of limb events. For these reasons, we suggest to use narrower band passes (Δλ ≤ 2 Å) for the observations of solar eruptions with future coronagraphs.
This research has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 647214) and from the UK Science and Technology Facilities Council.
2017-10-01T00:00:00ZBemporad, AlessandroPagano, PaoloGiordano, SilvioFineschi, SilvanoObservations of the solar corona in the FeXIV 530.3 nm “green line” have been very important in the past, and are planned for future coronagraphs on-board forthcoming space missions such as PROBA-3 and Aditya. For these instruments, a very important parameter to be optimized is the spectral width of the band-pass filter to be centred over the “green line”. Focusing on solar eruptions, motions occurring along the line of sight will Doppler shift the line profiles producing an emission that will partially fall out of the narrower pass-band, while broader pass-band will provide observations with reduced spectral purity. To address these issues, we performed numerical (MHD) simulation of CME emission in the “green line” and produced synthetic images assuming 4 different widths of the pass-band (Δλ = 20 Å, 10 Å, 5 Å, and 2 Å). It turns out that, as expected, during solar eruptions a significant fraction of “green line” emission will be lost using narrower filters; on the other hand these images will have a higher spectral purity and will contain emission coming from parcels of plasma expanding only along the plane of the sky. This will provide a better definition of single filamentary features and will help isolating single slices of plasma through the eruption, thus reducing the problem of superposition of different features along the line of sight and helping physical interpretation of limb events. For these reasons, we suggest to use narrower band passes (Δλ ≤ 2 Å) for the observations of solar eruptions with future coronagraphs.Attributing changes in the distribution of species abundance to weather variables using the example of British breeding birdsOedekoven, Cornelia S.Elston, David A.Harrison, Philip J.Brewer, Mark J.Buckland, Stephen T.Johnston, AlisonFoster, SimonPearce-Higgins, James W.http://hdl.handle.net/10023/141382019-01-06T03:16:24Z2017-12-01T00:00:00Z1. Modelling spatio-temporal changes in species abundance and attributing those changes to potential drivers such as climate, is an important but difficult problem. The standard approach for incorporating climatic variables into such models is to include each weather variable as a single covariate whose effect is expressed through a low-order polynomial or smoother in an additive model. This, however, confounds the spatial and temporal effects of the covariates. 2. We developed a novel approach to distinguish between three types of change in any particular weather covariate. We decomposed the weather covariate into three new covariates by separating out temporal variation in weather (averaging over space), spatial variation in weather (averaging over years) and a space-time anomaly term (residual variation). These three covariates were each fitted separately in the models. We illustrate the approach using generalized additive models applied to count data for a selection of species from the UK’s Breeding Bird Survey, 1994-2013. The weather covariates considered were the mean temperatures during the preceding winter and temperatures and rainfall during the preceding breeding season. We compare models that include these covariates directly with models including decomposed components of the same covariates, considering both linear and smooth relationships. 3. The lowest QAIC values were always associated with a decomposed weather covariate model. Different relationships between counts and the three new covariates provided strong evidence that the effects of changes in covariate values depended on whether changes took place in space, in time, or in the space-time anomaly. These results promote caution in predicting species distribution and abundance in future climate, based on relationships that are largely determined by environmental variation over space. 4. Our methods estimate the effect of temporal changes in weather, whilst accounting for spatial effects of long-term climate, improving inference on overall and/or localised effects of climate change. With increasing availability of large-scale data sets, need is growing for appropriate analytical tools. The proposed decomposition of the weather variables represents an important advance by eliminating the confounding issue often inherent in large-scale data sets.
The BBS is undertaken by the British Trust for Ornithology (BTO) and jointly funded by the BTO, the Joint Nature Conservation Committee and the Royal Society for the Protection of Birds.
2017-12-01T00:00:00ZOedekoven, Cornelia S.Elston, David A.Harrison, Philip J.Brewer, Mark J.Buckland, Stephen T.Johnston, AlisonFoster, SimonPearce-Higgins, James W.1. Modelling spatio-temporal changes in species abundance and attributing those changes to potential drivers such as climate, is an important but difficult problem. The standard approach for incorporating climatic variables into such models is to include each weather variable as a single covariate whose effect is expressed through a low-order polynomial or smoother in an additive model. This, however, confounds the spatial and temporal effects of the covariates. 2. We developed a novel approach to distinguish between three types of change in any particular weather covariate. We decomposed the weather covariate into three new covariates by separating out temporal variation in weather (averaging over space), spatial variation in weather (averaging over years) and a space-time anomaly term (residual variation). These three covariates were each fitted separately in the models. We illustrate the approach using generalized additive models applied to count data for a selection of species from the UK’s Breeding Bird Survey, 1994-2013. The weather covariates considered were the mean temperatures during the preceding winter and temperatures and rainfall during the preceding breeding season. We compare models that include these covariates directly with models including decomposed components of the same covariates, considering both linear and smooth relationships. 3. The lowest QAIC values were always associated with a decomposed weather covariate model. Different relationships between counts and the three new covariates provided strong evidence that the effects of changes in covariate values depended on whether changes took place in space, in time, or in the space-time anomaly. These results promote caution in predicting species distribution and abundance in future climate, based on relationships that are largely determined by environmental variation over space. 4. Our methods estimate the effect of temporal changes in weather, whilst accounting for spatial effects of long-term climate, improving inference on overall and/or localised effects of climate change. With increasing availability of large-scale data sets, need is growing for appropriate analytical tools. The proposed decomposition of the weather variables represents an important advance by eliminating the confounding issue often inherent in large-scale data sets.WKB estimates to the critical length of twisted solar coronal loopsHerbert, Simon I.http://hdl.handle.net/10023/140922018-06-21T23:17:28Z1995-01-01T00:00:00ZThe solar corona exhibits many different phenomena, observable from the Earth or space. Magnetohydrodynamic stability theory provides a method of investigating these phenomena by using it to test proposed mathematical models. WKB is a way of approximating the solutions of second order linear homogeneous differential equations with large parameters and so together with MHD stability theory, models for solar coronal loops can be investigated. In this thesis, the problem of a line tied twisted coronal loop is studied within the framework of ideal MHD using a WKB approximation to estimate the critical length at which the various magnetic fields become unstable. The problem will be split into two halves: (i) force-free and (ii) non force-free fields. Using a finite element/Fourier method, the full MHD equations will be solved numerically and the results compared with analytical solutions.
1995-01-01T00:00:00ZHerbert, Simon I.The solar corona exhibits many different phenomena, observable from the Earth or space. Magnetohydrodynamic stability theory provides a method of investigating these phenomena by using it to test proposed mathematical models. WKB is a way of approximating the solutions of second order linear homogeneous differential equations with large parameters and so together with MHD stability theory, models for solar coronal loops can be investigated. In this thesis, the problem of a line tied twisted coronal loop is studied within the framework of ideal MHD using a WKB approximation to estimate the critical length at which the various magnetic fields become unstable. The problem will be split into two halves: (i) force-free and (ii) non force-free fields. Using a finite element/Fourier method, the full MHD equations will be solved numerically and the results compared with analytical solutions.Solar coronal stability problemsHardie, Ian S.http://hdl.handle.net/10023/140902018-06-14T23:20:33Z1993-01-01T00:00:00ZMagnetohydrodynamic stability theory provides a powerful tool for understanding and testing hypothesized mathematical and physical models of observed phenomena on the surface of the Sun. In this thesis, the problem of applying the 'correct' boundary conditions at the photospheric/coronal interface used in modelling coronal arcades is tackled. Then some aspects of the stability of coronal loops and arcades are investigated using a Fourier truncated series approximation for the equation of motion. The problem involving the boundary conditions has been the subject of a controversy for the past decade with two principal conditions suggested, the 'rigid-wall' conditions where all perturbations vanish at the interface, and 'flow-through' conditions where flows parallel to the equilibrium magnetic field take place. By modelling the photosphere and corona as two different density regions and then varying the ratio of the densities of the two regions, growth rates and eigen-functions of both ideal and resistive modes are investigated in order to follow the evolution of the modes as the density ratio is increased. In order to simplify the analysis, the 2-D equations are reduced to 1-D equations by taking a WKB approximation for the spatial variations across the field to give a localized ballooning approach with ordinary differential equations along the fieldlines. Stability of coronal loops to kink modes transformed to localized modes by increasing the poloidal wavenumber, m, is investigated. Two fields generated numerically from the Grad-Shafranov equation and three analytic fields are investigated in detail and the effect of pressure on the marginal loop length is found, both for near force-free conditions such as is found in the solar corona, and away from force-free conditions. It was found that for near force-free conditions, kink modes are the most unstable with localized modes the most stable. As pressure and pressure gradients become important, there is a reversal in the most unstable modes with localized modes the most unstable.
1993-01-01T00:00:00ZHardie, Ian S.Magnetohydrodynamic stability theory provides a powerful tool for understanding and testing hypothesized mathematical and physical models of observed phenomena on the surface of the Sun. In this thesis, the problem of applying the 'correct' boundary conditions at the photospheric/coronal interface used in modelling coronal arcades is tackled. Then some aspects of the stability of coronal loops and arcades are investigated using a Fourier truncated series approximation for the equation of motion. The problem involving the boundary conditions has been the subject of a controversy for the past decade with two principal conditions suggested, the 'rigid-wall' conditions where all perturbations vanish at the interface, and 'flow-through' conditions where flows parallel to the equilibrium magnetic field take place. By modelling the photosphere and corona as two different density regions and then varying the ratio of the densities of the two regions, growth rates and eigen-functions of both ideal and resistive modes are investigated in order to follow the evolution of the modes as the density ratio is increased. In order to simplify the analysis, the 2-D equations are reduced to 1-D equations by taking a WKB approximation for the spatial variations across the field to give a localized ballooning approach with ordinary differential equations along the fieldlines. Stability of coronal loops to kink modes transformed to localized modes by increasing the poloidal wavenumber, m, is investigated. Two fields generated numerically from the Grad-Shafranov equation and three analytic fields are investigated in detail and the effect of pressure on the marginal loop length is found, both for near force-free conditions such as is found in the solar corona, and away from force-free conditions. It was found that for near force-free conditions, kink modes are the most unstable with localized modes the most stable. As pressure and pressure gradients become important, there is a reversal in the most unstable modes with localized modes the most unstable.Instability and wave-growth within some oscillatory fluid flowsForster, Graham Keithhttp://hdl.handle.net/10023/140872018-06-14T23:18:59Z1996-01-01T00:00:00ZOscillatory fluid flows arise naturally in many systems. Whether or not these systems are stable is an important question and external periodic forcing of the flow may result in rich and complicated behaviours. Here three distinct oscillatory fluid flows are examined in detail, with the stability of each being established using a range of analytical and computational methods. The first system comprises standing surface capillary-gravity waves in second-harmonic resonance subject to Faraday excitation. Using the perturbation technique of multiple scales, the amplitude equations for the system are derived. At exact resonance, and with the absence of damping, the only fixed point of the equations is found to be the origin. A computational approach reveals that the amplitudes of the two waves remain either bounded or grow to infinity depending on initial data. With the introduction of detuning and damping into the system families of fixed points now exist and some special cases are considered. The second class of flows are unbounded time-periodic flows with fixed ellipsoidal stream surfaces, and having spatially uniform but time-periodic strain rates. Using a recently developed method based on theoretical study of the Schrodinger equation with quasi-periodic potential, a computational approach is adopted which determines the stability of the flow to three-dimensional plane wave disturbances. Results for the growth rate and winding number of the disturbance clearly reveal the regions of instability. It is found that almost all these flows are highly unstable. The third class is another set of three-dimensional time-periodic flows with spatially uniform strain rates. These flows are non-axisymmetric and have sinusoidally-fluctuating rates of strain directed along the fixed coordinate axes. The same computational method is employed and it is found that instability increases along with the non-axisymmetric nature of the flow.
1996-01-01T00:00:00ZForster, Graham KeithOscillatory fluid flows arise naturally in many systems. Whether or not these systems are stable is an important question and external periodic forcing of the flow may result in rich and complicated behaviours. Here three distinct oscillatory fluid flows are examined in detail, with the stability of each being established using a range of analytical and computational methods. The first system comprises standing surface capillary-gravity waves in second-harmonic resonance subject to Faraday excitation. Using the perturbation technique of multiple scales, the amplitude equations for the system are derived. At exact resonance, and with the absence of damping, the only fixed point of the equations is found to be the origin. A computational approach reveals that the amplitudes of the two waves remain either bounded or grow to infinity depending on initial data. With the introduction of detuning and damping into the system families of fixed points now exist and some special cases are considered. The second class of flows are unbounded time-periodic flows with fixed ellipsoidal stream surfaces, and having spatially uniform but time-periodic strain rates. Using a recently developed method based on theoretical study of the Schrodinger equation with quasi-periodic potential, a computational approach is adopted which determines the stability of the flow to three-dimensional plane wave disturbances. Results for the growth rate and winding number of the disturbance clearly reveal the regions of instability. It is found that almost all these flows are highly unstable. The third class is another set of three-dimensional time-periodic flows with spatially uniform strain rates. These flows are non-axisymmetric and have sinusoidally-fluctuating rates of strain directed along the fixed coordinate axes. The same computational method is employed and it is found that instability increases along with the non-axisymmetric nature of the flow.The effects of magnetic fields on oscillations in the solar atmosphereEvans, David J.http://hdl.handle.net/10023/140822018-06-14T23:20:32Z1990-01-01T00:00:00ZA study has been made of wave propagation in two regions of the solar atmosphere in which magnetic forces are significant. Sunspot observations indicate a rich variety of characteristic modes of oscillation roughly divided into three categories: three minute umbral oscillations, five minute umbral oscillations and penumbral waves. Outside of intense magnetic flux concentrations the oscillation spectrum is dominated by the five minute period. These waves are trapped in a cavity whose upper boundary may be affected by the magnetism of the chromosphere. A sunspot has been modelled by a uniform cylindrical flux tube. The allowable modes of oscillation are found to vary as the atmospheric parameters change with depth. Umbral three minute oscillations are interpreted as slow body modes. The umbral five minute oscillations arise through a complicated interaction with acoustic waves outside the sunspot. This drives fast body modes as well as waves simply passing through the flux tube. The former may propagate upwards and become fast surface waves. Fast and slow surface waves may explain some of the oscillations of the penumbra. The magnetic structure of the chromosphere has been modelled as an isothermal atmosphere permeated by a uniform and horizontal magnetic field. A dispersion relation for the trapped waves below such an atmosphere has been derived and both asymptotic and numerical solutions found. The effect of a uniform magnetic field is to increase the frequency of the trapped modes. A physical explanation for these changes in frequency has been put forward. Observational evidence may indicate that such effects are indeed seen. This model has been further generalised to take some account of the variation in canopy height which has been observed.
1990-01-01T00:00:00ZEvans, David J.A study has been made of wave propagation in two regions of the solar atmosphere in which magnetic forces are significant. Sunspot observations indicate a rich variety of characteristic modes of oscillation roughly divided into three categories: three minute umbral oscillations, five minute umbral oscillations and penumbral waves. Outside of intense magnetic flux concentrations the oscillation spectrum is dominated by the five minute period. These waves are trapped in a cavity whose upper boundary may be affected by the magnetism of the chromosphere. A sunspot has been modelled by a uniform cylindrical flux tube. The allowable modes of oscillation are found to vary as the atmospheric parameters change with depth. Umbral three minute oscillations are interpreted as slow body modes. The umbral five minute oscillations arise through a complicated interaction with acoustic waves outside the sunspot. This drives fast body modes as well as waves simply passing through the flux tube. The former may propagate upwards and become fast surface waves. Fast and slow surface waves may explain some of the oscillations of the penumbra. The magnetic structure of the chromosphere has been modelled as an isothermal atmosphere permeated by a uniform and horizontal magnetic field. A dispersion relation for the trapped waves below such an atmosphere has been derived and both asymptotic and numerical solutions found. The effect of a uniform magnetic field is to increase the frequency of the trapped modes. A physical explanation for these changes in frequency has been put forward. Observational evidence may indicate that such effects are indeed seen. This model has been further generalised to take some account of the variation in canopy height which has been observed.Magnetic helicity and force-free equilibria in the solar corona and in laboratory devicesDixon, Andrew Michaelhttp://hdl.handle.net/10023/140802018-06-14T23:19:04Z1988-01-01T00:00:00ZForce-free equilibria are believed to be important in both an astrophysical and a laboratory context as minimum-energy configurations (see, for example, Woltjer, 1958; Taylor, 1974). Associated is the study of magnetic helicity and its invariance. In Chapter Two of this thesis we put forward a means of heating the corona by the rotation of the foot-points of a coronal "sunspot" magnetic field anchored in the photosphere. The method adopted is essentially that of Heyvaerts and Priest (1984), employing Taylor's Hypothesis (Taylor, 1974) and a magnetic helicity evolution equation. A characteristic of the Reversed-Field Pinch device is the appearance, at high enough values of the quantity "volt-seconds over toroidal flux", of a helical distortion to the basic axi-symmetric state. In Chapter Three we look for corresponding behaviour in the "sunspot equilibrium" of the previous chapter, with limited success. However, we go on to formulate a method of calculating general axi-symmetric fields above a sunspot given the normal field component at the photosphere. Chapters Four, Five and Six are concerned with equilibrium force-free fields in a sphere. The main aim here is the calculation minimum-energy configurations having magnetic flux crossing the boundary, and so we employ "relative helicity" (Berger and Field, 1984). In Chapter Four we consider the "P1(cos𝜃)" boundary radial field, finding that the minimum-energy state is always purely symmetric. In Chapter Five we treat the "P2(cos𝜃)" boundary condition. We find in this case that a "mixed state" is theoretically possible for high enough values of the helicity. In Chapter Six, we consider a general boundary field, which we use to model point sources of magnetic flux at the boundary of a spheromak, finding that in practice an axi-symmetric configuration is always the minimum-energy state. Finally, in Chapter Seven we present an extension to the theorem of Woltjer (1958), concerning the minimization of the magnetic energy of a magnetic structure, to include the case of a free boundary subjected to external pressure forces. To illustrate the theory, we have provided three applications, the first to a finite cylindrical flux and the remainder to possible spheromak configurations.
1988-01-01T00:00:00ZDixon, Andrew MichaelForce-free equilibria are believed to be important in both an astrophysical and a laboratory context as minimum-energy configurations (see, for example, Woltjer, 1958; Taylor, 1974). Associated is the study of magnetic helicity and its invariance. In Chapter Two of this thesis we put forward a means of heating the corona by the rotation of the foot-points of a coronal "sunspot" magnetic field anchored in the photosphere. The method adopted is essentially that of Heyvaerts and Priest (1984), employing Taylor's Hypothesis (Taylor, 1974) and a magnetic helicity evolution equation. A characteristic of the Reversed-Field Pinch device is the appearance, at high enough values of the quantity "volt-seconds over toroidal flux", of a helical distortion to the basic axi-symmetric state. In Chapter Three we look for corresponding behaviour in the "sunspot equilibrium" of the previous chapter, with limited success. However, we go on to formulate a method of calculating general axi-symmetric fields above a sunspot given the normal field component at the photosphere. Chapters Four, Five and Six are concerned with equilibrium force-free fields in a sphere. The main aim here is the calculation minimum-energy configurations having magnetic flux crossing the boundary, and so we employ "relative helicity" (Berger and Field, 1984). In Chapter Four we consider the "P1(cos𝜃)" boundary radial field, finding that the minimum-energy state is always purely symmetric. In Chapter Five we treat the "P2(cos𝜃)" boundary condition. We find in this case that a "mixed state" is theoretically possible for high enough values of the helicity. In Chapter Six, we consider a general boundary field, which we use to model point sources of magnetic flux at the boundary of a spheromak, finding that in practice an axi-symmetric configuration is always the minimum-energy state. Finally, in Chapter Seven we present an extension to the theorem of Woltjer (1958), concerning the minimization of the magnetic energy of a magnetic structure, to include the case of a free boundary subjected to external pressure forces. To illustrate the theory, we have provided three applications, the first to a finite cylindrical flux and the remainder to possible spheromak configurations.MHD flows in the solar atmosphereDel Zanna, Lucahttp://hdl.handle.net/10023/140752018-06-14T23:19:03Z1997-01-01T00:00:00ZIn this thesis, different aspects of the physics of flows in the solar atmosphere are examined. These are described by means of the set of (ideal) magnetohydrodynamics (MHD) and throughout the thesis there is a progressive refinement in the mathematical methods to solve these equations. First, an analysis of symmetric MHD equilibria is presented and the difficulties that are found in solving the steady equations, both analytically and numerically, are discussed in detail. A novel method to find exact solutions in the incompressible case is presented and families of solutions are given in different geometries. Then, attention is turned to flows in coronal magnetic structures, namely quiescent prominences (closed fieldlines) and polar plumes (open fieldlines), and MHD models for these structures are developed by following two different methods: for the former a semi- analytic approach while for the latter a linearisation through a low 𝛽 assumption. In the prominence model, the effects of a subsonic flow along the fieldlines supporting the structure are studied and the results are compared both with a previous static model and with the observed flow speeds. For the plume model, flows are supposed to be transonic along the open fieldlines and their behaviour is studied for different distributions of temperature, density and magnetic flux. However, here the main goal is to demonstrate that coronal plumes are essentially magnetic features and some results of the model are compared with observations. Finally, a time dependent MHD code in spherical coordinates is presented. The aim is to study the interaction of the solar wind with the large scale coronal magnetic structures and the propagation of MHD waves. As a test in 1-D, simulations of the dynamic response of a spherically symmetric extended corona to changes at the outer pressure are studied, following a previous analytic work.
1997-01-01T00:00:00ZDel Zanna, LucaIn this thesis, different aspects of the physics of flows in the solar atmosphere are examined. These are described by means of the set of (ideal) magnetohydrodynamics (MHD) and throughout the thesis there is a progressive refinement in the mathematical methods to solve these equations. First, an analysis of symmetric MHD equilibria is presented and the difficulties that are found in solving the steady equations, both analytically and numerically, are discussed in detail. A novel method to find exact solutions in the incompressible case is presented and families of solutions are given in different geometries. Then, attention is turned to flows in coronal magnetic structures, namely quiescent prominences (closed fieldlines) and polar plumes (open fieldlines), and MHD models for these structures are developed by following two different methods: for the former a semi- analytic approach while for the latter a linearisation through a low 𝛽 assumption. In the prominence model, the effects of a subsonic flow along the fieldlines supporting the structure are studied and the results are compared both with a previous static model and with the observed flow speeds. For the plume model, flows are supposed to be transonic along the open fieldlines and their behaviour is studied for different distributions of temperature, density and magnetic flux. However, here the main goal is to demonstrate that coronal plumes are essentially magnetic features and some results of the model are compared with observations. Finally, a time dependent MHD code in spherical coordinates is presented. The aim is to study the interaction of the solar wind with the large scale coronal magnetic structures and the propagation of MHD waves. As a test in 1-D, simulations of the dynamic response of a spherically symmetric extended corona to changes at the outer pressure are studied, following a previous analytic work.The nonuniform magnetohydrodynamic nature of the solar atmosphereDe Ville, Andrewhttp://hdl.handle.net/10023/140732018-06-14T23:19:34Z1991-01-01T00:00:00ZThe nonuniform structure observed in the solar atmosphere, and in particular the corona, is thought to arise due to the interaction between the magnetic field and plasma. Using a linear theory, the nature of these interactions is investigated, and it is shown how coronal structure may be modelled in a simple way by extended standing disturbances. The effect of inertial forces in considered in both a Cartesian and cylindrical geometries, and a first correction due to gravity is calculated. The restrictions of a linear theory may be overcome by finding exact solutions. Solutions are presented which may model plasma flows in closed, partially open and open magnetic field line structures. A new method for finding particular classes of exact steady solutions in a gravitationally stratified, isothermal atmosphere is presented, along with some examples of possible solutions.
1991-01-01T00:00:00ZDe Ville, AndrewThe nonuniform structure observed in the solar atmosphere, and in particular the corona, is thought to arise due to the interaction between the magnetic field and plasma. Using a linear theory, the nature of these interactions is investigated, and it is shown how coronal structure may be modelled in a simple way by extended standing disturbances. The effect of inertial forces in considered in both a Cartesian and cylindrical geometries, and a first correction due to gravity is calculated. The restrictions of a linear theory may be overcome by finding exact solutions. Solutions are presented which may model plasma flows in closed, partially open and open magnetic field line structures. A new method for finding particular classes of exact steady solutions in a gravitationally stratified, isothermal atmosphere is presented, along with some examples of possible solutions.Aspects of solar coronal stability theoryDe Bruyne, Peter J. J.http://hdl.handle.net/10023/140712018-06-14T23:18:47Z1991-01-01T00:00:00ZSolar coronal stability theory is a powerful tool for understanding the complex behaviour of the Sun's atmosphere. It enables one to discover the driving forces behind some intriguing phenomena and to gauge the soundness of theoretical models for observed structures. In this thesis, the linear stability analysis of line-tied symmetric magnetohydrostatic equilibria is studied within the framework of ideal MHD, aimed at its application to the solar corona. Firstly, a tractable stability procedure based on a variational method is devised. It provides a necessary condition for stability to disturbances localised about a particular flux surface, and a sufficient condition for stability to all accessible perturbations that vanish at the photosphere. The tests require the minimisation of a line integral along the magnetic field lines. For 1-D equilibria, this can be performed analytically, and simple stability criteria are obtained. The necessary condition then serves as an extended Suydam criterion, incorporating the stabilising effect of line-tying. For 2-D equilibria, the minimisation requires the integration of a system of ordinary differential equations along the field lines. This stability technique is applied to arcade, loop, and prominence models, yielding tight bounds on the equilibrium parameters. Secondly, global modes in 1-D coronal loops are investigated using a normal mode method, in order to clarify their link with localised interchange modes. For nearly force-free fields it is shown that instability to localised modes implies the existence of a fast growing global kink mode driven in the neighbourhood of the radius predicted by the local analysis. This confers a new significance on the study of localised interchange modes and the associated extended Suydam criterion.
1991-01-01T00:00:00ZDe Bruyne, Peter J. J.Solar coronal stability theory is a powerful tool for understanding the complex behaviour of the Sun's atmosphere. It enables one to discover the driving forces behind some intriguing phenomena and to gauge the soundness of theoretical models for observed structures. In this thesis, the linear stability analysis of line-tied symmetric magnetohydrostatic equilibria is studied within the framework of ideal MHD, aimed at its application to the solar corona. Firstly, a tractable stability procedure based on a variational method is devised. It provides a necessary condition for stability to disturbances localised about a particular flux surface, and a sufficient condition for stability to all accessible perturbations that vanish at the photosphere. The tests require the minimisation of a line integral along the magnetic field lines. For 1-D equilibria, this can be performed analytically, and simple stability criteria are obtained. The necessary condition then serves as an extended Suydam criterion, incorporating the stabilising effect of line-tying. For 2-D equilibria, the minimisation requires the integration of a system of ordinary differential equations along the field lines. This stability technique is applied to arcade, loop, and prominence models, yielding tight bounds on the equilibrium parameters. Secondly, global modes in 1-D coronal loops are investigated using a normal mode method, in order to clarify their link with localised interchange modes. For nearly force-free fields it is shown that instability to localised modes implies the existence of a fast growing global kink mode driven in the neighbourhood of the radius predicted by the local analysis. This confers a new significance on the study of localised interchange modes and the associated extended Suydam criterion.Hysteresis and mode competition in Faraday wavesDecent, Stephen Paulhttp://hdl.handle.net/10023/140542018-07-26T09:21:00Z1996-01-01T00:00:00ZFaraday waves arise on the surface of a liquid in a container that is undergoing vertical periodic oscillations. We investigate two-dimensional Faraday waves in a long rectangular container, both theoretically and experimentally. Hysteresis occurs when both finite amplitude solutions and the flat surface solution are available. We derive a nonlinear model of a standing wave, extending the Lagrangian method of Miles (1976). The model is used to investigate hysteresis. It is found necessary to retain cubic damping, cubic forcing and the fifth-order conservative term in order to achieve agreement with experiments. The fifth-order conservative term was omitted from all previous studies of Faraday waves. Stable limit cycles are found to arise from this single-mode equation. We examine the structure of this new solution in detail, both analytically and numerically. We describe local bifurcations using a multiple time scales analysis and global bifurcations using Melnikov's method. The coefficients of linear and cubic damping are derived for a standing wave in a rectangular container by considering energy dissipation in the main body of the fluid (due to potential flow and streaming) and in boundary layers at the sidewalls and at the surface. Surface contamination, due to the presence of a thin viscoelastic surface film, creates a boundary layer at the surface which causes enhanced dissipation comparable to, or greater than, that caused by the boundary layers at the walls of the container. Three-mode interaction equations are used to model intermittency and complex modulations which are found to arise from a sideband instability mechanism similar to that of Eckhaus (1963) and Benjamin & Feir (1967). The role of cubic and fifth-order nonlinear terms on this instability mechanism is examined. Theoretical results are found to compare quite favourably with experimental data.
1996-01-01T00:00:00ZDecent, Stephen PaulFaraday waves arise on the surface of a liquid in a container that is undergoing vertical periodic oscillations. We investigate two-dimensional Faraday waves in a long rectangular container, both theoretically and experimentally. Hysteresis occurs when both finite amplitude solutions and the flat surface solution are available. We derive a nonlinear model of a standing wave, extending the Lagrangian method of Miles (1976). The model is used to investigate hysteresis. It is found necessary to retain cubic damping, cubic forcing and the fifth-order conservative term in order to achieve agreement with experiments. The fifth-order conservative term was omitted from all previous studies of Faraday waves. Stable limit cycles are found to arise from this single-mode equation. We examine the structure of this new solution in detail, both analytically and numerically. We describe local bifurcations using a multiple time scales analysis and global bifurcations using Melnikov's method. The coefficients of linear and cubic damping are derived for a standing wave in a rectangular container by considering energy dissipation in the main body of the fluid (due to potential flow and streaming) and in boundary layers at the sidewalls and at the surface. Surface contamination, due to the presence of a thin viscoelastic surface film, creates a boundary layer at the surface which causes enhanced dissipation comparable to, or greater than, that caused by the boundary layers at the walls of the container. Three-mode interaction equations are used to model intermittency and complex modulations which are found to arise from a sideband instability mechanism similar to that of Eckhaus (1963) and Benjamin & Feir (1967). The role of cubic and fifth-order nonlinear terms on this instability mechanism is examined. Theoretical results are found to compare quite favourably with experimental data.The influence of thermal and magnetic layers on solar oscillation frequenciesDaniell, Markhttp://hdl.handle.net/10023/140512018-06-14T23:20:28Z1998-01-01T00:00:00ZIn this thesis, a study is made of the global solar oscillations known as p-modes, modelled by a plane-parallel stratified plasma, within which is embedded a horizontal layered magnetic field. A magnetohydrodynamic formalism is used to investigate the models. The main aim of the thesis is to model the turnover effect in the frequency shifts of the p-modes observed over the course of the solar cycle. Radial oscillations (modes of degree zero) of the Sun are studied for several atmospheric temperature and magnetic field profiles. It is found that the turnover in frequency shifts may be obtained by an increase in the strength of the atmospheric horizontal magnetic field (assumed to be uniform), coupled with a simultaneous increase in atmospheric temperature. The effect of a thin superadiabatic layer in the upper convection zone on p-mode frequencies is also considered. For this model we study modes of general degree, and find that the observed rise and subsequent downturn in the frequency shifts can be duplicated, in the absence of a magnetic field, by simultaneously steepening the temperature gradient of the superadiabatic layer and increasing the atmospheric temperature. In the presence of a magnetic field, where the atmosphere is permeated by a uniform horizontal magnetic field, turnover is reproduced by a combination of an increase in magnetic field strength, a steepening of the temperature gradient in the superadiabatic region, and an increase in atmospheric temperature. The unstable superadiabatic layer also gives rise to convective modes, which are considered briefly. Finally, a model incorporating a magnetic layer residing at the base of the convection zone is constructed and its influence on the frequencies of p-modes assessed. By simply changing the magnetic field strength of this layer, we are unable to reproduce the observed solar cycle variations in p-mode frequencies. The buried magnetic layer supports surface and body magnetoacoustic waves, and a brief study is made of their properties.
1998-01-01T00:00:00ZDaniell, MarkIn this thesis, a study is made of the global solar oscillations known as p-modes, modelled by a plane-parallel stratified plasma, within which is embedded a horizontal layered magnetic field. A magnetohydrodynamic formalism is used to investigate the models. The main aim of the thesis is to model the turnover effect in the frequency shifts of the p-modes observed over the course of the solar cycle. Radial oscillations (modes of degree zero) of the Sun are studied for several atmospheric temperature and magnetic field profiles. It is found that the turnover in frequency shifts may be obtained by an increase in the strength of the atmospheric horizontal magnetic field (assumed to be uniform), coupled with a simultaneous increase in atmospheric temperature. The effect of a thin superadiabatic layer in the upper convection zone on p-mode frequencies is also considered. For this model we study modes of general degree, and find that the observed rise and subsequent downturn in the frequency shifts can be duplicated, in the absence of a magnetic field, by simultaneously steepening the temperature gradient of the superadiabatic layer and increasing the atmospheric temperature. In the presence of a magnetic field, where the atmosphere is permeated by a uniform horizontal magnetic field, turnover is reproduced by a combination of an increase in magnetic field strength, a steepening of the temperature gradient in the superadiabatic region, and an increase in atmospheric temperature. The unstable superadiabatic layer also gives rise to convective modes, which are considered briefly. Finally, a model incorporating a magnetic layer residing at the base of the convection zone is constructed and its influence on the frequencies of p-modes assessed. By simply changing the magnetic field strength of this layer, we are unable to reproduce the observed solar cycle variations in p-mode frequencies. The buried magnetic layer supports surface and body magnetoacoustic waves, and a brief study is made of their properties.Energy-balance models of the solar coronaWragg, M. A.http://hdl.handle.net/10023/140472018-06-14T23:20:29Z1982-01-01T00:00:00ZSolar coronal observations have shown that the corona has a highly complex structure which presumably owes its existence to the magnetic field. Models in thermal and hydrostatic equilibrium are here calculated in order to try and explain many of these observations. Coronal holes occur where open field lines reach out into space. The model of McWhirter, et al. (1975) for the inner corona in such a configuration is generalised to allow different types and magnitudes of heating as well as different area divergences and flows. It is found that hot, fast upflows cannot always exist in thermal equilibrium. The choice of boundary conditions can appreciably alter the results, and so different choices are compared. Most of the corona, especially in active regions, appears to consist of coronal loops. Subtle relations for energy balance models of such loops are found to exist between the physical parameters of a loop's length, base density, and heat input. No solution exists at coronal temperatures in certain cases, which may explain the observations of very cool loops. The effect of a loop's geometry and field line divergence on the structure is found. Results predicted from scaling laws are compared, and the uniqueness of the solution for a loop with a fixed mass is studied. The error in the predicted emission measure through assuming uniform pressure is shown to be considerable. The life-time of a loop can often be many days, suggesting the existence of a thermally stable state. A global stability analysis is performed, and it is found that a loop's stability may depend critically upon its length. Thermally isolated loops, which are the most unstable type, can be thermally stable, provided their pressure falls off sufficiently rapidly with height (due to hydrostatic equilibrium).
1982-01-01T00:00:00ZWragg, M. A.Solar coronal observations have shown that the corona has a highly complex structure which presumably owes its existence to the magnetic field. Models in thermal and hydrostatic equilibrium are here calculated in order to try and explain many of these observations. Coronal holes occur where open field lines reach out into space. The model of McWhirter, et al. (1975) for the inner corona in such a configuration is generalised to allow different types and magnitudes of heating as well as different area divergences and flows. It is found that hot, fast upflows cannot always exist in thermal equilibrium. The choice of boundary conditions can appreciably alter the results, and so different choices are compared. Most of the corona, especially in active regions, appears to consist of coronal loops. Subtle relations for energy balance models of such loops are found to exist between the physical parameters of a loop's length, base density, and heat input. No solution exists at coronal temperatures in certain cases, which may explain the observations of very cool loops. The effect of a loop's geometry and field line divergence on the structure is found. Results predicted from scaling laws are compared, and the uniqueness of the solution for a loop with a fixed mass is studied. The error in the predicted emission measure through assuming uniform pressure is shown to be considerable. The life-time of a loop can often be many days, suggesting the existence of a thermally stable state. A global stability analysis is performed, and it is found that a loop's stability may depend critically upon its length. Thermally isolated loops, which are the most unstable type, can be thermally stable, provided their pressure falls off sufficiently rapidly with height (due to hydrostatic equilibrium).Three-dimensional topology of solar coronal magnetic fieldsBrown, Daniel Stephenhttp://hdl.handle.net/10023/140362018-07-26T09:22:30Z1999-01-01T00:00:00ZThis thesis investigates the topology of the magnetic field in the solar corona. It is important have an understanding of how the highly complex coronal magnetic field behaves in order to study many fundamental coronal phenomena, such as coronal heating events, solar flares and polar plumes. The magnetic fields due to three or four discrete sources are investigated and the corresponding topological states are found. The locations of these states in parameter space is calculated and the bifurcations between states are analysed. A complete analysis has been undertaken for the three-source case and a selective one for the four-source case in order to identify new non-generic behaviour. The thesis goes on to study the topological behaviour of a coronal bright point. Different phases during the lifetime of the bright point are identified and the responsible topological behaviour due to the movement of the magnetic fragments in the photosphere is discussed.
1999-01-01T00:00:00ZBrown, Daniel StephenThis thesis investigates the topology of the magnetic field in the solar corona. It is important have an understanding of how the highly complex coronal magnetic field behaves in order to study many fundamental coronal phenomena, such as coronal heating events, solar flares and polar plumes. The magnetic fields due to three or four discrete sources are investigated and the corresponding topological states are found. The locations of these states in parameter space is calculated and the bifurcations between states are analysed. A complete analysis has been undertaken for the three-source case and a selective one for the four-source case in order to identify new non-generic behaviour. The thesis goes on to study the topological behaviour of a coronal bright point. Different phases during the lifetime of the bright point are identified and the responsible topological behaviour due to the movement of the magnetic fragments in the photosphere is discussed.External and internal magnetohydrostatic models of quiescent solar prominencesCartledge, Nicholas P.http://hdl.handle.net/10023/140292018-06-13T23:19:57Z1996-01-01T00:00:00ZQuiescent solar prominences are amongst the most interesting and yet least understood of the phenomena observed on the Sun and provide both the theorist and the observer with equally demanding challenges. The theoretical study of prominences is an important branch of solar physics as it contributes significantly to the overall understanding of the Sun and its atmosphere. One only needs to be presented with the illuminating fact that there is more mass contained in these bodies than in the remainder of the entire corona to be convinced of their importance. Although many of the physical mechanisms associated with prominence theory are important in their own right, they are also of much wider relevance for various other astrophysical phenomena. For example, radiative and magnetic instabilities are explored in detail in the context of solar prominences; yet clearly these are important processes that relate to many other branches of astrophysics. Prominences are intimately associated with solar flares which occur when a prominence loses equilibrium. Also, prominence eruptions are very important as they are closely connected with coronal mass ejections. These account for a large fraction of the total mass lost from the Sun and so are extremely important events, particularly when one considers the consequences as this plasma interacts with the Earth's environment. It is the period of global equilibrium of quiescent prominences, though, that is the focus of this thesis. Various models are proposed to help understand both the topology and supporting mechanisms of the external, coronal magnetic field, and also the internal prominence structure and the way in which the two regimes fit together. In Chapter 3 we extend a model for the equilibrium of a prominence sheet in a twisted magnetic flux-tube, given by Ridgway, Priest and Amari (1991), to incorporate a current sheet of finite height. This removes the discontinuity at the edge of the tube and provides a shear-free outer boundary which enables the tube to be matched onto a background potential field. In addition, internal prominence solutions are found by expanding the sheet to a finite width and matching suitable magnetic profiles across this region. Next we consider a global model for the magnetic field structure surrounding a polar-crown prominence. We examine potential configurations generated from typical distributions of photospheric flux, and select solutions for which there is a location of dipped magnetic field where prominence material may collect and form. Once such a configuration is available, it is necessary to construct the ensuing prominence solution. We achieve this in Chapter 4 by considering a simplified form for the photospheric field. We show that the equilibrium contains a weighted, curved prominence sheet supported in the location of dipped magnetic field. The equilibrium requires an enhanced magnetic pressure below the sheet to support the component of weight in the normal direction. The internal equilibrium of curved or inclined prominence material has not been considered previously and so we formulate, in Chapter 6, a simple one-dimensional isothermal solution for a cut across the prominence. This is developed to allow for variations along the sheet and in this way an internal solution for the curved prominence of Chapter 4 is given, which matches onto the external potential polar-crown field. Finally, in Chapter 7, we rewrite this solution in terms of its constituent internal and external components and show how the composite solution switches between the two in a region of overlap, or transition region. From this, the internal plasma properties are deduced and realistic profiles for the pressure, density and temperature are obtained.
1996-01-01T00:00:00ZCartledge, Nicholas P.Quiescent solar prominences are amongst the most interesting and yet least understood of the phenomena observed on the Sun and provide both the theorist and the observer with equally demanding challenges. The theoretical study of prominences is an important branch of solar physics as it contributes significantly to the overall understanding of the Sun and its atmosphere. One only needs to be presented with the illuminating fact that there is more mass contained in these bodies than in the remainder of the entire corona to be convinced of their importance. Although many of the physical mechanisms associated with prominence theory are important in their own right, they are also of much wider relevance for various other astrophysical phenomena. For example, radiative and magnetic instabilities are explored in detail in the context of solar prominences; yet clearly these are important processes that relate to many other branches of astrophysics. Prominences are intimately associated with solar flares which occur when a prominence loses equilibrium. Also, prominence eruptions are very important as they are closely connected with coronal mass ejections. These account for a large fraction of the total mass lost from the Sun and so are extremely important events, particularly when one considers the consequences as this plasma interacts with the Earth's environment. It is the period of global equilibrium of quiescent prominences, though, that is the focus of this thesis. Various models are proposed to help understand both the topology and supporting mechanisms of the external, coronal magnetic field, and also the internal prominence structure and the way in which the two regimes fit together. In Chapter 3 we extend a model for the equilibrium of a prominence sheet in a twisted magnetic flux-tube, given by Ridgway, Priest and Amari (1991), to incorporate a current sheet of finite height. This removes the discontinuity at the edge of the tube and provides a shear-free outer boundary which enables the tube to be matched onto a background potential field. In addition, internal prominence solutions are found by expanding the sheet to a finite width and matching suitable magnetic profiles across this region. Next we consider a global model for the magnetic field structure surrounding a polar-crown prominence. We examine potential configurations generated from typical distributions of photospheric flux, and select solutions for which there is a location of dipped magnetic field where prominence material may collect and form. Once such a configuration is available, it is necessary to construct the ensuing prominence solution. We achieve this in Chapter 4 by considering a simplified form for the photospheric field. We show that the equilibrium contains a weighted, curved prominence sheet supported in the location of dipped magnetic field. The equilibrium requires an enhanced magnetic pressure below the sheet to support the component of weight in the normal direction. The internal equilibrium of curved or inclined prominence material has not been considered previously and so we formulate, in Chapter 6, a simple one-dimensional isothermal solution for a cut across the prominence. This is developed to allow for variations along the sheet and in this way an internal solution for the curved prominence of Chapter 4 is given, which matches onto the external potential polar-crown field. Finally, in Chapter 7, we rewrite this solution in terms of its constituent internal and external components and show how the composite solution switches between the two in a region of overlap, or transition region. From this, the internal plasma properties are deduced and realistic profiles for the pressure, density and temperature are obtained.Dynamical processes in the solar atmosphereCargill, P. (Peter)http://hdl.handle.net/10023/140242018-06-13T23:18:51Z1982-01-01T00:00:00ZIt has become clear that the closed-field regions of the solar atmosphere are not static (as was once thought) but that many types of steady and unsteady flows and other dynamical, processes such as flares are continually occurring, in them. This thesis investigates some theoretical aspects of these dynamical phenomena. Steady, one-dimensional flow along a coronal loop is investigated first of all. Such a flow may be driven by a pressure difference between the foot points, and a wide range of shocked and unshocked flows are found. The presence of steady flows removes the symmetry present in most static loop models, and these models are shown to form only one class of a much wider range of dynamic solutions to the equations of motion. Thermal non-equilibrium in hot coronal loops occurs if the pressure in a loop becomes too big. The non-linear evolution of this non-equilibrium state is followed, and the loop is found to cool from of order 10[super]6 K to below 10[super]5 K in a few hours. An upflow is driven, and non-equilibrium is suggested as a means of formation of either cool loop cores or prominences. Thermal non-equilibrium is also discussed as a possible mechanism for the simple-loop flare. It is suggested that a cool equilibrium at a temperature of a few times 10[super]4 K can flare to over. 10[super]7 K if the mechanical heating in the cool loop becomes too large. The evolution is followed and the loop is found to flare to over 10[super]7 K in approximately 5 minutes. Magnetohydrodynamic shock waves have long been regarded as a potentially efficient heating mechanism. Their behaviour is re-examined here, and it is found that certain types of shock can release very large amounts of energy. These results are then applied to the heating of "post"-flare loops, for which temperatures of 10[super]7 K at the loop summit may be obtained. Finally, some solutions of the magnetostatic equation are discussed, and it is pointed out that if the gas pressure is too big then magnetostatic equilibrium will break down. It is suggested that the subsequent evolution may give rise to a surge or other mass ejection.
1982-01-01T00:00:00ZCargill, P. (Peter)It has become clear that the closed-field regions of the solar atmosphere are not static (as was once thought) but that many types of steady and unsteady flows and other dynamical, processes such as flares are continually occurring, in them. This thesis investigates some theoretical aspects of these dynamical phenomena. Steady, one-dimensional flow along a coronal loop is investigated first of all. Such a flow may be driven by a pressure difference between the foot points, and a wide range of shocked and unshocked flows are found. The presence of steady flows removes the symmetry present in most static loop models, and these models are shown to form only one class of a much wider range of dynamic solutions to the equations of motion. Thermal non-equilibrium in hot coronal loops occurs if the pressure in a loop becomes too big. The non-linear evolution of this non-equilibrium state is followed, and the loop is found to cool from of order 10[super]6 K to below 10[super]5 K in a few hours. An upflow is driven, and non-equilibrium is suggested as a means of formation of either cool loop cores or prominences. Thermal non-equilibrium is also discussed as a possible mechanism for the simple-loop flare. It is suggested that a cool equilibrium at a temperature of a few times 10[super]4 K can flare to over. 10[super]7 K if the mechanical heating in the cool loop becomes too large. The evolution is followed and the loop is found to flare to over 10[super]7 K in approximately 5 minutes. Magnetohydrodynamic shock waves have long been regarded as a potentially efficient heating mechanism. Their behaviour is re-examined here, and it is found that certain types of shock can release very large amounts of energy. These results are then applied to the heating of "post"-flare loops, for which temperatures of 10[super]7 K at the loop summit may be obtained. Finally, some solutions of the magnetostatic equation are discussed, and it is pointed out that if the gas pressure is too big then magnetostatic equilibrium will break down. It is suggested that the subsequent evolution may give rise to a surge or other mass ejection.Topological configurations of coronal magnetic fields and current sheetsBungey, Timothy N.http://hdl.handle.net/10023/140212018-06-13T23:18:40Z1996-01-01T00:00:00ZThe question of topology in the coronal magnetic field is addressed in this thesis. Magnetic reconnection, which plays a major role in many of the fascinating phenomena seen in the solar atmosphere, is likely to occur at the boundaries between different topological regions of the magnetic field. By modelling the coronal field using discrete sources of flux, to represent the concentrations seen at the photospheric surface, we study the varying topological structures present in the field. We generate a criterion for determining the presence of null points above the photospheric surface and establish that any separatrix surfaces present in the field are due to the presence of either null points, or regions where the field tangentially grazes the surface. We follow the evolution of these separatrix surfaces and, in particular, determine the existence of a well-defined separator field line in the absence of coronal null points. Finally, we look locally at the configuration of the magnetic field in the region surrounding a straight current sheet. We derive an analytical expression to describe the topology of both potential and constant-current force-free fields in the neighbourhood of a sheet, and in so doing generalise the previously known expressions.
1996-01-01T00:00:00ZBungey, Timothy N.The question of topology in the coronal magnetic field is addressed in this thesis. Magnetic reconnection, which plays a major role in many of the fascinating phenomena seen in the solar atmosphere, is likely to occur at the boundaries between different topological regions of the magnetic field. By modelling the coronal field using discrete sources of flux, to represent the concentrations seen at the photospheric surface, we study the varying topological structures present in the field. We generate a criterion for determining the presence of null points above the photospheric surface and establish that any separatrix surfaces present in the field are due to the presence of either null points, or regions where the field tangentially grazes the surface. We follow the evolution of these separatrix surfaces and, in particular, determine the existence of a well-defined separator field line in the absence of coronal null points. Finally, we look locally at the configuration of the magnetic field in the region surrounding a straight current sheet. We derive an analytical expression to describe the topology of both potential and constant-current force-free fields in the neighbourhood of a sheet, and in so doing generalise the previously known expressions.Parametric instabilities in inhomogenous plasmasBegg, Iain M.http://hdl.handle.net/10023/140162018-06-13T23:19:31Z1976-01-01T00:00:00ZThis thesis will deal with certain problems of parametric instabilities in the inhomogeneous plasma. A large amplitude, 'pump' wave can deposit some of its energy into the plasma through resonance with two lower frequency waves (which may be damped). This type of process is a parametric decay of the pump wave and has applications in many fields. We consider, predominantly, that of laser fusion, in which the pump wave is electromagnetic and incident on the plasma. The objective is to deposit as much energy as possible within the plasma. Instabilities reducing this energy input are therefore of importance and it is, mostly, to these that this thesis will turn. They are mostly scattering processes in which one of the decay modes is electromagnetic. We examine the stimulated Brillouin backscattering process (the other decay mode being an ion accoustic wave) from a reference frame in which the plasma is streaming outwards. It is found that, if this velocity is near the sound velocity, the ion acoustic wave has a frequency Doppler-shifted to zero, the electromagnetic waves then having equal frequencies. In such a situation, any reflection of the pump wave at the critical surface will enhance the initial level of the backscattered wave. We find that, allowing for this, there is considerable enhancement of backscatter from the plasma, with consequent energy loss to the pump. Since the effect is noticeably unaffected by 'off- resonance' situations, it is felt that this process could mount a barrier to possible applications. We next consider the stimulated Compton scattering process, where the pump is scattered off the 'bare' or thermal electrons in the plasma. It is found that this rather weak instability occurs predominantly only when electron plasma waves are heavily dampled. Substantial reflection only occurs for high pump powers. Whilst there is little loss to the pump energy, there is substantial perturbation to the background distribution function. However, at the high powers involved filamentation and modulation of the pump can occur with a resulting enhancement of the scattering. Finally, we consider the effect on the decay instability (photon → plasmon + phonon) of the presence of substantial filamentation of the critical surface. It is found that the growth rate is substantially reduced.
1976-01-01T00:00:00ZBegg, Iain M.This thesis will deal with certain problems of parametric instabilities in the inhomogeneous plasma. A large amplitude, 'pump' wave can deposit some of its energy into the plasma through resonance with two lower frequency waves (which may be damped). This type of process is a parametric decay of the pump wave and has applications in many fields. We consider, predominantly, that of laser fusion, in which the pump wave is electromagnetic and incident on the plasma. The objective is to deposit as much energy as possible within the plasma. Instabilities reducing this energy input are therefore of importance and it is, mostly, to these that this thesis will turn. They are mostly scattering processes in which one of the decay modes is electromagnetic. We examine the stimulated Brillouin backscattering process (the other decay mode being an ion accoustic wave) from a reference frame in which the plasma is streaming outwards. It is found that, if this velocity is near the sound velocity, the ion acoustic wave has a frequency Doppler-shifted to zero, the electromagnetic waves then having equal frequencies. In such a situation, any reflection of the pump wave at the critical surface will enhance the initial level of the backscattered wave. We find that, allowing for this, there is considerable enhancement of backscatter from the plasma, with consequent energy loss to the pump. Since the effect is noticeably unaffected by 'off- resonance' situations, it is felt that this process could mount a barrier to possible applications. We next consider the stimulated Compton scattering process, where the pump is scattered off the 'bare' or thermal electrons in the plasma. It is found that this rather weak instability occurs predominantly only when electron plasma waves are heavily dampled. Substantial reflection only occurs for high pump powers. Whilst there is little loss to the pump energy, there is substantial perturbation to the background distribution function. However, at the high powers involved filamentation and modulation of the pump can occur with a resulting enhancement of the scattering. Finally, we consider the effect on the decay instability (photon → plasmon + phonon) of the presence of substantial filamentation of the critical surface. It is found that the growth rate is substantially reduced.Magnetic annihilation and reconnectionAnderson, Craighttp://hdl.handle.net/10023/140142018-06-13T23:18:39Z1994-01-01T00:00:00ZThis thesis presents several analytical models of magnetic annihilation and reconnection and studies their properties. The models investigated are 1. Steady-state magnetic annihilation. The assumption of straight field lines reduces the resistive, viscous MHD equations to two ordinary differential equations, one for the flow and one for the magnetic field. These equations can be solved exactly (for the case of a simple stagnation-point flow) and asymptotically (for a more general stagnation-point flow). In both cases the reconnection rates can be fast due to advection effects which create large magnetic gradients. 2. Time-dependent magnetic annihilation. The assumption of straight field lines whose strength can vary with time reduces the MHD equations to two partial differential equations, one for the flow and one for the magnetic field. The time-modulated simple stagnation-point flow is shown to be an exact solution and the equation for the magnetic field is then solved on infinite and finite intervals. For the infinite interval the reconnection rates are shown to be dependent on the nature of the advected initial field. Also examined are self-similar solutions and the effect of variation of diffusivity with time. 3. Annihilation in a compressible, inviscid plasma. Here, the assumption of straight field lines and an inviscid, compressible flow reduce the MHD equations to a pair of non-linear coupled partial differential equations. Further assuming that the density only varies in one direction and the flow is of a stagnation-point type allow these equations to be solved approximately analytically and exactly numerically. It is shown that the magnetic field and reconnection rates are the same in both the compressible and incompressible cases and that the density of the plasma is greatest within the current sheet. 4. Steady-state magnetic reconnection. For an incompressible flow the MHD equations can be reduced to two coupled non-linear partial differential equations. These two equations are studied by seeking asymptotic solutions around the annihilation solution and then looking for series solutions to the first-order equations. It is found that the magnetic field always has a magnetic cusp and never possesses an x-type neutral point. 5. Reconnection in a viscous plasma. Assuming that the viscous forces dominate, the induction equation and equation of motion decouple and become linear. The magnetic field is obtained for the case of a simple stagnation-point flow. It is shown that if the inflow magnetic field is taken to be straight then the magnetic field within the region tends towards the annihilation solution as the magnetic Reynolds number increases. 6. Magnetic flipping. A previous ideal model of magnetic flipping is refined so that it becomes an exact solution of the MHD equations. In the refined model the streamlines are straight rather than curved. Assuming straight streamlines, the MHD equations reduce to two linear ordinary differential equations, one for the flow and one for the magnetic field. These are then solved exactly analytically to find a flow containing a viscous boundary layer and a magnetic field that contains an x-type neutral point. The angle between the separatrices of the field is determined by the Reynolds and magnetic Reynolds numbers. It is shown that most of the ohmic heating occurs within the viscous boundary layer.
1994-01-01T00:00:00ZAnderson, CraigThis thesis presents several analytical models of magnetic annihilation and reconnection and studies their properties. The models investigated are 1. Steady-state magnetic annihilation. The assumption of straight field lines reduces the resistive, viscous MHD equations to two ordinary differential equations, one for the flow and one for the magnetic field. These equations can be solved exactly (for the case of a simple stagnation-point flow) and asymptotically (for a more general stagnation-point flow). In both cases the reconnection rates can be fast due to advection effects which create large magnetic gradients. 2. Time-dependent magnetic annihilation. The assumption of straight field lines whose strength can vary with time reduces the MHD equations to two partial differential equations, one for the flow and one for the magnetic field. The time-modulated simple stagnation-point flow is shown to be an exact solution and the equation for the magnetic field is then solved on infinite and finite intervals. For the infinite interval the reconnection rates are shown to be dependent on the nature of the advected initial field. Also examined are self-similar solutions and the effect of variation of diffusivity with time. 3. Annihilation in a compressible, inviscid plasma. Here, the assumption of straight field lines and an inviscid, compressible flow reduce the MHD equations to a pair of non-linear coupled partial differential equations. Further assuming that the density only varies in one direction and the flow is of a stagnation-point type allow these equations to be solved approximately analytically and exactly numerically. It is shown that the magnetic field and reconnection rates are the same in both the compressible and incompressible cases and that the density of the plasma is greatest within the current sheet. 4. Steady-state magnetic reconnection. For an incompressible flow the MHD equations can be reduced to two coupled non-linear partial differential equations. These two equations are studied by seeking asymptotic solutions around the annihilation solution and then looking for series solutions to the first-order equations. It is found that the magnetic field always has a magnetic cusp and never possesses an x-type neutral point. 5. Reconnection in a viscous plasma. Assuming that the viscous forces dominate, the induction equation and equation of motion decouple and become linear. The magnetic field is obtained for the case of a simple stagnation-point flow. It is shown that if the inflow magnetic field is taken to be straight then the magnetic field within the region tends towards the annihilation solution as the magnetic Reynolds number increases. 6. Magnetic flipping. A previous ideal model of magnetic flipping is refined so that it becomes an exact solution of the MHD equations. In the refined model the streamlines are straight rather than curved. Assuming straight streamlines, the MHD equations reduce to two linear ordinary differential equations, one for the flow and one for the magnetic field. These are then solved exactly analytically to find a flow containing a viscous boundary layer and a magnetic field that contains an x-type neutral point. The angle between the separatrices of the field is determined by the Reynolds and magnetic Reynolds numbers. It is shown that most of the ohmic heating occurs within the viscous boundary layer.Plasma drift waves and instabilitiesAllan, Williamhttp://hdl.handle.net/10023/140112018-06-13T23:18:44Z1974-01-01T00:00:00ZThe work of this thesis is concerned with the investigation of the propagation of waves in a magnetized plasma containing various parameter gradients, and with the stability of ion acoustic waves in a weakly collisional plasma with a strong temperature gradient. The thesis is divided into three sections. In the first section the intention is to derive in a compact and unambiguous tensor form the dispersion relation describing the propagation of waves in a magnetized plasma containing three-dimensional density and temperature gradients, an E̲⏜ B̲ drift, and differing temperatures parallel and perpendicular to the magnetic field. This is achieved by introducing and extending the polarized co-ordinate system first proposed by Buneman in 1961, and then carrying through the standard procedure of integration along unperturbed trajectories. The "local" approximation of Krall and Rosenbluth is used in order that an analytic result may be derived. The dispersion relation obtained includes certain moment tensors whose elements may be evaluated independently of the gradients involved in the problem. These elements may then be listed and the list referred to in order to obtain the elements required for a specific problem. The second section is concerned with the use of the theory and results of J.P. Dougherty to show that in the high-frequency regime the introduction of a small amount of collisions into a plasma is sufficient to disrupt the gyro-resonances which allow the existence of Bernstein waves at multiples of the gyro-frequencies perpendicular and near- perpendicular to the magnetic field. It is shown that a collision frequency v such that (k 𝜌) ⁻² ≲ v/Ω < (k 𝜌) ⁻¹ where k 𝜌 >> 1 is sufficient to do this; k is the wave-number, 𝜌 the Larmor radius, and the gyro-frequency. It is also shown that in this case the ion-acoustic dispersion relation is valid even for propagation perpendicular to the magnetic field. In the final section the result of the second section is used to derive a dispersion relation for high-frequency wave propagation in a weakly-collisional plasma containing an electron temperature gradient. The dispersion relation is solved numerically for various electron-ion temperature ratios and electron temperature gradient drift velocities. Earlier predictions, based on analytic calculations for small temperature ratios and drift velocities, are confirmed and some new results presented. In particular, it is shown that a temperature gradient is a more effective destabilizing agent then a simple drift between ions and electrons. Dispersion plots are given, along with analytic and physical explanations of their form; finally neutral stability curves are presented. The thesis concludes with a summary of the results obtained.
1974-01-01T00:00:00ZAllan, WilliamThe work of this thesis is concerned with the investigation of the propagation of waves in a magnetized plasma containing various parameter gradients, and with the stability of ion acoustic waves in a weakly collisional plasma with a strong temperature gradient. The thesis is divided into three sections. In the first section the intention is to derive in a compact and unambiguous tensor form the dispersion relation describing the propagation of waves in a magnetized plasma containing three-dimensional density and temperature gradients, an E̲⏜ B̲ drift, and differing temperatures parallel and perpendicular to the magnetic field. This is achieved by introducing and extending the polarized co-ordinate system first proposed by Buneman in 1961, and then carrying through the standard procedure of integration along unperturbed trajectories. The "local" approximation of Krall and Rosenbluth is used in order that an analytic result may be derived. The dispersion relation obtained includes certain moment tensors whose elements may be evaluated independently of the gradients involved in the problem. These elements may then be listed and the list referred to in order to obtain the elements required for a specific problem. The second section is concerned with the use of the theory and results of J.P. Dougherty to show that in the high-frequency regime the introduction of a small amount of collisions into a plasma is sufficient to disrupt the gyro-resonances which allow the existence of Bernstein waves at multiples of the gyro-frequencies perpendicular and near- perpendicular to the magnetic field. It is shown that a collision frequency v such that (k 𝜌) ⁻² ≲ v/Ω < (k 𝜌) ⁻¹ where k 𝜌 >> 1 is sufficient to do this; k is the wave-number, 𝜌 the Larmor radius, and the gyro-frequency. It is also shown that in this case the ion-acoustic dispersion relation is valid even for propagation perpendicular to the magnetic field. In the final section the result of the second section is used to derive a dispersion relation for high-frequency wave propagation in a weakly-collisional plasma containing an electron temperature gradient. The dispersion relation is solved numerically for various electron-ion temperature ratios and electron temperature gradient drift velocities. Earlier predictions, based on analytic calculations for small temperature ratios and drift velocities, are confirmed and some new results presented. In particular, it is shown that a temperature gradient is a more effective destabilizing agent then a simple drift between ions and electrons. Dispersion plots are given, along with analytic and physical explanations of their form; finally neutral stability curves are presented. The thesis concludes with a summary of the results obtained.Aspects of current sheet theoryTur, T. J.http://hdl.handle.net/10023/140002018-06-13T23:19:42Z1977-01-01T00:00:00ZCurrent sheets are widely believed to play an important role in astrophysics when regions of magnetic flux are in motion. Several models based on the formation of current sheets have been proposed to explain such phenomena as geomagnetic storms, solar flares and prominences. In this thesis three aspects of current sheet theory are studied with particular reference to the solar flare problem. Firstly the development of two-dimensional current sheets is investigated for several simple configurations. These include converging line current sources, converging and diverging line dipole sources and a dipole of increasing moment situated in either a uniform magnetic field or a constant dipole field. These last two may be thought of as modelling the emergence of bipolar flux from beneath the photosphere, a phenomena frequently observed prior to solar flares. The length, position and shape of the current sheet is determined from the requirement that the magnetic field be frozen-into the plasma. The sheet is found to be curved, except in the symmetrical case of converging line sources. In addition, the extra energy due to the presence of the current sheet is determined. Comparison with estimates of the energy dissipated during a flare indicate that the formation of current sheets may store an adequate amount of preflare magnetic energy, provided no reconnection occurs during the formation process. A three-dimensional axi-symmetric model for current sheet formation is then considered. Two equal and co-directional dipoles approach along the axis of symmetry to form an annular current sheet between them. The equations determining the magnetic field for this configuration are reduced to a single integral equation for the current density in the sheet as a function of radial distance from the axis. A numerical method is used to solve this integral equation. The inner and outer radii of the sheet are then determined from the conditions of flux conservation as for the two-dimensional case. Finally the energetics of a current sheet that forms between newly emerging flux and an ambient field are considered. As more and more flux emerges, so the sheet rises in the solar atmosphere. The various contributions to the thermal energy balance in the sheet are approximated and the resulting equation is solved for the internal temperature of the sheet. It is found that, for certain choices of the ambient magnetic field strength and velocity, the internal temperature increases until, when the sheet reaches some critical height, no neighbouring stable state exists. The temperature then increases rapidly seeking a hotter branch of the solution curve. During this dynamic heating the threshold temperature for the onset of microinstabilities may be attained. It is suggested that this may be a suitable trigger mechanism for the recently proposed "emerging flux" model of a solar flare.
1977-01-01T00:00:00ZTur, T. J.Current sheets are widely believed to play an important role in astrophysics when regions of magnetic flux are in motion. Several models based on the formation of current sheets have been proposed to explain such phenomena as geomagnetic storms, solar flares and prominences. In this thesis three aspects of current sheet theory are studied with particular reference to the solar flare problem. Firstly the development of two-dimensional current sheets is investigated for several simple configurations. These include converging line current sources, converging and diverging line dipole sources and a dipole of increasing moment situated in either a uniform magnetic field or a constant dipole field. These last two may be thought of as modelling the emergence of bipolar flux from beneath the photosphere, a phenomena frequently observed prior to solar flares. The length, position and shape of the current sheet is determined from the requirement that the magnetic field be frozen-into the plasma. The sheet is found to be curved, except in the symmetrical case of converging line sources. In addition, the extra energy due to the presence of the current sheet is determined. Comparison with estimates of the energy dissipated during a flare indicate that the formation of current sheets may store an adequate amount of preflare magnetic energy, provided no reconnection occurs during the formation process. A three-dimensional axi-symmetric model for current sheet formation is then considered. Two equal and co-directional dipoles approach along the axis of symmetry to form an annular current sheet between them. The equations determining the magnetic field for this configuration are reduced to a single integral equation for the current density in the sheet as a function of radial distance from the axis. A numerical method is used to solve this integral equation. The inner and outer radii of the sheet are then determined from the conditions of flux conservation as for the two-dimensional case. Finally the energetics of a current sheet that forms between newly emerging flux and an ambient field are considered. As more and more flux emerges, so the sheet rises in the solar atmosphere. The various contributions to the thermal energy balance in the sheet are approximated and the resulting equation is solved for the internal temperature of the sheet. It is found that, for certain choices of the ambient magnetic field strength and velocity, the internal temperature increases until, when the sheet reaches some critical height, no neighbouring stable state exists. The temperature then increases rapidly seeking a hotter branch of the solution curve. During this dynamic heating the threshold temperature for the onset of microinstabilities may be attained. It is suggested that this may be a suitable trigger mechanism for the recently proposed "emerging flux" model of a solar flare.Thermal and resistive instabilities in the solar atmosphereSmith, E. A.http://hdl.handle.net/10023/139982018-06-13T23:19:37Z1977-01-01T00:00:00ZThe magnetic field greatly influences the plasma in the solar atmosphere and in this thesis we consider the effect of the field on the stability of the plasma. The many observations that have been made suggest that two types of field structure play a major role. Firstly a current sheet - this has field lines which change direction in a thin, current forming region, but are fairly uniform outside. We consider the case where the field strength is zero along the neutral line so that a gas pressure gradient is required across the sheet to balance the magnetic pressure gradient. Secondly a force-free field - here the magnetic force is zero, which requires the magnetic pressure to be much larger than the gas pressure. In the neutral current sheet we examine the thermal instability and the tearing-mode instability. While in the force-free magnetic arch system we look for a thermal instability which can occur when the foot points of the arch are sheared. When we investigated the thermal stability of the current sheet we found that as its length increases it passes through a series of stable equilibria until a value, L[sub]max, is reached when the sheet cools down to a max new stable equilibrium. For coronal conditions, values for L[sub]max and max cooling time are in fair agreement with the observed values for quiescent prominences. We calculate the growth rate of the tearing-mode instability in a neutral current sheet with no energy sources or sinks and find that the maximum growth rate can be significantly larger in the current sheet than in the sheared field of constant magnitude considered by others. Also the growth rate decreases when the ratio of gas to magnetic pressure is reduced. We find that the growth rate is significantly inhibited if the current sheet has a transverse magnetic field which is large enough. Lastly we examine the thermal balance in a sheared, force-free magnetic field and show that thermal instability can occur if the field is sheared enough. We assume thermal equilibrium between radiative loss and thermal conduction and we take gravity balanced by a pressure gradient. If, for example, the density at the base of the field is ten times larger than the normal coronal value, as it may be in coronal condensations, then there is instability if the shear angle is greater than 63 °. The presence of a large enough mechanical heating is found to prevent the instability occurring.
1977-01-01T00:00:00ZSmith, E. A.The magnetic field greatly influences the plasma in the solar atmosphere and in this thesis we consider the effect of the field on the stability of the plasma. The many observations that have been made suggest that two types of field structure play a major role. Firstly a current sheet - this has field lines which change direction in a thin, current forming region, but are fairly uniform outside. We consider the case where the field strength is zero along the neutral line so that a gas pressure gradient is required across the sheet to balance the magnetic pressure gradient. Secondly a force-free field - here the magnetic force is zero, which requires the magnetic pressure to be much larger than the gas pressure. In the neutral current sheet we examine the thermal instability and the tearing-mode instability. While in the force-free magnetic arch system we look for a thermal instability which can occur when the foot points of the arch are sheared. When we investigated the thermal stability of the current sheet we found that as its length increases it passes through a series of stable equilibria until a value, L[sub]max, is reached when the sheet cools down to a max new stable equilibrium. For coronal conditions, values for L[sub]max and max cooling time are in fair agreement with the observed values for quiescent prominences. We calculate the growth rate of the tearing-mode instability in a neutral current sheet with no energy sources or sinks and find that the maximum growth rate can be significantly larger in the current sheet than in the sheared field of constant magnitude considered by others. Also the growth rate decreases when the ratio of gas to magnetic pressure is reduced. We find that the growth rate is significantly inhibited if the current sheet has a transverse magnetic field which is large enough. Lastly we examine the thermal balance in a sheared, force-free magnetic field and show that thermal instability can occur if the field is sheared enough. We assume thermal equilibrium between radiative loss and thermal conduction and we take gravity balanced by a pressure gradient. If, for example, the density at the base of the field is ten times larger than the normal coronal value, as it may be in coronal condensations, then there is instability if the shear angle is greater than 63 °. The presence of a large enough mechanical heating is found to prevent the instability occurring.Numerical studies of the Fokker-Planck equationMcGowan, Alastair Davidhttp://hdl.handle.net/10023/139952018-06-13T23:19:58Z1992-01-01T00:00:00ZJorna and Wood recently developed a program that numerically solved the Fokker-Planck equation in spherical geometry. In this thesis, we describe how the original program has been redeveloped to produce a program that is an order of magnitude quicker and that has superior energy and density conservation. The revised version of the program has been used to extend the work of Jorna and Wood on thermal conduction in laser produced plasmas. It has been shown that the effect of curvature on heat flow can be described from a purely geometrical argument and that for aspect ratios similar to those found in targets, the heat flow is reduced by approximately 10%. Also, it has been shown, in contradiction with Jorna and Wood, that the inclusion of the anisotropic portion of the Rosenbluth potentials does not have a significant effect on the heat flow. Even for highly anisotropic plasmas, the inclusion of the anisotropic portion only increases the heat flow by 10%. In addition, the revised version of the program has been used to study the energy relaxation of model distributions It has been shown that the relaxation time of most non - thermal distributions depends on the detailed structure of the distribution and that the normal Spitzer collision time can under-estimate or over-estimate the time required for energy relaxation.
1992-01-01T00:00:00ZMcGowan, Alastair DavidJorna and Wood recently developed a program that numerically solved the Fokker-Planck equation in spherical geometry. In this thesis, we describe how the original program has been redeveloped to produce a program that is an order of magnitude quicker and that has superior energy and density conservation. The revised version of the program has been used to extend the work of Jorna and Wood on thermal conduction in laser produced plasmas. It has been shown that the effect of curvature on heat flow can be described from a purely geometrical argument and that for aspect ratios similar to those found in targets, the heat flow is reduced by approximately 10%. Also, it has been shown, in contradiction with Jorna and Wood, that the inclusion of the anisotropic portion of the Rosenbluth potentials does not have a significant effect on the heat flow. Even for highly anisotropic plasmas, the inclusion of the anisotropic portion only increases the heat flow by 10%. In addition, the revised version of the program has been used to study the energy relaxation of model distributions It has been shown that the relaxation time of most non - thermal distributions depends on the detailed structure of the distribution and that the normal Spitzer collision time can under-estimate or over-estimate the time required for energy relaxation.Nonlinear plasma waves and their applicationsAmin, Mohamed Ruhulhttp://hdl.handle.net/10023/139932018-06-13T23:19:50Z1999-01-01T00:00:00ZThe possibility of beat wave current drive in tokamaks is considered in this thesis in steady state 2D geometry. The problem is considered by including in the analysis the 2D toroidal inhomogeneity effect and the effect of finite spatial width of the pump microwave pulses on the beat wave excitation. Both a Langmuir beat wave as well as an obliquely propagating upper-hybrid cyclotron beat wave are considered in this study. The three wave coupled system of equations in a magnetized plasma has been derived and solved numerically for this purpose. It has been found that Langmuir type beat wave excited by two almost antiparallel pump microwaves is more efficient for action transfer than a cyclotron beat wave. It has also been found that for the same input parameters, right hand polarized pumps are more efficient than left hand polarized pump microwaves for depositing power in the beat wave. The second part of the thesis considers the relativistic excitation mechanism of a large amplitude plasma wake field by a single ultra-short laser pulse. This type of large amplitude wake field has been proposed for particle acceleration to very high energies for future generation of accelerators. The problem has been modeled self consistently in ID geometry and the relevant coupled system of equations have been solved numerically. It has been found that the shape of the laser pulse profile and the ratio of the ambient plasma frequency to the incident laser frequency play an important role for the excitation of the wake-field and the stability of the laser pulse profile.
1999-01-01T00:00:00ZAmin, Mohamed RuhulThe possibility of beat wave current drive in tokamaks is considered in this thesis in steady state 2D geometry. The problem is considered by including in the analysis the 2D toroidal inhomogeneity effect and the effect of finite spatial width of the pump microwave pulses on the beat wave excitation. Both a Langmuir beat wave as well as an obliquely propagating upper-hybrid cyclotron beat wave are considered in this study. The three wave coupled system of equations in a magnetized plasma has been derived and solved numerically for this purpose. It has been found that Langmuir type beat wave excited by two almost antiparallel pump microwaves is more efficient for action transfer than a cyclotron beat wave. It has also been found that for the same input parameters, right hand polarized pumps are more efficient than left hand polarized pump microwaves for depositing power in the beat wave. The second part of the thesis considers the relativistic excitation mechanism of a large amplitude plasma wake field by a single ultra-short laser pulse. This type of large amplitude wake field has been proposed for particle acceleration to very high energies for future generation of accelerators. The problem has been modeled self consistently in ID geometry and the relevant coupled system of equations have been solved numerically. It has been found that the shape of the laser pulse profile and the ratio of the ambient plasma frequency to the incident laser frequency play an important role for the excitation of the wake-field and the stability of the laser pulse profile.The effects of the Kelvin-Helmholtz instability of the magnetosphereMills, Katharine J.http://hdl.handle.net/10023/139902018-06-13T23:18:53Z1999-01-01T00:00:00ZIn this thesis, the behaviour of Kelvin-Helmholtz unstable modes on the magnetospheric flanks and in the magnetotail are investigated. A model of a straight bounded magnetosphere connected to a semi-infinite field-free magnetosheath which is flowing with a uniform speed is used. First the magnetosphere is taken to be uniform with the magnetic field perpendicular to the flow in the magnetosheath and it is shown that the increase in Pc5 wave power observed for high solar wind flow speeds correlates well with the onset of instability of the fast body modes. A condition for the exact onset of instability of these modes is derived and the behaviour of fast surface and slow body and surface modes is also investigated. Using a non-uniform magnetosphere, it is shown that these unstable body modes may couple to field line resonances. The fastest growing modes are found to have a common azimuthal phase speed which depends only on the local conditions at the magnetopause and may be predicted using the theory of over-reflection. A finite width boundary layer is then added to the uniform magnetosphere model to investigate the space-time evolution of wave-packets on the magnetopause. Fast surface mode wave-packets are found to grow rapidly as they convect around the flanks so that non-linear effects will be important. Fast cavity mode wave-packets will remain relatively small on the flanks, explaining the robustness of the body of the magnetosphere here. Slow modes are found to grow very little in this region. Finally, a uniform magnetosphere with the magnetic field parallel to the flow in the magnetosheath is considered. Here, the fast modes are unlikely to be Kelvin-Helmholtz unstable for realistic flow speeds, and the magnetopause boundary may be reasonably assumed to be perfectly reflecting. The low value of the plasma pressure is this region suggests that slow modes will be unimportant.
1999-01-01T00:00:00ZMills, Katharine J.In this thesis, the behaviour of Kelvin-Helmholtz unstable modes on the magnetospheric flanks and in the magnetotail are investigated. A model of a straight bounded magnetosphere connected to a semi-infinite field-free magnetosheath which is flowing with a uniform speed is used. First the magnetosphere is taken to be uniform with the magnetic field perpendicular to the flow in the magnetosheath and it is shown that the increase in Pc5 wave power observed for high solar wind flow speeds correlates well with the onset of instability of the fast body modes. A condition for the exact onset of instability of these modes is derived and the behaviour of fast surface and slow body and surface modes is also investigated. Using a non-uniform magnetosphere, it is shown that these unstable body modes may couple to field line resonances. The fastest growing modes are found to have a common azimuthal phase speed which depends only on the local conditions at the magnetopause and may be predicted using the theory of over-reflection. A finite width boundary layer is then added to the uniform magnetosphere model to investigate the space-time evolution of wave-packets on the magnetopause. Fast surface mode wave-packets are found to grow rapidly as they convect around the flanks so that non-linear effects will be important. Fast cavity mode wave-packets will remain relatively small on the flanks, explaining the robustness of the body of the magnetosphere here. Slow modes are found to grow very little in this region. Finally, a uniform magnetosphere with the magnetic field parallel to the flow in the magnetosheath is considered. Here, the fast modes are unlikely to be Kelvin-Helmholtz unstable for realistic flow speeds, and the magnetopause boundary may be reasonably assumed to be perfectly reflecting. The low value of the plasma pressure is this region suggests that slow modes will be unimportant.Study of solitary waves in space plasmasMamun, A. A.http://hdl.handle.net/10023/139872018-06-13T23:18:32Z1997-01-01T00:00:00ZTheoretical investigations have been made of arbitrary amplitude electrostatic solitary waves in non-thermal plasmas, which may be of relevance to ionospheric and magnetospheric plasmas, and dusty plasmas, which are most common in earth's and cometary environments as well as in planetary rings, for understanding the nonlinear features of localised electrostatic disturbances in such space plasma systems. This thesis starts with an introductory chapter where a very brief historical review of solitary waves in plasmas has been presented. The study of arbitrary amplitude electrostatic solitary waves in non-thermal plasma has considered a plasma system consisting of warm adiabatic ions and non- thermal electrons. It is found that a non-thermal electron distribution may change the nature of ion-acoustic solitary waves. If the ions are assumed to respond as a fluid to perturbations in the potential, with no significant trapping in a potential well, then a thermal plasma only supports solitary waves with a density peak. However, with a suitable distribution of non-thermal electrons, solitary waves with both density peaks and density depressions may exist. This study has also included a numerical analysis showing how these electrostatic solitary structures evolve with time. The investigation has then been extended to magnetised plasmas to study the effects of magnetic field on obliquely propagating electrostatic solitary structures. This attempt first employed the reductive perturbation method and investigated the nonlinear properties of small but finite amplitude obliquely propagating solitary waves in this magnetised non-thermal plasma model. This study is then generalised to arbitrary amplitude solitary waves by the numerical solution of the full nonlinear system of equations. This numerical method has also been utilised to present a similar study in another popular plasma model, namely the two-electron-temperature plasma model. The study of arbitrary amplitude solitary waves in a dusty plasma has considered another plasma system which consists of an inertial dust fluid and ions with Maxwellian distribution and has investigated the nonlinear properties of dust- acoustic solitary waves. A numerical study has also been made to show how these dust-acoustic solitary waves evolve with time. The effects of non-thermal and vortex-like ion distributions are then incorporated into this study. The study of arbitrary amplitude electrostatic solitary waves in this thesis has finally been concluded with some brief discussion of our results and proposal for further studies, which are expected to generalise and develop our present work to some other extents, in this versatile area of research.
1997-01-01T00:00:00ZMamun, A. A.Theoretical investigations have been made of arbitrary amplitude electrostatic solitary waves in non-thermal plasmas, which may be of relevance to ionospheric and magnetospheric plasmas, and dusty plasmas, which are most common in earth's and cometary environments as well as in planetary rings, for understanding the nonlinear features of localised electrostatic disturbances in such space plasma systems. This thesis starts with an introductory chapter where a very brief historical review of solitary waves in plasmas has been presented. The study of arbitrary amplitude electrostatic solitary waves in non-thermal plasma has considered a plasma system consisting of warm adiabatic ions and non- thermal electrons. It is found that a non-thermal electron distribution may change the nature of ion-acoustic solitary waves. If the ions are assumed to respond as a fluid to perturbations in the potential, with no significant trapping in a potential well, then a thermal plasma only supports solitary waves with a density peak. However, with a suitable distribution of non-thermal electrons, solitary waves with both density peaks and density depressions may exist. This study has also included a numerical analysis showing how these electrostatic solitary structures evolve with time. The investigation has then been extended to magnetised plasmas to study the effects of magnetic field on obliquely propagating electrostatic solitary structures. This attempt first employed the reductive perturbation method and investigated the nonlinear properties of small but finite amplitude obliquely propagating solitary waves in this magnetised non-thermal plasma model. This study is then generalised to arbitrary amplitude solitary waves by the numerical solution of the full nonlinear system of equations. This numerical method has also been utilised to present a similar study in another popular plasma model, namely the two-electron-temperature plasma model. The study of arbitrary amplitude solitary waves in a dusty plasma has considered another plasma system which consists of an inertial dust fluid and ions with Maxwellian distribution and has investigated the nonlinear properties of dust- acoustic solitary waves. A numerical study has also been made to show how these dust-acoustic solitary waves evolve with time. The effects of non-thermal and vortex-like ion distributions are then incorporated into this study. The study of arbitrary amplitude electrostatic solitary waves in this thesis has finally been concluded with some brief discussion of our results and proposal for further studies, which are expected to generalise and develop our present work to some other extents, in this versatile area of research.Steady models for magnetic reconnectionJardine, Moirahttp://hdl.handle.net/10023/139852018-06-13T23:18:40Z1989-01-01T00:00:00ZMagnetic reconnection is a fundamental physical process by which stored magnetic energy may be released. It is already known that different reconnection regimes result from changes in the nature of the plasma inflow towards the reconnection site. In this thesis, we examine both how the outflow region responds to changes both in the inflow and outflow boundary conditions and also how introducing compressibility affects the results. We find that if the inflow is converging, the outflow velocity is least, the width of the outflow region is greatest and the ratio of outflowing thermal to kinetic energy is greatest. Also, there is one free outflow parameter which would naturally be specified by the velocity of plasma leaving the reconnection site. We suggest that reverse currents seen in numerical simulations may result from the specification of an extra boundary condition. In addition, we find that the main effects of including compressibility are: to enhance convergence or divergence of the inflow; to increase the maximum reconnection rate where the inflow is converging; to increase the flow speed near the reconnection site where the inflow is diverging; to give faster, narrower outflow jets; to increase variations between regimes in the energy conversion and to increase the ratio of thermal to kinetic energy in the outflow jet.
1989-01-01T00:00:00ZJardine, MoiraMagnetic reconnection is a fundamental physical process by which stored magnetic energy may be released. It is already known that different reconnection regimes result from changes in the nature of the plasma inflow towards the reconnection site. In this thesis, we examine both how the outflow region responds to changes both in the inflow and outflow boundary conditions and also how introducing compressibility affects the results. We find that if the inflow is converging, the outflow velocity is least, the width of the outflow region is greatest and the ratio of outflowing thermal to kinetic energy is greatest. Also, there is one free outflow parameter which would naturally be specified by the velocity of plasma leaving the reconnection site. We suggest that reverse currents seen in numerical simulations may result from the specification of an extra boundary condition. In addition, we find that the main effects of including compressibility are: to enhance convergence or divergence of the inflow; to increase the maximum reconnection rate where the inflow is converging; to increase the flow speed near the reconnection site where the inflow is diverging; to give faster, narrower outflow jets; to increase variations between regimes in the energy conversion and to increase the ratio of thermal to kinetic energy in the outflow jet.Microinstabilities in high power electron cyclotron heating of plasmasMiller, Andrew Gilberthttp://hdl.handle.net/10023/139772018-06-12T23:19:06Z1991-01-01T00:00:00ZElectron cyclotron resonance heating has been successfully used in a number of experiments, firstly to raise the plasma temperature and secondly to drive currents noninductively. Recently the microwaves in tokamak experiment (MTX) has been proposed at the Lawrence Livermore Laboratory, which will involve pulsed heating at powers much higher than have previously been possible, using a Free Electron Laser (PEL). The physics of such an experiment differs greatly from the physics of experiments using less powerful but continuous operation gyrotron sources. An analytical model of the interaction between a wave and an electron is presented on the assumption that the wave amplitude experienced along the electron guiding centre changes slowly with time as it passes through the beam. This model is tested numerically by integrating the equations of motion governing the electron's motion as it interacts with the wave. Finally this model is used to predict the possible growth of instabilities in a plasma heated by a FEL. The growth rates of these waves may be large enough to act on the plasma in time scales much shorter than typical electron collision times.
1991-01-01T00:00:00ZMiller, Andrew GilbertElectron cyclotron resonance heating has been successfully used in a number of experiments, firstly to raise the plasma temperature and secondly to drive currents noninductively. Recently the microwaves in tokamak experiment (MTX) has been proposed at the Lawrence Livermore Laboratory, which will involve pulsed heating at powers much higher than have previously been possible, using a Free Electron Laser (PEL). The physics of such an experiment differs greatly from the physics of experiments using less powerful but continuous operation gyrotron sources. An analytical model of the interaction between a wave and an electron is presented on the assumption that the wave amplitude experienced along the electron guiding centre changes slowly with time as it passes through the beam. This model is tested numerically by integrating the equations of motion governing the electron's motion as it interacts with the wave. Finally this model is used to predict the possible growth of instabilities in a plasma heated by a FEL. The growth rates of these waves may be large enough to act on the plasma in time scales much shorter than typical electron collision times.A gyrokinetic analysis of electron plasma waves at resonance in magnetic field gradientsMcDonald, Darrenhttp://hdl.handle.net/10023/139752018-06-12T23:19:12Z1995-01-01T00:00:00ZTo produce nuclear fusion in a Tokamak reactor requires the heating of a plasma to a temperature of the order of 10 keV. Electron cyclotron resonant heating (ECRH), in which the plasma is heated by radio waves in resonance with the Larmor frequency of the plasma's electrons, is one scheme under consideration for achieving this. A description of such a heating scheme requires a theory to explain the propagation and absorption of high frequency waves in a plasma in the presence of a magnetic field gradient. A WKB analysis can describe some of the processes involved but a complete explanation requires the use of full wave equations. In this thesis we shall develop a technique for deriving such equations which will be shown to be simpler and more general than calculations performed by earlier workers. The technique relies on including the effect of the magnetic gradient across the Larmor orbit of the electrons in the resonance condition of the wave, the so called Gyrokinetic correction, which has been ignored in calculations by previous workers. Once derived, the equations are solved numerically and the results applied to a number of experiments currently being performed on Tokamak fusion. In addition, we shall also look at the energy loss processes of runaway electrons, which have been shown experimentally to be shorter than would be expected.
1995-01-01T00:00:00ZMcDonald, DarrenTo produce nuclear fusion in a Tokamak reactor requires the heating of a plasma to a temperature of the order of 10 keV. Electron cyclotron resonant heating (ECRH), in which the plasma is heated by radio waves in resonance with the Larmor frequency of the plasma's electrons, is one scheme under consideration for achieving this. A description of such a heating scheme requires a theory to explain the propagation and absorption of high frequency waves in a plasma in the presence of a magnetic field gradient. A WKB analysis can describe some of the processes involved but a complete explanation requires the use of full wave equations. In this thesis we shall develop a technique for deriving such equations which will be shown to be simpler and more general than calculations performed by earlier workers. The technique relies on including the effect of the magnetic gradient across the Larmor orbit of the electrons in the resonance condition of the wave, the so called Gyrokinetic correction, which has been ignored in calculations by previous workers. Once derived, the equations are solved numerically and the results applied to a number of experiments currently being performed on Tokamak fusion. In addition, we shall also look at the energy loss processes of runaway electrons, which have been shown experimentally to be shorter than would be expected.The theory of electron heating in collisonless plasma shock wavesBuckner, A. J. F.http://hdl.handle.net/10023/139732018-06-12T23:19:09Z1993-01-01T00:00:00ZEquations are derived to describe the evolution of an electron distribution function under the action of electromagnetic instabilities in a non-uniform plasma using an extension of the quasilinear theory of Kennel and Engelmann. Variations in both the electron density and temperature and the background magnetic field are taken into account. These equations are simplified in the limit of small electron beta so that an electrostatic approximation is justified. Methods are then presented which allow the solution of these equations (or, in principle, the more complex electromagnetic equations). In particular, a method of solving the kinetic dispersion relation for an arbitrary background (first-order) distribution function with the minimum of additional assumptions and approximations is described in detail. The electrostatic equations are solved for a number of different cases in order to study the action of the modified two stream instability on the electron distribution function. Throughout, realistic values of the ratios of electron to ion mass and electron plasma to cyclotron frequency ratio are used. The applications to collisionless plasma shock waves are discussed, and it is found that the modified two stream instability can produce the (relatively small) amounts of electron heating observed at quasi-perpendicular terrestrial bow shocks, and the flat-topped electron distribution functions seen to evolve. Extensions to the model which would greatly improve its applicability and accuracy, as well as the amount of computational effort required, are discussed.
1993-01-01T00:00:00ZBuckner, A. J. F.Equations are derived to describe the evolution of an electron distribution function under the action of electromagnetic instabilities in a non-uniform plasma using an extension of the quasilinear theory of Kennel and Engelmann. Variations in both the electron density and temperature and the background magnetic field are taken into account. These equations are simplified in the limit of small electron beta so that an electrostatic approximation is justified. Methods are then presented which allow the solution of these equations (or, in principle, the more complex electromagnetic equations). In particular, a method of solving the kinetic dispersion relation for an arbitrary background (first-order) distribution function with the minimum of additional assumptions and approximations is described in detail. The electrostatic equations are solved for a number of different cases in order to study the action of the modified two stream instability on the electron distribution function. Throughout, realistic values of the ratios of electron to ion mass and electron plasma to cyclotron frequency ratio are used. The applications to collisionless plasma shock waves are discussed, and it is found that the modified two stream instability can produce the (relatively small) amounts of electron heating observed at quasi-perpendicular terrestrial bow shocks, and the flat-topped electron distribution functions seen to evolve. Extensions to the model which would greatly improve its applicability and accuracy, as well as the amount of computational effort required, are discussed.Rotational flow in fluid dynamicsMurray, J. D. (James Dickson)http://hdl.handle.net/10023/139672018-06-12T23:18:46Z1955-01-01T00:00:00ZThe thesis is divided into four chapters. Chapter I gives a brief résumé of the state of rotational flow theory up to 1955. Chapter II contains a study of the constant shear flow past cylinders with various cross sections. Chapter III contains a method for obtaining the stream functions for cylinders in a variable shear flow when the latter approximates firstly to a linear vorticity distribution, and secondly to the rotational flow present in a boundary layer. Further, it illustrates the nature of the difficulties likely to be encountered in trying to obtain analytical solutions of problems where the rotation is of a more complicated nature. Finally, Chapter IV contains a relaxation solution to the two-dimensional isentropic compressible rotational flow of a gas through a channel containing a constriction, it also illustrates the complexity of the numerical work required in obtaining relaxation solutions of compressible flow problems with rotation.
1955-01-01T00:00:00ZMurray, J. D. (James Dickson)The thesis is divided into four chapters. Chapter I gives a brief résumé of the state of rotational flow theory up to 1955. Chapter II contains a study of the constant shear flow past cylinders with various cross sections. Chapter III contains a method for obtaining the stream functions for cylinders in a variable shear flow when the latter approximates firstly to a linear vorticity distribution, and secondly to the rotational flow present in a boundary layer. Further, it illustrates the nature of the difficulties likely to be encountered in trying to obtain analytical solutions of problems where the rotation is of a more complicated nature. Finally, Chapter IV contains a relaxation solution to the two-dimensional isentropic compressible rotational flow of a gas through a channel containing a constriction, it also illustrates the complexity of the numerical work required in obtaining relaxation solutions of compressible flow problems with rotation.Mode conversion of plasma wavesWoods, Anna Mariahttp://hdl.handle.net/10023/139652018-06-12T23:18:41Z1987-01-01T00:00:00ZLinear mode conversion processes are much studied in plasma physics because they determine the efficiency of any radio frequency heating scheme. Mode coupling model equations, extracted with varying degrees of rigour from the Maxwell-linearized kinetic equations, are usually fourth or higher order O.D.E's. These are solved by complicated methods to obtain transmission, conversion, reflection and absorption coefficients. Recently, Fuchs et al and Cairns and Lashmore-Davies (C.L-D.) have postulated second order O.D.E's to describe pairwise coupling events. The second order theories have reproduced results previously obtained by much more sophisticated treatments. In this thesis, we firstly examine the hybrid resonances in a cold plasma and show that they have a mode conversion interpretation in the framework of the C.L-D. model. The Budden tunnelling coefficients are recovered for this case. Next, mode conversion between the fast and slow electromagnetic waves in the lower hybrid frequency range is considered. This phenomenon determines the accessibility of the lower hybrid resonance to the slow wave, and is also of theoretical interest because the mode coupling differs in certain aspects from cases previously investigated by C.L-D. A second order approximation to the dispersion relation is used in the mode conversion region leading to Weber's equation from which transmission coefficients are then obtained in various cases. Finally, we provide justification for the use of Weber's equation. The exact fourth order system of O.D.E's for the problem is set down, and a linear transformation, which is an extension of that given by Heading, reveals the second order nature of the coupling process. Numerical solutions of the fourth order system yield transmission coefficients in excellent agreement with the second order theory, and also demonstrate that the electric field variation across the mode conversion region is well approximated, via the above transformation, by our second order theory.
1987-01-01T00:00:00ZWoods, Anna MariaLinear mode conversion processes are much studied in plasma physics because they determine the efficiency of any radio frequency heating scheme. Mode coupling model equations, extracted with varying degrees of rigour from the Maxwell-linearized kinetic equations, are usually fourth or higher order O.D.E's. These are solved by complicated methods to obtain transmission, conversion, reflection and absorption coefficients. Recently, Fuchs et al and Cairns and Lashmore-Davies (C.L-D.) have postulated second order O.D.E's to describe pairwise coupling events. The second order theories have reproduced results previously obtained by much more sophisticated treatments. In this thesis, we firstly examine the hybrid resonances in a cold plasma and show that they have a mode conversion interpretation in the framework of the C.L-D. model. The Budden tunnelling coefficients are recovered for this case. Next, mode conversion between the fast and slow electromagnetic waves in the lower hybrid frequency range is considered. This phenomenon determines the accessibility of the lower hybrid resonance to the slow wave, and is also of theoretical interest because the mode coupling differs in certain aspects from cases previously investigated by C.L-D. A second order approximation to the dispersion relation is used in the mode conversion region leading to Weber's equation from which transmission coefficients are then obtained in various cases. Finally, we provide justification for the use of Weber's equation. The exact fourth order system of O.D.E's for the problem is set down, and a linear transformation, which is an extension of that given by Heading, reveals the second order nature of the coupling process. Numerical solutions of the fourth order system yield transmission coefficients in excellent agreement with the second order theory, and also demonstrate that the electric field variation across the mode conversion region is well approximated, via the above transformation, by our second order theory.Some exact solutions in the one-dimensional unsteady motion of a gasWeir, David Gordonhttp://hdl.handle.net/10023/139642018-06-12T23:19:05Z1961-01-01T00:00:00ZIn this thesis, we present certain exact solutions of the mathematical equations governing the one-dimensional unsteady flow of a compressible fluid. In Chapter 2 we introduce the well-known simplification of the equations (1.1.10), (1.1.11) and (1.1.12) which occurs when the entropy is assumed to be constant, and conditions for parching solutions of the equations along characteristics are obtained. These results are used to generalise a problem solved by Mackie. In chapter 3 we meet the concept of a shook, and exact solutions are obtained for two problems in which shocks occur in non-uniform flows. In chapter 4 the case of waves in shallow water which has differential equations similar to those of gas flow is discussed. The results of the previous section are applied to this case and a problem attacked which permits a comparison to be made of the results obtained by this theory and a simpler linearized theory. Finally in chapter 5 we examine a method introduced by Martin for dealing with certain non-isentropic flows. Some new exact solutions of non-isentropic flows are thus obtained.
1961-01-01T00:00:00ZWeir, David GordonIn this thesis, we present certain exact solutions of the mathematical equations governing the one-dimensional unsteady flow of a compressible fluid. In Chapter 2 we introduce the well-known simplification of the equations (1.1.10), (1.1.11) and (1.1.12) which occurs when the entropy is assumed to be constant, and conditions for parching solutions of the equations along characteristics are obtained. These results are used to generalise a problem solved by Mackie. In chapter 3 we meet the concept of a shook, and exact solutions are obtained for two problems in which shocks occur in non-uniform flows. In chapter 4 the case of waves in shallow water which has differential equations similar to those of gas flow is discussed. The results of the previous section are applied to this case and a problem attacked which permits a comparison to be made of the results obtained by this theory and a simpler linearized theory. Finally in chapter 5 we examine a method introduced by Martin for dealing with certain non-isentropic flows. Some new exact solutions of non-isentropic flows are thus obtained.Stability of some free-surface flowsSmith, Frank Ian Pitthttp://hdl.handle.net/10023/139602018-06-12T23:19:08Z1969-01-01T00:00:00ZThe subject matter of this thesis is concerned with the stability of fluid flows; more particularly , with the stability of liquid films which have an interface with air. We will therefore begin by formulating the basic equations and ideas which pertain to this class of problems. Later in this chapter, a summary will be given of the topics dealt with in this dissertation.
1969-01-01T00:00:00ZSmith, Frank Ian PittThe subject matter of this thesis is concerned with the stability of fluid flows; more particularly , with the stability of liquid films which have an interface with air. We will therefore begin by formulating the basic equations and ideas which pertain to this class of problems. Later in this chapter, a summary will be given of the topics dealt with in this dissertation.The unsteady expansion of a gas into a near vacuumMcLaughlin, Raymondhttp://hdl.handle.net/10023/139562018-06-12T23:18:13Z1975-01-01T00:00:00ZThis thesis is concerned with the unsteady expansion of an initially uniform, stationary gas into a low density, stationary atmosphere, studied from the viewpoint of inviscid gasdynamics. It is found that, there are two regions in the k-𝜎 parameter space having distinct forms for the large time solution, when the atmospheric density is initially proportional to r⁻[super]k, r being the spatial coordinate, k being constant and 𝜎, the geometry index, has its usual meaning. First of all a constant asymptotic shock velocity is assumed and matched expansions, for large r, are constructed. Inner expansions, valid near the shock, are matched to zeroth and first orders with the outer expansions which are valid near the contact front. Zeroth order matching, which, yields the constant asymptotic shock velocity, is possible only in a restricted region of the k-𝜎parameter space and this situation is clarified by appealing to the similarity solutions which are extended to cover cases which have not been dealt with previously.
In the other region of the k-𝜎 parameter space the asymptotic shock velocity is proportional to r[super]∈ where ∈, a positive constant, is found from the similarity solutions as a function of k, γ ,𝜎. An attempt is made at constructing matched asymptotic expansions for large r. The inner solution can be obtained, apart from the evaluation of certain constants, to zeroth and first orders but the outer solution is inaccessible and can only be determined from the full inviscid solution. However it is shown that there exists a solution to the outer equations which matches with the inner solution up to first order. In both cases matching of the first order inner terms to the outer solution produces an eigenvalue problem, the solution of which is not attempted here. Finally full numerical solutions of the inviscid equations, one for each case, were produced using the method of backward drawn characteristics, devised by Hartree, and it will be seen that they compare most favourably with the asymptotic analysis.
1975-01-01T00:00:00ZMcLaughlin, RaymondThis thesis is concerned with the unsteady expansion of an initially uniform, stationary gas into a low density, stationary atmosphere, studied from the viewpoint of inviscid gasdynamics. It is found that, there are two regions in the k-𝜎 parameter space having distinct forms for the large time solution, when the atmospheric density is initially proportional to r⁻[super]k, r being the spatial coordinate, k being constant and 𝜎, the geometry index, has its usual meaning. First of all a constant asymptotic shock velocity is assumed and matched expansions, for large r, are constructed. Inner expansions, valid near the shock, are matched to zeroth and first orders with the outer expansions which are valid near the contact front. Zeroth order matching, which, yields the constant asymptotic shock velocity, is possible only in a restricted region of the k-𝜎parameter space and this situation is clarified by appealing to the similarity solutions which are extended to cover cases which have not been dealt with previously.
In the other region of the k-𝜎 parameter space the asymptotic shock velocity is proportional to r[super]∈ where ∈, a positive constant, is found from the similarity solutions as a function of k, γ ,𝜎. An attempt is made at constructing matched asymptotic expansions for large r. The inner solution can be obtained, apart from the evaluation of certain constants, to zeroth and first orders but the outer solution is inaccessible and can only be determined from the full inviscid solution. However it is shown that there exists a solution to the outer equations which matches with the inner solution up to first order. In both cases matching of the first order inner terms to the outer solution produces an eigenvalue problem, the solution of which is not attempted here. Finally full numerical solutions of the inviscid equations, one for each case, were produced using the method of backward drawn characteristics, devised by Hartree, and it will be seen that they compare most favourably with the asymptotic analysis.Hodograph methods applied to flow past finite wedgesMackie, A. G. (Andrew George)http://hdl.handle.net/10023/139462018-06-12T23:18:31Z1953-01-01T00:00:00Z1953-01-01T00:00:00ZMackie, A. G. (Andrew George)Two parameter integral methods in laminar boundary layer theoryLister, William Macraehttp://hdl.handle.net/10023/139442018-06-12T23:18:51Z1971-01-01T00:00:00ZThe work of this thesis is concerned, with the investigation and attempted improvement of an integral method for solving the two dimensional, incompressible laminar boundary layer equations of fluid dynamics. The method which is based on a theoretical two parameter representation of well-known boundary layer properties was first produced by Professor S. N. Curle. Its range of application, reliability and accuracy rely on four universal functions which have been derived from known exact solutions to the boundary layer equations, and are given tabulated in terms of a pressure gradient parameter 𝞴. This thesis seeks to improve these properties by making adjustments to the tabulated functions and also considers the extension of the method to certain compressible boundary layer problems. The first chapter contains the development of, and background to the method and gives a critical assessment of the existing functions. This analysis indicates that the method may be improved by supplying more data for certain ranges of 𝞴 from which the functions may be calculated; by improving the fitting process; and by the provision for small values of 𝞴 of an analytic form for a shape parameter H which the method involves.
To supply more data two new solutions for the flows u₁ = U₀ (1+𝜉) and u₁ = u₀ (𝜉+𝜉³) where 𝜉 is a non-dimensional co-ordinate in the direction of the flow, are investigated. The resulting work produces some interesting examples of the use of series expansions in boundary layer theory and these, and the results produced, are given in the second chapter. The fitting of the functions is carried out in chapter three. Polynomial models in terms of 𝞴 are fitted by least squares techniques to data from seven solutions and are adjusted to ensure an analytic form for H for small values of 𝞴. A comparison of results using new and old tables Indicates that an improvement has been made. The transformation relating certain compressible and incompressible flows is next examined and the extension of the method to such problems considered. An idea due to Stewartson for assessing the relative accuracies of methods under such circumstances indicates that the method should be highly accurate, a result confirmed by the calculation of the compressible flow u₁ = u₀ (1-𝜉) at a leading edge Mach number of four. The thesis is concluded with a review of the work carried out and the results obtained.
1971-01-01T00:00:00ZLister, William MacraeThe work of this thesis is concerned, with the investigation and attempted improvement of an integral method for solving the two dimensional, incompressible laminar boundary layer equations of fluid dynamics. The method which is based on a theoretical two parameter representation of well-known boundary layer properties was first produced by Professor S. N. Curle. Its range of application, reliability and accuracy rely on four universal functions which have been derived from known exact solutions to the boundary layer equations, and are given tabulated in terms of a pressure gradient parameter 𝞴. This thesis seeks to improve these properties by making adjustments to the tabulated functions and also considers the extension of the method to certain compressible boundary layer problems. The first chapter contains the development of, and background to the method and gives a critical assessment of the existing functions. This analysis indicates that the method may be improved by supplying more data for certain ranges of 𝞴 from which the functions may be calculated; by improving the fitting process; and by the provision for small values of 𝞴 of an analytic form for a shape parameter H which the method involves.
To supply more data two new solutions for the flows u₁ = U₀ (1+𝜉) and u₁ = u₀ (𝜉+𝜉³) where 𝜉 is a non-dimensional co-ordinate in the direction of the flow, are investigated. The resulting work produces some interesting examples of the use of series expansions in boundary layer theory and these, and the results produced, are given in the second chapter. The fitting of the functions is carried out in chapter three. Polynomial models in terms of 𝞴 are fitted by least squares techniques to data from seven solutions and are adjusted to ensure an analytic form for H for small values of 𝞴. A comparison of results using new and old tables Indicates that an improvement has been made. The transformation relating certain compressible and incompressible flows is next examined and the extension of the method to such problems considered. An idea due to Stewartson for assessing the relative accuracies of methods under such circumstances indicates that the method should be highly accurate, a result confirmed by the calculation of the compressible flow u₁ = u₀ (1-𝜉) at a leading edge Mach number of four. The thesis is concluded with a review of the work carried out and the results obtained.The evaporation kinetics of liquid helium IIHunter, George Huttonhttp://hdl.handle.net/10023/139412018-06-12T23:18:42Z1968-01-01T00:00:00ZThis work is concerned with the evaporation and condensation processes occurring when liquid helium II is in equilibrium with its saturated vapour. We define the condensation coefficient a as the fraction of atoms incident on the liquid vapour interface which cross it to form part of the liquid. Experiments to measure are described, and the results are discussed in terms of microscopic condensation processes. The measurements are made by reflecting second sound pulses from the liquid vapour surface at normal incidence and measuring the reflection coefficient. An account is given of the phenomenological theories of Osborne (1962a) and Chernikova (1964), which describe the reflection of second sound from the surface and the associated effect, its transformation into first sound in the gas. Neither of these agree with the experimental results, and Osborne's theory is modified by taking account of the conditions in the gas a small fraction of a mean free path above the surface (rather than many mean free paths above the surface, as in Osborne's original theory). Thus modified, the theory is shown to be in agreement with the measurements of the reflection coefficient. Also described are measurements made in second sound pulses generated at the interface by first sound pulses, themselves generated at the interface by second sound, propagated up the tube, and reflected from its closed and back to the surface. From the time intervals between these pulses the velocity of first sound in the vapour is deduced, and found to be in agreement with previous work. Measurements of pulse amplitude corroborate the reflection coefficient measurements, and taking the two sets of measurements together wo have concluded that a is probably 1 and not less than 0.8 between 1.0°K and 2.14°K. The microscopic processes by which condensation can take place are considered. Experiments due to beaker (unpublished, see Osborne, 1962a) and Osborne (1962b) are described, which indicate that the vapour exchanges momentum with normal fluid only. We have therefore supposed that processes in which a gas atom condenses to form excitations must conserve energy and momentum. Processes involving both bulk excitations and surface excitations are considered, but effects due to the finite lifetime of the excitations and the linewidth of the excitations spectrum are neglected. No attempt has been made to calculate the matrix elements for condensation processes, but plausible estimates have been made of their relative magnitudes. In particular, only processes involving one gas atom and one or two excitations have been considered. Using the requirements of conservation of energy and momentum, it is shown that as the temperature decreases, a decreasing fraction of the incident atom have enough energy to form two excitations, and condensation must take place by the collision of an atom with an existing excitation. A rough estimate of the collision probability for such a process leads to the conclusion that at 1°K, a should be about 0.2. This disagreement with experiment has not been resolved. Finally, some remarks are made about the implications for other work on liquid helium II, and some suggestions for future work.
1968-01-01T00:00:00ZHunter, George HuttonThis work is concerned with the evaporation and condensation processes occurring when liquid helium II is in equilibrium with its saturated vapour. We define the condensation coefficient a as the fraction of atoms incident on the liquid vapour interface which cross it to form part of the liquid. Experiments to measure are described, and the results are discussed in terms of microscopic condensation processes. The measurements are made by reflecting second sound pulses from the liquid vapour surface at normal incidence and measuring the reflection coefficient. An account is given of the phenomenological theories of Osborne (1962a) and Chernikova (1964), which describe the reflection of second sound from the surface and the associated effect, its transformation into first sound in the gas. Neither of these agree with the experimental results, and Osborne's theory is modified by taking account of the conditions in the gas a small fraction of a mean free path above the surface (rather than many mean free paths above the surface, as in Osborne's original theory). Thus modified, the theory is shown to be in agreement with the measurements of the reflection coefficient. Also described are measurements made in second sound pulses generated at the interface by first sound pulses, themselves generated at the interface by second sound, propagated up the tube, and reflected from its closed and back to the surface. From the time intervals between these pulses the velocity of first sound in the vapour is deduced, and found to be in agreement with previous work. Measurements of pulse amplitude corroborate the reflection coefficient measurements, and taking the two sets of measurements together wo have concluded that a is probably 1 and not less than 0.8 between 1.0°K and 2.14°K. The microscopic processes by which condensation can take place are considered. Experiments due to beaker (unpublished, see Osborne, 1962a) and Osborne (1962b) are described, which indicate that the vapour exchanges momentum with normal fluid only. We have therefore supposed that processes in which a gas atom condenses to form excitations must conserve energy and momentum. Processes involving both bulk excitations and surface excitations are considered, but effects due to the finite lifetime of the excitations and the linewidth of the excitations spectrum are neglected. No attempt has been made to calculate the matrix elements for condensation processes, but plausible estimates have been made of their relative magnitudes. In particular, only processes involving one gas atom and one or two excitations have been considered. Using the requirements of conservation of energy and momentum, it is shown that as the temperature decreases, a decreasing fraction of the incident atom have enough energy to form two excitations, and condensation must take place by the collision of an atom with an existing excitation. A rough estimate of the collision probability for such a process leads to the conclusion that at 1°K, a should be about 0.2. This disagreement with experiment has not been resolved. Finally, some remarks are made about the implications for other work on liquid helium II, and some suggestions for future work.Hydrodynamics of liquid helium IIGriffiths, D. J. (Derek John)http://hdl.handle.net/10023/139382018-06-20T10:29:59Z1964-01-01T00:00:00ZObservations have been made of the behaviour of a fine quartz fibre, weighted at its lower end and suspended inside a short, horizontal tunnel in which counterblow of the normal and superfluid components of liquid helium II can be produced by a heater. Section I of this thesis is an introduction to the hydrodynamic of liquid helium II. In section II the interaction with such a fibre of quantized vortex lines in the superfluid is discussed, and the effect of a short heat pulse on the fibre when it is carrying superfluid circulation in calculated approximately. The different responses of the fibre to turbulence in the normal fluid and in the superfluid are contrasted.
In section III, after a description of the apparatus and the experimental method, measurements, deduced from the response to heat pulses, of the circulation about the fibre from 1.3°K to2.1°K are reported. At all temperatures circulations of the expected order from magnitude are observed to grow and decay with time. At 1.3°K apparent circulations of up to about 1/5 quantum occur. In undisturbed helium the largest circulations are more stable than other values, persisting for up to five minutes. Measurement of the same circulation both by the heat-pulse method and by the deflection of the fibre in a steady heat current suggests that the large, persistent circulations may in fact be equal to one quantum. The sense of the observed circulations about the fibre at 1.3°K is strongly biased, this bias being probably associated with the heater geometry. In small heat current no change in the bins or persistence of circulation can be detected, but in currents above 11/2-3 mW/cm², depending on the heater, the circulation about the fibre is both more variable and of the opposite bias to that in undisturbed helium. This behaviour continues for 100 sec or more after the heat current has been switched off. At higher temperatures there are indications that the behaviour might be similar if it were possible for the helium to region its undisturbed condition after being stirred up by turbulent heat currents. In fact this seems other to be impossible, or to require many hundreds of seconds, and the situation is therefore rather confused.
In still higher heat currents measurement of superfluid circulation by heat pulses is impossible because the fibre is continuously agitated in a random way. From measurements of the rms deflections of the bob on the end of the fibre a critical heat current for the onset of such turbulence is found at 1.3°K. At higher temperatures the sensitivity is too low for the transition itself, if any, to be detected, but an upper limit to the critical heat current is given. At 1.3°K and 2.1°K the rms deflection increases monotonically with increasing heat currents, but at intermediate temperatures it is variable, because the bob is often hardly agitated for long periods during apparently supercritical heat currents. This is called quiescent behaviour.
When a supercritical heat current is a delay before agitation of the fibre begins. The delay time, which is often not very well defined, has been measured as a function of the heat current. When the current is switched off the agitation of the bob decoys in a few seconds, but at 1.3°K the circulation about the fibre is small and variable for 100 sec or more, until the persistence and bias characteristic of undisturbed helium regained. These results are discussed in section IV.
1964-01-01T00:00:00ZGriffiths, D. J. (Derek John)Observations have been made of the behaviour of a fine quartz fibre, weighted at its lower end and suspended inside a short, horizontal tunnel in which counterblow of the normal and superfluid components of liquid helium II can be produced by a heater. Section I of this thesis is an introduction to the hydrodynamic of liquid helium II. In section II the interaction with such a fibre of quantized vortex lines in the superfluid is discussed, and the effect of a short heat pulse on the fibre when it is carrying superfluid circulation in calculated approximately. The different responses of the fibre to turbulence in the normal fluid and in the superfluid are contrasted.
In section III, after a description of the apparatus and the experimental method, measurements, deduced from the response to heat pulses, of the circulation about the fibre from 1.3°K to2.1°K are reported. At all temperatures circulations of the expected order from magnitude are observed to grow and decay with time. At 1.3°K apparent circulations of up to about 1/5 quantum occur. In undisturbed helium the largest circulations are more stable than other values, persisting for up to five minutes. Measurement of the same circulation both by the heat-pulse method and by the deflection of the fibre in a steady heat current suggests that the large, persistent circulations may in fact be equal to one quantum. The sense of the observed circulations about the fibre at 1.3°K is strongly biased, this bias being probably associated with the heater geometry. In small heat current no change in the bins or persistence of circulation can be detected, but in currents above 11/2-3 mW/cm², depending on the heater, the circulation about the fibre is both more variable and of the opposite bias to that in undisturbed helium. This behaviour continues for 100 sec or more after the heat current has been switched off. At higher temperatures there are indications that the behaviour might be similar if it were possible for the helium to region its undisturbed condition after being stirred up by turbulent heat currents. In fact this seems other to be impossible, or to require many hundreds of seconds, and the situation is therefore rather confused.
In still higher heat currents measurement of superfluid circulation by heat pulses is impossible because the fibre is continuously agitated in a random way. From measurements of the rms deflections of the bob on the end of the fibre a critical heat current for the onset of such turbulence is found at 1.3°K. At higher temperatures the sensitivity is too low for the transition itself, if any, to be detected, but an upper limit to the critical heat current is given. At 1.3°K and 2.1°K the rms deflection increases monotonically with increasing heat currents, but at intermediate temperatures it is variable, because the bob is often hardly agitated for long periods during apparently supercritical heat currents. This is called quiescent behaviour.
When a supercritical heat current is a delay before agitation of the fibre begins. The delay time, which is often not very well defined, has been measured as a function of the heat current. When the current is switched off the agitation of the bob decoys in a few seconds, but at 1.3°K the circulation about the fibre is small and variable for 100 sec or more, until the persistence and bias characteristic of undisturbed helium regained. These results are discussed in section IV.Approximate methods in high speed flowBurnside, Robert R.http://hdl.handle.net/10023/139312018-06-18T14:04:50Z1962-01-01T00:00:00ZIn many problems arising in the theory of compressible flow, the equations characterising the solution of the system are so intractable that recourse must be made to some approximate method which allows the essential features of the flow to be preserved, whilst to some degree, simplifying the mathematics. It is with certain methods of this type that this thesis is concerned.
In the subsequent work, we shall assume that the effects due to viscosity and heat conduction are so small as to be negligible. These assumptions may be shown to be largely valid except in those domains of the flow-field where the modified system of equations predicts regions in which the solution is in general multivalued. In the modified system, however, such ‘regions’ are avoided by the introduction of mathematical discontinuities and, assuming that the jump conditions across them can be determines, are sufficient to provide single-valued solutions valid everywhere, except at the discontinuity. The methods to be presented are formulated in the plane consisting of one space variable and one time variable.
1962-01-01T00:00:00ZBurnside, Robert R.In many problems arising in the theory of compressible flow, the equations characterising the solution of the system are so intractable that recourse must be made to some approximate method which allows the essential features of the flow to be preserved, whilst to some degree, simplifying the mathematics. It is with certain methods of this type that this thesis is concerned.
In the subsequent work, we shall assume that the effects due to viscosity and heat conduction are so small as to be negligible. These assumptions may be shown to be largely valid except in those domains of the flow-field where the modified system of equations predicts regions in which the solution is in general multivalued. In the modified system, however, such ‘regions’ are avoided by the introduction of mathematical discontinuities and, assuming that the jump conditions across them can be determines, are sufficient to provide single-valued solutions valid everywhere, except at the discontinuity. The methods to be presented are formulated in the plane consisting of one space variable and one time variable.Aspects of natural convention and of non-linear hydridynamic stabilityUsher, J. R.http://hdl.handle.net/10023/139222018-06-12T23:17:26Z1974-01-01T00:00:00ZIn Part I of this thesis, steady and time-dependent, natural-convection similarity flows with mass transfer are discussed. Similarity flows for natural convection on families of two-dimensional bodies with closed lower ends are enumerated, when both a temperature distribution and a suction velocity distribution are prescribed at the body surface. For steady similarity flow on a heated vertical flat plate, with mass transfer at the surface, a numerical procedure is introduced for determining the velocity and temperature profiles. These results are presented in Figs. 2 and 3. Other similarity flows may be found by the same method.
A simplification, valid for “strong” suction, is discussed. An extension of Mangler’s transformation [1948] is given which reduces the equations governing axisymmetric flow to those for two-dimensional flow in steady natural convection.
In Part II non-linear resonant instability in parallel shear flows is discussed. A.D.D.Craik’s (see Usher and Craik [I]) modified version of Bateman’s [1956] variational formulation for viscous flows is employed to derive the second-order interaction equations governing the temporal evolution of a resonant wave triad in a sheer flow. (An extension of Craik’s variational formulation to free surface flows is presented but is not required in the subsequent analysis for the resonance problem). This problem was treated previously using a ‘direct’ approach (employing the Navier-Stokes equations) by Craik [1971]. The major advantage of the present method over the ‘direct’ method is the substantial reduction in algebraic complexity. Also, a justification of the validity of Craik’s previous analysis is given.
For this same resonance problem, third-order interaction equations are derived by the *direct* method since, to this order of approximation, little advantage is to be gained from the variational formulation. The resonance theory is thereby developed to the same order of approximation as the non-resonant third-order theory of Stuart [1960, 1962].
An asymptotic analysis for large Reynolds numbers reveals that the magnitudes of the third-order interaction coefficients – like certain of those at second-order – are remarkably large. Such results lead to a discussion of the regions of validity of the perturbation analysis. Also some light is shed on the roles played by resonance and three-dimensionality in the non-linear instability of shear flows.
1974-01-01T00:00:00ZUsher, J. R.In Part I of this thesis, steady and time-dependent, natural-convection similarity flows with mass transfer are discussed. Similarity flows for natural convection on families of two-dimensional bodies with closed lower ends are enumerated, when both a temperature distribution and a suction velocity distribution are prescribed at the body surface. For steady similarity flow on a heated vertical flat plate, with mass transfer at the surface, a numerical procedure is introduced for determining the velocity and temperature profiles. These results are presented in Figs. 2 and 3. Other similarity flows may be found by the same method.
A simplification, valid for “strong” suction, is discussed. An extension of Mangler’s transformation [1948] is given which reduces the equations governing axisymmetric flow to those for two-dimensional flow in steady natural convection.
In Part II non-linear resonant instability in parallel shear flows is discussed. A.D.D.Craik’s (see Usher and Craik [I]) modified version of Bateman’s [1956] variational formulation for viscous flows is employed to derive the second-order interaction equations governing the temporal evolution of a resonant wave triad in a sheer flow. (An extension of Craik’s variational formulation to free surface flows is presented but is not required in the subsequent analysis for the resonance problem). This problem was treated previously using a ‘direct’ approach (employing the Navier-Stokes equations) by Craik [1971]. The major advantage of the present method over the ‘direct’ method is the substantial reduction in algebraic complexity. Also, a justification of the validity of Craik’s previous analysis is given.
For this same resonance problem, third-order interaction equations are derived by the *direct* method since, to this order of approximation, little advantage is to be gained from the variational formulation. The resonance theory is thereby developed to the same order of approximation as the non-resonant third-order theory of Stuart [1960, 1962].
An asymptotic analysis for large Reynolds numbers reveals that the magnitudes of the third-order interaction coefficients – like certain of those at second-order – are remarkably large. Such results lead to a discussion of the regions of validity of the perturbation analysis. Also some light is shed on the roles played by resonance and three-dimensionality in the non-linear instability of shear flows.Ion dynamics in collisionless shock wavesSherwell, Davidhttp://hdl.handle.net/10023/139172018-06-11T23:17:53Z1976-01-01T00:00:00ZIn a laminar model of a collisionless magnetosonic shock wave, ion equations of motion are integrated through shock-like profiles. Conservation relations and Maxwell's equations allow a self-consistent determination of unknown downstream ion distribution functions fᵢ, ion temperature Tᵢ, and electric potential jump 𝛷. Favourable comparison of model Tᵢ, 𝛷.
Favourable comparison of model Tᵢ, 𝛷 , with experiment establishes (at low 𝛽 ≲ O.3, 𝛽=8 π N
[sub] l k (T[sub]e₂+Tᵢ[sub]l)/B₁²)
importance of laminar ion dynamics. Heating is due to distortion of Maxwellian distributions when entropy is conserved; in particular shock dynamics is dominated by a fast "tail" of reflected ions. The solutions for fᵢ are considered. The "stability" of the model to its assumptions (linear profiles, shock thickness (L[sub]s)) is shown. When reflections occur a self-consistent length emerges. The solutions Tᵢ, 𝛷 are extensively studied at various Mach numbers for different values of 𝛽. Laminar ion heating is very efficient and at high 𝛽 can exceed proper conservation levels due to ion reflections; at high 𝛽(≥ 𝛽 *) the electric potential is unable to slow the ions to conservation levels. The model predicts significant reflected ion currents in the plane of the shock. The boundary 𝛽 * is determined. Then laminar ion dynamics on the scale of the electron heating length (~10 C/w[sub]p ₑ) cannot occur for 𝛽 > 𝛽 *. Dependence on L[sub]s and T ₑ₁,/Tᵢ₁ is considered. The nature of non-laminar 𝛽 >𝛽* shocks is considered. Collisions are found to be important in laboratory shocks, and are efficient in slowing the reflected ions. In the absence of collisions, ion instabilities must be considered. It is shown that turbulent slowing of the fast ions cannot take place in L[sub]s alone. Further it is shown possible to construct a shock so that non-laminar mechanisms cannot occur significantly. Then the laminar model is re-instated. A decoupling of ion and electron heating lengths is proposed. Reflection heating in the Earth's Bow Shock (𝛽>𝛽*) is modelled, and is comparable with experiment.
1976-01-01T00:00:00ZSherwell, DavidIn a laminar model of a collisionless magnetosonic shock wave, ion equations of motion are integrated through shock-like profiles. Conservation relations and Maxwell's equations allow a self-consistent determination of unknown downstream ion distribution functions fᵢ, ion temperature Tᵢ, and electric potential jump 𝛷. Favourable comparison of model Tᵢ, 𝛷.
Favourable comparison of model Tᵢ, 𝛷 , with experiment establishes (at low 𝛽 ≲ O.3, 𝛽=8 π N
[sub] l k (T[sub]e₂+Tᵢ[sub]l)/B₁²)
importance of laminar ion dynamics. Heating is due to distortion of Maxwellian distributions when entropy is conserved; in particular shock dynamics is dominated by a fast "tail" of reflected ions. The solutions for fᵢ are considered. The "stability" of the model to its assumptions (linear profiles, shock thickness (L[sub]s)) is shown. When reflections occur a self-consistent length emerges. The solutions Tᵢ, 𝛷 are extensively studied at various Mach numbers for different values of 𝛽. Laminar ion heating is very efficient and at high 𝛽 can exceed proper conservation levels due to ion reflections; at high 𝛽(≥ 𝛽 *) the electric potential is unable to slow the ions to conservation levels. The model predicts significant reflected ion currents in the plane of the shock. The boundary 𝛽 * is determined. Then laminar ion dynamics on the scale of the electron heating length (~10 C/w[sub]p ₑ) cannot occur for 𝛽 > 𝛽 *. Dependence on L[sub]s and T ₑ₁,/Tᵢ₁ is considered. The nature of non-laminar 𝛽 >𝛽* shocks is considered. Collisions are found to be important in laboratory shocks, and are efficient in slowing the reflected ions. In the absence of collisions, ion instabilities must be considered. It is shown that turbulent slowing of the fast ions cannot take place in L[sub]s alone. Further it is shown possible to construct a shock so that non-laminar mechanisms cannot occur significantly. Then the laminar model is re-instated. A decoupling of ion and electron heating lengths is proposed. Reflection heating in the Earth's Bow Shock (𝛽>𝛽*) is modelled, and is comparable with experiment.Investigations on classical symmetries theory of quantizationGuest, P. B.http://hdl.handle.net/10023/139132018-06-11T23:17:38Z1972-01-01T00:00:00ZThe thesis divides naturally into two parts. Part I raises, and in some cases answers, questions concerning symmetry in classical mechanics. The main result (Theorem 6.4) shows that the assumption of the existence of a realization puts an upper limit on the rank of the algebra.
The heart of the thesis (covering three-quarters of the volume) is section II on the quantization of classical systems. §1 lists axioms desirable in any quantization rule for the 'functions of the q's'. The momentum observables are introduced in §2 prior to their quantization in §4. §5 essentially shows how conventional quantum mechanics fits into this scheme of things. By progressive specialization from a general manifold to a vector space, from a general quantization scheme to one which is linear on the linear momentum functions, and finally to an entirely well-behaved (admissible) quantization rule, into which conventional quantum mechanics fits nicely, we obtain in §7-§9 results which become progressively more and more powerful. The final theorem (Theorem 9.2) is perhaps the most significant of all. This result states that there exists a class of functions, which contains all functions of the q's and functions of the p's and all momentum observables and which is closed with respect to any linear canonical transformation L; a rule A assigning a unique self-adjoint operator to each such function f; a unitary operator WL corresponding to L and an equation
𝛬(𝑓 ∘ 𝐿) = 𝑊[sub]𝐿⁻ 𝛬 𝑓 𝑊[sub]𝐿
1972-01-01T00:00:00ZGuest, P. B.The thesis divides naturally into two parts. Part I raises, and in some cases answers, questions concerning symmetry in classical mechanics. The main result (Theorem 6.4) shows that the assumption of the existence of a realization puts an upper limit on the rank of the algebra.
The heart of the thesis (covering three-quarters of the volume) is section II on the quantization of classical systems. §1 lists axioms desirable in any quantization rule for the 'functions of the q's'. The momentum observables are introduced in §2 prior to their quantization in §4. §5 essentially shows how conventional quantum mechanics fits into this scheme of things. By progressive specialization from a general manifold to a vector space, from a general quantization scheme to one which is linear on the linear momentum functions, and finally to an entirely well-behaved (admissible) quantization rule, into which conventional quantum mechanics fits nicely, we obtain in §7-§9 results which become progressively more and more powerful. The final theorem (Theorem 9.2) is perhaps the most significant of all. This result states that there exists a class of functions, which contains all functions of the q's and functions of the p's and all momentum observables and which is closed with respect to any linear canonical transformation L; a rule A assigning a unique self-adjoint operator to each such function f; a unitary operator WL corresponding to L and an equation
𝛬(𝑓 ∘ 𝐿) = 𝑊[sub]𝐿⁻ 𝛬 𝑓 𝑊[sub]𝐿An algebraic formulation of asmptotically separable quantum mechanicsMcLean, R. G. Derekhttp://hdl.handle.net/10023/139092018-06-11T23:17:34Z1984-01-01T00:00:00ZThis thesis explores the possibility of an algebraic formulation of non-relativistic quantum theory in which certain paradoxes associated with non-locality may be resolved. It is shown that the localisation of a free quantum mechanical wave function at large time coincides approximately with the localisation of an ensemble of classical particles having the same momentum range. This result is used to give a formal definition of spatially separating states and spatially separating particles. We then study certain C*-algebras on which expectation values converge in an infinite time limit. By considering such algebras which contain local observables it is possible to introduce states at infinity as limits of states described by wave functions. In such a state at infinity there is zero probability of a position measurement finding the system in any bounded region in configuration space. It is shown that a C*-algebra exists on which any coherent superposition of spatially separating states will converge in an infinite time limit to a mixture of disjoint states. This allows us to obtain an asymptotic resolution of de Broglie's paradox and the Einstein, Podolsy and Rosen paradox. These results are obtained for the simplest types of quantum systems i.e. a one particle system without spin having configuration space IRⁿ and a system consisting of two such particles which may be distinguished from each other.
1984-01-01T00:00:00ZMcLean, R. G. DerekThis thesis explores the possibility of an algebraic formulation of non-relativistic quantum theory in which certain paradoxes associated with non-locality may be resolved. It is shown that the localisation of a free quantum mechanical wave function at large time coincides approximately with the localisation of an ensemble of classical particles having the same momentum range. This result is used to give a formal definition of spatially separating states and spatially separating particles. We then study certain C*-algebras on which expectation values converge in an infinite time limit. By considering such algebras which contain local observables it is possible to introduce states at infinity as limits of states described by wave functions. In such a state at infinity there is zero probability of a position measurement finding the system in any bounded region in configuration space. It is shown that a C*-algebra exists on which any coherent superposition of spatially separating states will converge in an infinite time limit to a mixture of disjoint states. This allows us to obtain an asymptotic resolution of de Broglie's paradox and the Einstein, Podolsy and Rosen paradox. These results are obtained for the simplest types of quantum systems i.e. a one particle system without spin having configuration space IRⁿ and a system consisting of two such particles which may be distinguished from each other.Geometrical and topological properties of fractal percolationOrzechowski, Mark E.http://hdl.handle.net/10023/139072018-07-26T09:20:36Z1998-01-01T00:00:00ZThe basic 'fractal percolation' process was first proposed by Mandelbrot in 1974 and takes the following form. Let M ≥2 and P ∈ [0,1]; we start with the unit square C₀ = [0,1]²; Divide C₀ into M² equal closed squares, each of side-length M⁻¹ , in the natural way and retain each of these squares with probability p, or else remove it with probability 1 - p. We let C₁ be the union of those squares retained. The process is now repeated within each square of C₁ to give a new set C₂⊆C₁, consisting of squares of side-length M⁻². Iterating the construction in the obvious way, we obtain a decreasing sequence of sets C₀⊇ C₁ ⊇ C₂ ⊇ … with limit C[sub]∞ = ∩[sub]n≥₁C[sub]n.
The set C[sub]∞ is an example of a random Cantor set, and is typically highly intricate in nature. It may be empty, dust-like or highly connected, depending on the value of p; percolation is said to occur if C[sub]∞ contains large connected components linking opposite sides of the unit square.
In this thesis we shall investigate some of the geometrical and topological properties of C[sub]∞ that hold either almost surely (with probability 1) or with non-zero probability. In particular, the following results are established. We obtain (almost sure) lower and upper bounds on the box-counting dimension of the 'straightest' crossings in C[sub]∞ whenever percolation occurs; we also look at the distribution of the sizes of the connected components and the probability of percolation. In the three-dimensional version of the process, we establish the existence of two distinct phases of percolation, corresponding to the occurrence of paths and surfaces (or 'sheets') in the limit set, and study the limiting behaviour of the phase transition to sheet percolation as M → ∞. We also consider the results of some computer simulations of fractal percolation and present a number of generalisations of the basic process and other closely related constructions.
1998-01-01T00:00:00ZOrzechowski, Mark E.The basic 'fractal percolation' process was first proposed by Mandelbrot in 1974 and takes the following form. Let M ≥2 and P ∈ [0,1]; we start with the unit square C₀ = [0,1]²; Divide C₀ into M² equal closed squares, each of side-length M⁻¹ , in the natural way and retain each of these squares with probability p, or else remove it with probability 1 - p. We let C₁ be the union of those squares retained. The process is now repeated within each square of C₁ to give a new set C₂⊆C₁, consisting of squares of side-length M⁻². Iterating the construction in the obvious way, we obtain a decreasing sequence of sets C₀⊇ C₁ ⊇ C₂ ⊇ … with limit C[sub]∞ = ∩[sub]n≥₁C[sub]n.
The set C[sub]∞ is an example of a random Cantor set, and is typically highly intricate in nature. It may be empty, dust-like or highly connected, depending on the value of p; percolation is said to occur if C[sub]∞ contains large connected components linking opposite sides of the unit square.
In this thesis we shall investigate some of the geometrical and topological properties of C[sub]∞ that hold either almost surely (with probability 1) or with non-zero probability. In particular, the following results are established. We obtain (almost sure) lower and upper bounds on the box-counting dimension of the 'straightest' crossings in C[sub]∞ whenever percolation occurs; we also look at the distribution of the sizes of the connected components and the probability of percolation. In the three-dimensional version of the process, we establish the existence of two distinct phases of percolation, corresponding to the occurrence of paths and surfaces (or 'sheets') in the limit set, and study the limiting behaviour of the phase transition to sheet percolation as M → ∞. We also consider the results of some computer simulations of fractal percolation and present a number of generalisations of the basic process and other closely related constructions.Graph directed self-conformal multifractalsCole, Julianhttp://hdl.handle.net/10023/139032018-06-11T23:17:58Z1999-01-01T00:00:00ZIn this thesis we study the multifractal structure of graph directed self-conformal measures. We begin by introducing a number of notions from geometric measure theory. In particular, several notions of dimension, graph directed iterated function schemes, and the thermodynamic formalism. We then give an historical introduction to multifractal analysis. Finally, we develop our own contribution to multifractal analysis. Our own contribution to multifractal analysis can be broken into three parts; the proof of two multifractal density theorems, the calculation of the multifractal spectrum of self-conformal measures coded by graph directed iterated function schemes, and the introduction of a relative multifractal formalism together with an investigation of the relative multifractal structure of one graph directed self-conformal measure with respect to another. Specifically, in Chapter 5 we show that by interpreting the multifractal Hausdorff and packing measures Olsen introduced in [0195] as Henstock-Thomson variation measures we are able to obtain two stronger density theorems than those obtained by Olsen. In Chapter 6 we give full details of the calculation of the multifractal spectrum of graph directed self-conformal measures satisfying the strong open set condition and show that the multifractal Hausdorff and packing measures introduced by Olsen in [0195] take positive and finite values at the critical dimension provided that the self-conformal measures satisfy the strong separation condition. In Chapter 7 we formalise the idea of performing multifractal analysis with respect to an arbitrary reference measure by developing a formalism for the multifractal analysis of one measure with respect to another. This formalism is based on the ideas of the 'multifractal formalism' as first introduced by Halsey et. al. [HJKPS86] and closely parallels Olsen's formal treatment of this formalism in [0195]. In Chapter 8 we illustrate our relative multifractal formalism by investigating the relative multifractal structure of one graph directed self-conformal measure with respect to another where the two measures are based on the same graph directed self-conformal iterated function scheme which satisfies the strong open set condition.
1999-01-01T00:00:00ZCole, JulianIn this thesis we study the multifractal structure of graph directed self-conformal measures. We begin by introducing a number of notions from geometric measure theory. In particular, several notions of dimension, graph directed iterated function schemes, and the thermodynamic formalism. We then give an historical introduction to multifractal analysis. Finally, we develop our own contribution to multifractal analysis. Our own contribution to multifractal analysis can be broken into three parts; the proof of two multifractal density theorems, the calculation of the multifractal spectrum of self-conformal measures coded by graph directed iterated function schemes, and the introduction of a relative multifractal formalism together with an investigation of the relative multifractal structure of one graph directed self-conformal measure with respect to another. Specifically, in Chapter 5 we show that by interpreting the multifractal Hausdorff and packing measures Olsen introduced in [0195] as Henstock-Thomson variation measures we are able to obtain two stronger density theorems than those obtained by Olsen. In Chapter 6 we give full details of the calculation of the multifractal spectrum of graph directed self-conformal measures satisfying the strong open set condition and show that the multifractal Hausdorff and packing measures introduced by Olsen in [0195] take positive and finite values at the critical dimension provided that the self-conformal measures satisfy the strong separation condition. In Chapter 7 we formalise the idea of performing multifractal analysis with respect to an arbitrary reference measure by developing a formalism for the multifractal analysis of one measure with respect to another. This formalism is based on the ideas of the 'multifractal formalism' as first introduced by Halsey et. al. [HJKPS86] and closely parallels Olsen's formal treatment of this formalism in [0195]. In Chapter 8 we illustrate our relative multifractal formalism by investigating the relative multifractal structure of one graph directed self-conformal measure with respect to another where the two measures are based on the same graph directed self-conformal iterated function scheme which satisfies the strong open set condition.Parametric models of surfacesRobertson, Stewart A. (Stewart Alexander)http://hdl.handle.net/10023/138972018-06-11T23:17:17Z1957-01-01T00:00:00Z1957-01-01T00:00:00ZRobertson, Stewart A. (Stewart Alexander)Finite difference techniques of improved accuracyLambert, J. D.http://hdl.handle.net/10023/138882018-06-11T23:16:39Z1963-01-01T00:00:00ZIt is the major purpose of this thesis to propose finite difference techniques of improved accuracy for the numerical solution of ordinary differential equations, and for the numerical evaluation of definite integrals, the former problem being discussed in Chapter II, and the latter in Chapter IV. In Chapter III the stability of the formulae evolved in Chapter II is studied.
1963-01-01T00:00:00ZLambert, J. D.It is the major purpose of this thesis to propose finite difference techniques of improved accuracy for the numerical solution of ordinary differential equations, and for the numerical evaluation of definite integrals, the former problem being discussed in Chapter II, and the latter in Chapter IV. In Chapter III the stability of the formulae evolved in Chapter II is studied.Polynomial interpolation on a triangular regionYahaya, Daudhttp://hdl.handle.net/10023/138872018-06-11T23:17:02Z1994-01-01T00:00:00ZIt is well known that given f there is a unique polynomial of degree at most n which interpolates f on the standard triangle with uniform nodes (i, j), i, j ≥ 0, i + j ≤n. This leads us to the study of polynomial interpolation on a "triangular" domain with the nodes,
S = {([i], [j]): i, j ≥ 0, i + j ≤n}, [k] = [k][sub]q = (1-qᵏ)/(1-q), q > 0, which includes the standard triangle as a special case. In Chapter 2 of this thesis we derive a forward difference formula (of degree at most n) in the x and y directions for the interpolating polynomial P[sub]n on S. We also construct a Lagrange form of an interpolating polynomial which uses hyperbolas (although its coefficients are of degree up to 2n) and discuss a Neville-Aitken algorithm. In Chapter 3 we derive the Newton formula for the interpolating polynomial P[sub]n on the set of distinct points {(xᵢ, y[sub]j): i, j ≥ 0, i + j ≤n}. In particular if xᵢ = [i][sub]p and y[sub]j = [j]q, we show that Newton's form of P[sub]n reduces to a forward difference formula. We show further that we can express the interpolating polynomial on S itself in a Lagrange form and although its coefficients Ln/ij are not as simple as those of the first Lagrange form, they all have degree n. Moreover, Ln/ij can all be expressed in terms of Lm/0,0, 0 ≤ m ≤ n. In Chapter 4 we show that P[sub]n has a limit when both p, q → 0. We then verify that the interpolation properties of the limit form depend on the appropriate partial derivatives of f(x, y). In Chapter 5 we study integration rules I[sub]n of interpolatory type on the triangle S[sub] = {(x, y): 0 ≤ x ≤y ≤ [n]). For 1 ≤ n ≤5, we calculate the weights wn/ij for I[sub]n in terms of the parameter q and study certain general properties which govern wn/ij on S[sub]n. Finally, Chapter 6 deals with the behaviour of the Lebesgue functions 𝜆[sub]n(x, y; q) and the corresponding Lebesgue constant. We prove a property concerning where 𝜆[sub]n takes the value 1 at points other than the interpolation nodes. We also analyse the discontinuity of the directional derivative of 𝜆[sub]n on S[sub]n.
1994-01-01T00:00:00ZYahaya, DaudIt is well known that given f there is a unique polynomial of degree at most n which interpolates f on the standard triangle with uniform nodes (i, j), i, j ≥ 0, i + j ≤n. This leads us to the study of polynomial interpolation on a "triangular" domain with the nodes,
S = {([i], [j]): i, j ≥ 0, i + j ≤n}, [k] = [k][sub]q = (1-qᵏ)/(1-q), q > 0, which includes the standard triangle as a special case. In Chapter 2 of this thesis we derive a forward difference formula (of degree at most n) in the x and y directions for the interpolating polynomial P[sub]n on S. We also construct a Lagrange form of an interpolating polynomial which uses hyperbolas (although its coefficients are of degree up to 2n) and discuss a Neville-Aitken algorithm. In Chapter 3 we derive the Newton formula for the interpolating polynomial P[sub]n on the set of distinct points {(xᵢ, y[sub]j): i, j ≥ 0, i + j ≤n}. In particular if xᵢ = [i][sub]p and y[sub]j = [j]q, we show that Newton's form of P[sub]n reduces to a forward difference formula. We show further that we can express the interpolating polynomial on S itself in a Lagrange form and although its coefficients Ln/ij are not as simple as those of the first Lagrange form, they all have degree n. Moreover, Ln/ij can all be expressed in terms of Lm/0,0, 0 ≤ m ≤ n. In Chapter 4 we show that P[sub]n has a limit when both p, q → 0. We then verify that the interpolation properties of the limit form depend on the appropriate partial derivatives of f(x, y). In Chapter 5 we study integration rules I[sub]n of interpolatory type on the triangle S[sub] = {(x, y): 0 ≤ x ≤y ≤ [n]). For 1 ≤ n ≤5, we calculate the weights wn/ij for I[sub]n in terms of the parameter q and study certain general properties which govern wn/ij on S[sub]n. Finally, Chapter 6 deals with the behaviour of the Lebesgue functions 𝜆[sub]n(x, y; q) and the corresponding Lebesgue constant. We prove a property concerning where 𝜆[sub]n takes the value 1 at points other than the interpolation nodes. We also analyse the discontinuity of the directional derivative of 𝜆[sub]n on S[sub]n.Some contributions to the theory and application of polynomial approximationPhillips, G. M. (George McArtney)http://hdl.handle.net/10023/138832018-06-11T23:17:11Z1969-01-01T00:00:00ZThe fundamental theorem, as far as this work is concerned, is Weierstrass' theorem (1885) on the approximability of continuous functions by polynomials. Since the time of Weierstrass (1815-97) and his equally important contemporary Chebyshev (1821-94), the topic of approximation has grown enormously into a subject of considerable interest to both pure and applied mathematicians. The subject matter of this thesis, being exclusively concerned with polynomial approximations to a single-valued, function of one real variable, is on the side of 'applied' side of approximation theory. The first chapter lists the definitions and theorems required subsequently. Chapter is devoted to estimates for the maximum error in minimax polynomial approximations. Extensions of this are used to obtain crude error estimates for cubic spline approximations. The following chapter extends the minimax results to deal also with best L[sub]p polynomial approximations, which include beat least squares (L₂) and best modulus of integral (L₁) approximations as special cases. Chapter 4 is different in character. It is on the practical problem of approximating to convex or nearly convex data.
1969-01-01T00:00:00ZPhillips, G. M. (George McArtney)The fundamental theorem, as far as this work is concerned, is Weierstrass' theorem (1885) on the approximability of continuous functions by polynomials. Since the time of Weierstrass (1815-97) and his equally important contemporary Chebyshev (1821-94), the topic of approximation has grown enormously into a subject of considerable interest to both pure and applied mathematicians. The subject matter of this thesis, being exclusively concerned with polynomial approximations to a single-valued, function of one real variable, is on the side of 'applied' side of approximation theory. The first chapter lists the definitions and theorems required subsequently. Chapter is devoted to estimates for the maximum error in minimax polynomial approximations. Extensions of this are used to obtain crude error estimates for cubic spline approximations. The following chapter extends the minimax results to deal also with best L[sub]p polynomial approximations, which include beat least squares (L₂) and best modulus of integral (L₁) approximations as special cases. Chapter 4 is different in character. It is on the practical problem of approximating to convex or nearly convex data.Some consequences of symmetry in strong Stieltjes distributionsBracciali, Cleonice Fátima Braccialihttp://hdl.handle.net/10023/138812018-06-11T23:17:06Z1998-01-01T00:00:00ZThe main purpose of this work is to study a class of strong Stieltjes distributions 𝜓(t), defined on an interval (a, b) ⊆ (0, ∞), where 0 < 𝛽 < b ≤ ∞ and a = 𝛽²/b which satisfy the symmetric property
(dψ(t))/t[super]ω=-(dψ(β^2/t))/((β^2/t)[super]ω), tε (a,b), 2ωε𝓩
We investigate the consequences of this symmetric property on the orthogonal L-polynomials related to distributions ψ(t)and which are the denominators of the two-point Pade approximants for the power series that arise in the moment problem. We examine relations involving the coefficients of the continued fractions that correspond to these power series. We also study the consequences of the symmetry on the associated quadrature formulae.
1998-01-01T00:00:00ZBracciali, Cleonice Fátima BraccialiThe main purpose of this work is to study a class of strong Stieltjes distributions 𝜓(t), defined on an interval (a, b) ⊆ (0, ∞), where 0 < 𝛽 < b ≤ ∞ and a = 𝛽²/b which satisfy the symmetric property
(dψ(t))/t[super]ω=-(dψ(β^2/t))/((β^2/t)[super]ω), tε (a,b), 2ωε𝓩
We investigate the consequences of this symmetric property on the orthogonal L-polynomials related to distributions ψ(t)and which are the denominators of the two-point Pade approximants for the power series that arise in the moment problem. We examine relations involving the coefficients of the continued fractions that correspond to these power series. We also study the consequences of the symmetry on the associated quadrature formulae.Some aspects of the Jacobian conjecture: (the geometry of automorphisms of ℂ²)Ali, A. Hamid A. Hussainhttp://hdl.handle.net/10023/138782018-06-11T23:16:44Z1987-01-01T00:00:00ZWe consider the affine varieties which arise by considering invertible polynomial maps from ℂ² to itself of less than or equal to a given-degree. These varieties arise naturally in the investigation of the long-standing Jacobian Conjecture. We start with some calculations in the lower degree cases. These calculations provide a proof of the Jacobian conjecture in these cases and suggest how the investigation in the higher degree cases should proceed. We then show how invertible polynomial maps can be decomposed as products of what we call triangular maps and we are able to prove a uniqueness result which gives a stronger version of Jung's theorem [j] which is one of the most important results in this area. Our proof also gives a new derivation of Jung's theorem from Segre's lemma. We give a different decomposition of an invertible polynomial map as a composition of "irreducible maps" and we are able to write down standard forms for these irreducibles. We use these standard forms to give a description of the structure of the varieties of invertible maps. We consider some interesting group actions on our varieties and show how these fit in with the structure we describe. Finally, we look at the problem of identifying polynomial maps of finite order. Our description of the structure of the above varieties allows us to solve this problem completely and we are able to show that the only elements of finite order are those which arise from conjugating linear elements of finite order.
1987-01-01T00:00:00ZAli, A. Hamid A. HussainWe consider the affine varieties which arise by considering invertible polynomial maps from ℂ² to itself of less than or equal to a given-degree. These varieties arise naturally in the investigation of the long-standing Jacobian Conjecture. We start with some calculations in the lower degree cases. These calculations provide a proof of the Jacobian conjecture in these cases and suggest how the investigation in the higher degree cases should proceed. We then show how invertible polynomial maps can be decomposed as products of what we call triangular maps and we are able to prove a uniqueness result which gives a stronger version of Jung's theorem [j] which is one of the most important results in this area. Our proof also gives a new derivation of Jung's theorem from Segre's lemma. We give a different decomposition of an invertible polynomial map as a composition of "irreducible maps" and we are able to write down standard forms for these irreducibles. We use these standard forms to give a description of the structure of the varieties of invertible maps. We consider some interesting group actions on our varieties and show how these fit in with the structure we describe. Finally, we look at the problem of identifying polynomial maps of finite order. Our description of the structure of the above varieties allows us to solve this problem completely and we are able to show that the only elements of finite order are those which arise from conjugating linear elements of finite order.On Riesz summabilityShawyer, Brucehttp://hdl.handle.net/10023/138252018-06-20T10:30:12Z1963-01-01T00:00:00ZThe thesis is divided into four chapters. The first contains notation and fundamental results. The others contain a number of theorems on Riesz summability, ordinary in the second, absolute in the third and strong in the fourth. The substance of chapter II has appeared in the Proceedings of the Glasgow Mathermatical Association [2].
1963-01-01T00:00:00ZShawyer, BruceThe thesis is divided into four chapters. The first contains notation and fundamental results. The others contain a number of theorems on Riesz summability, ordinary in the second, absolute in the third and strong in the fourth. The substance of chapter II has appeared in the Proceedings of the Glasgow Mathermatical Association [2].Global optimization using interval arithmeticMohd, Ismail Binhttp://hdl.handle.net/10023/138242018-06-07T23:16:42Z1987-01-01T00:00:00ZThis thesis contains a description of algorithm, MW, for bounding the global minimizers and globally minimum value of a twice continuously differentiable function f :Rⁿ → R¹ R1 in a compact sub-interval of Rⁿ. The algorithm MW is similar to the algorithm of Hansen (Han-80a] in that interval arithmetic is used together with certain of Hansen's ideas, but is different from Hansen's algorithm in that MW bounds the Kuhn Tucker points corresponding to the global minimizers of f in the given sab-interval. The Kuhn Tucker points are bounded with prescribed precision by using either of the algorithms KMSW [SheW-85c] or MAP [SheW-85b]. Numerical results which are obtained from Triplex [BaCM-82a] [MorC-83a] implementations of H and MW axe presented.
1987-01-01T00:00:00ZMohd, Ismail BinThis thesis contains a description of algorithm, MW, for bounding the global minimizers and globally minimum value of a twice continuously differentiable function f :Rⁿ → R¹ R1 in a compact sub-interval of Rⁿ. The algorithm MW is similar to the algorithm of Hansen (Han-80a] in that interval arithmetic is used together with certain of Hansen's ideas, but is different from Hansen's algorithm in that MW bounds the Kuhn Tucker points corresponding to the global minimizers of f in the given sab-interval. The Kuhn Tucker points are bounded with prescribed precision by using either of the algorithms KMSW [SheW-85c] or MAP [SheW-85b]. Numerical results which are obtained from Triplex [BaCM-82a] [MorC-83a] implementations of H and MW axe presented.Modifications of some algorithms for unconstrained optimizationMirnia-Harikandi, Khttp://hdl.handle.net/10023/138222018-06-07T23:17:02Z1979-01-01T00:00:00ZThis thesis contains an account of several modifications to two algorithms for unconstrained optimization, both of which are due to Gill and Murray. Chapter One contains a brief survey of unconstrained optimization and contains also some results which are used subsequently. Chapter Two contains an account of some work on iterative procedures for the solution of operator equations in Banach spaces due to Wolfe (1978a) in which it is suggested that it may be possible, in certain circumstances, to use high-order iterative procedures rather than Newton's method, thereby obtaining computational advantages. In Chapter Three the Newton-type algorithm of Gill and Murray (1974) is described and the ideas contained in Chapter Two are used to construct some modifications of this algorithm. Chapter Four contains some algorithms for the numerical estimation of both full and b and-type Hessian matrices. These algorithms may be used in conjunction with the optimization algorithms which are described in Chapters Three and Five. In Chapter Five the least-squares algorithm of Gill and Murray (1976) is described and the ideas contained in Chapter Two are used to construct some modifications of this algorithm. Chapter Six contains the computational results which were obtained by using the algorithms which are described in Chapters Three, Four and Five to solve the test problems which are listed in Appendices One and Two.
1979-01-01T00:00:00ZMirnia-Harikandi, KThis thesis contains an account of several modifications to two algorithms for unconstrained optimization, both of which are due to Gill and Murray. Chapter One contains a brief survey of unconstrained optimization and contains also some results which are used subsequently. Chapter Two contains an account of some work on iterative procedures for the solution of operator equations in Banach spaces due to Wolfe (1978a) in which it is suggested that it may be possible, in certain circumstances, to use high-order iterative procedures rather than Newton's method, thereby obtaining computational advantages. In Chapter Three the Newton-type algorithm of Gill and Murray (1974) is described and the ideas contained in Chapter Two are used to construct some modifications of this algorithm. Chapter Four contains some algorithms for the numerical estimation of both full and b and-type Hessian matrices. These algorithms may be used in conjunction with the optimization algorithms which are described in Chapters Three and Five. In Chapter Five the least-squares algorithm of Gill and Murray (1976) is described and the ideas contained in Chapter Two are used to construct some modifications of this algorithm. Chapter Six contains the computational results which were obtained by using the algorithms which are described in Chapters Three, Four and Five to solve the test problems which are listed in Appendices One and Two.Formal methods for deriving Green-type transitional and uniform asymptotic expansions from differential equationsJorna, Siebehttp://hdl.handle.net/10023/138212018-06-07T23:16:40Z1965-01-01T00:00:00ZIn the present work, we develop and illustrate powerful, but straightforward, formal methods for deriving asymptotic expansions from differential equations. In the second chapter, the ‘inverse Frobenius method’ for deriving Stokes expansions is exemplified. The main body of this thesis, however, consists of the development of the new Green-Liouville-Melin transform method, and its detailed application to modified Bessel functions, parabolic cylinder functions, Whittaker functions, Poiseuille functions, confluent hypergeometric functions, and also to periodic Mathieu functions and oblate spheroidal wave functions, all with at least one parameter large⁺. The wide scope of the method is evinced by the fact that treatment of the essentially eigenvalue problem posed by the two last-named cases requires no additional techniques. This method, as will be explained in detail in chapter 3, yields Green-type, transitional and uniform expansions.
The transitional expansions found in this way are usually of a simpler form than those derived by alternative processes (e.g. perturbation theory). To state an example, the asymptotic expansions for the periodic Mathieu functions ce(z,h) and se(z,h) valid near |z| = 1/2π that have been obtained in earlier work contain the complicated parabolic cylinder functions (c.f. Meixner 1948, Sips 1949, Dingle and Müller 1962). By contrast, our methods yield expansions of comparable applicability, but involving only elementary functions. To demonstrate their usefulness, we have fed these expansions into a digital computer and obtained extensive tables for ce(z,h) and se(z,h) in the range 50°≤ z ≤90° . Extracts from these tables and comparisons with correct results are given in §8.71.
Following the chapters on the introduction and applications of the Mellin transform technique, there is some preliminary work on a new method for determining the general term in Green-type expansions. The method is illustrated by detailed calculations for modified Bessel and parabolic cylinder functions.
In the final chapter, we present certain suggestions for further work.
1965-01-01T00:00:00ZJorna, SiebeIn the present work, we develop and illustrate powerful, but straightforward, formal methods for deriving asymptotic expansions from differential equations. In the second chapter, the ‘inverse Frobenius method’ for deriving Stokes expansions is exemplified. The main body of this thesis, however, consists of the development of the new Green-Liouville-Melin transform method, and its detailed application to modified Bessel functions, parabolic cylinder functions, Whittaker functions, Poiseuille functions, confluent hypergeometric functions, and also to periodic Mathieu functions and oblate spheroidal wave functions, all with at least one parameter large⁺. The wide scope of the method is evinced by the fact that treatment of the essentially eigenvalue problem posed by the two last-named cases requires no additional techniques. This method, as will be explained in detail in chapter 3, yields Green-type, transitional and uniform expansions.
The transitional expansions found in this way are usually of a simpler form than those derived by alternative processes (e.g. perturbation theory). To state an example, the asymptotic expansions for the periodic Mathieu functions ce(z,h) and se(z,h) valid near |z| = 1/2π that have been obtained in earlier work contain the complicated parabolic cylinder functions (c.f. Meixner 1948, Sips 1949, Dingle and Müller 1962). By contrast, our methods yield expansions of comparable applicability, but involving only elementary functions. To demonstrate their usefulness, we have fed these expansions into a digital computer and obtained extensive tables for ce(z,h) and se(z,h) in the range 50°≤ z ≤90° . Extracts from these tables and comparisons with correct results are given in §8.71.
Following the chapters on the introduction and applications of the Mellin transform technique, there is some preliminary work on a new method for determining the general term in Green-type expansions. The method is illustrated by detailed calculations for modified Bessel and parabolic cylinder functions.
In the final chapter, we present certain suggestions for further work.Presentations of linear groupsWilliams, Peter D.http://hdl.handle.net/10023/138142018-06-07T23:17:19Z1983-01-01T00:00:00ZIf d(M) denotes the rank of the Schur multiplicator of a finite group G, then a group is efficient if -def G = d(M). Efficient presentations of the simple groups PSL(2,p), p an odd prime > 3, were obtained by J.G. Sunday.This raised the question of whether or not all finite simple groups are efficient.
In this thesis, we investigate the deficiency of the groups PSL(2,pⁿ). J.A. Todd gave presentations for PSL(2,pⁿ) which use large numbers of generators and relations. Starting with these, we obtain, at best, deficiency -1 presentations for PSL(2,2ⁿ) (= SL(2,2ⁿ)) and deficiency -6 presentations for PSL(2,pⁿ), p an odd prime. If pⁿ = -1(mod 4), the latter can be reduced to a deficiency -4 presentation. Efficient presentations for PSL(2,25), PSL(2,27) and PSL(2,49) are obtained.
The Behr-Mennicke presentation for PSL(2,p) is one of the most fundamental in the sense that it forms the basis for others, such as those given by Sunday, Zassenhaus and Sidki. Behr and Mennicke derived their presentation indirectly, and it would be desirable to have a more direct proof. The groups G[sub]p(a) are defined as
< U, R, S | U³ = (UR)² = (US)² = Sᵖ = Rᵗ = (SaRU)³= 1, Sᵃ²R = RS >
where a ε GF(p)* and a²ᵗ = 1 (mod p) . We show that G[sub]p (2) is isomorphic with the Behr-Mennicke presentation for PSL(2,p), p > 3. Conditions are found to discover when Gp (a) is isomorphic with PSL(2,p) and, under these conditions, this provides a direct proof of the Behr-Mennicke presentations. For any odd positive integer m, we show that the groups SL(2,ℤ (m)) and PSL(2,ℤ(m)) are efficient.
1983-01-01T00:00:00ZWilliams, Peter D.If d(M) denotes the rank of the Schur multiplicator of a finite group G, then a group is efficient if -def G = d(M). Efficient presentations of the simple groups PSL(2,p), p an odd prime > 3, were obtained by J.G. Sunday.This raised the question of whether or not all finite simple groups are efficient.
In this thesis, we investigate the deficiency of the groups PSL(2,pⁿ). J.A. Todd gave presentations for PSL(2,pⁿ) which use large numbers of generators and relations. Starting with these, we obtain, at best, deficiency -1 presentations for PSL(2,2ⁿ) (= SL(2,2ⁿ)) and deficiency -6 presentations for PSL(2,pⁿ), p an odd prime. If pⁿ = -1(mod 4), the latter can be reduced to a deficiency -4 presentation. Efficient presentations for PSL(2,25), PSL(2,27) and PSL(2,49) are obtained.
The Behr-Mennicke presentation for PSL(2,p) is one of the most fundamental in the sense that it forms the basis for others, such as those given by Sunday, Zassenhaus and Sidki. Behr and Mennicke derived their presentation indirectly, and it would be desirable to have a more direct proof. The groups G[sub]p(a) are defined as
< U, R, S | U³ = (UR)² = (US)² = Sᵖ = Rᵗ = (SaRU)³= 1, Sᵃ²R = RS >
where a ε GF(p)* and a²ᵗ = 1 (mod p) . We show that G[sub]p (2) is isomorphic with the Behr-Mennicke presentation for PSL(2,p), p > 3. Conditions are found to discover when Gp (a) is isomorphic with PSL(2,p) and, under these conditions, this provides a direct proof of the Behr-Mennicke presentations. For any odd positive integer m, we show that the groups SL(2,ℤ (m)) and PSL(2,ℤ(m)) are efficient.A study of the infinite dimensional linear and symplectic groupsArrell, David G.http://hdl.handle.net/10023/138102018-06-07T23:16:43Z1979-01-01T00:00:00ZBy a linear group we shall mean essentially a group of invertible matrices over a ring. Thus, we include in our class of linear groups the 'classical' geometric groups. These are the general linear group, GL[sub]n(F), the orthogonal groups, 0[sub]n (F) and the syraplectic groups Sp[sub]n(F). The normal and subnormal subgroup structure of these groups is well known and has been the subject of much investigation since the turn of the century. We study here the normal and subnormal structure of some of their infinite dimensional counterparts, namely, the infinite dimensional linear group GL(Ω,R), for arbitrary rings R, and the infinite dimensional syraplectic group Sp(Ω,R), for commutative rings R with identity. We shall see that a key role in the classification of the normal and subnormal subgroups of GL(Ω,R) and Sp(Ω,R) is played by the 'elementary' normal subgroups E(Ω,R) and ESp(Ω,R). We shall also see that, in the case of the infinite dimensional linear group, the normal subgroup structure depends very much upon the way in which R is generated as a right R-module. We shall also give a presentation for the 'elementary' subgroup E(Ω,R) when R is a division ring.
1979-01-01T00:00:00ZArrell, David G.By a linear group we shall mean essentially a group of invertible matrices over a ring. Thus, we include in our class of linear groups the 'classical' geometric groups. These are the general linear group, GL[sub]n(F), the orthogonal groups, 0[sub]n (F) and the syraplectic groups Sp[sub]n(F). The normal and subnormal subgroup structure of these groups is well known and has been the subject of much investigation since the turn of the century. We study here the normal and subnormal structure of some of their infinite dimensional counterparts, namely, the infinite dimensional linear group GL(Ω,R), for arbitrary rings R, and the infinite dimensional syraplectic group Sp(Ω,R), for commutative rings R with identity. We shall see that a key role in the classification of the normal and subnormal subgroups of GL(Ω,R) and Sp(Ω,R) is played by the 'elementary' normal subgroups E(Ω,R) and ESp(Ω,R). We shall also see that, in the case of the infinite dimensional linear group, the normal subgroup structure depends very much upon the way in which R is generated as a right R-module. We shall also give a presentation for the 'elementary' subgroup E(Ω,R) when R is a division ring.A variable input boundary problem in contaminant transportWarner, G. C.http://hdl.handle.net/10023/138012018-06-07T23:16:52Z1997-01-01T00:00:00ZThis thesis considers the large-time behaviour of the equation
(∂(u+uᵖ) )/( ∂t ) + Q(t) ∂u/( ∂x) = ∂²u/∂x² p>0, r≥ -1
With 0 ≤ 𝓍 < ∞, t ≥ 0 and Q (t) ~ tʳ, t ∞. This equation models, after suitable scalings are introduced, the one-dimensional flow of a solute through a porous medium with the solute undergoing adsorption by the solid matrix. We consider two models for the contaminant input at 𝓍= 0, the first being continuous input and the second being an initial pulse of contaminant which terminates after a finite time. Thus the total mass of the solute both adsorbed and in solution is considered to be dependent on time. It is found that the asymptotic solution depends crucially on both p and r. In finding the asymptotic solution, a similarity variable is introduced which for p ≥ 1 may involve spatial translation. We also have that when p < 1 interfaces appear and hence we have bounded support, whilst for p≥1 we do not. The principal role of r is to determine the balance between diffusion and convection effects. In the continuous input case this balance is independent of p, whilst in the pulse problem p is also involved in determining the balance.
1997-01-01T00:00:00ZWarner, G. C.This thesis considers the large-time behaviour of the equation
(∂(u+uᵖ) )/( ∂t ) + Q(t) ∂u/( ∂x) = ∂²u/∂x² p>0, r≥ -1
With 0 ≤ 𝓍 < ∞, t ≥ 0 and Q (t) ~ tʳ, t ∞. This equation models, after suitable scalings are introduced, the one-dimensional flow of a solute through a porous medium with the solute undergoing adsorption by the solid matrix. We consider two models for the contaminant input at 𝓍= 0, the first being continuous input and the second being an initial pulse of contaminant which terminates after a finite time. Thus the total mass of the solute both adsorbed and in solution is considered to be dependent on time. It is found that the asymptotic solution depends crucially on both p and r. In finding the asymptotic solution, a similarity variable is introduced which for p ≥ 1 may involve spatial translation. We also have that when p < 1 interfaces appear and hence we have bounded support, whilst for p≥1 we do not. The principal role of r is to determine the balance between diffusion and convection effects. In the continuous input case this balance is independent of p, whilst in the pulse problem p is also involved in determining the balance.The prediction of thermal phase-change boundaries and associated temperature distributionsWood, A. S.http://hdl.handle.net/10023/137972018-06-07T23:16:53Z1984-01-01T00:00:00ZThe past three decades have seen a fast expanding interest in thermal problems exhibiting a change of phase, more commonly known as Stefan problems. With the rapid advance in computer technology the use and expansion of numerical simulation schemes has been responsible for large advances in this field. The increasing size of computers has led to more sophisticated and complex numerical solutions becoming feasible from a computational point of view. On the other hand, part of this interest has developed from industrial quarters where a knowledge of the location of a melting/freezing boundary may be of critical importance for certain processes. Much experimental work has been completed in this field. However, it is still useful to be able to obtain quick, accurate numerical solutions to such problems and it is with this in mind that this thesis is presented. Ideas from both of the above areas of interest are treated. In the first case a simple to program and computationally efficient numerical scheme is proposed for solving one dimensional Stefan problems and its merits are discussed in relation to several of the more common existing solution schemes. This scheme is then modified to cater for a two dimensional problem which crudely imitates a possible heating configuration in some industrial processes. The problem, with its attendant difficulties, is first approximated by a 'test' problem which is constructed so as to admit an analytic solution. This allows assessment of the numerical procedure in two dimensions. In the course of this work a pseudo-analytic solution was obtained for the original two dimensional problem. Finally, in collaboration with the British Gas Corporation, a complex industrial freezing problem is discussed concerning the flow of liquid through an enclosed channel. Some simplifying assumptions are proposed to reduce the problem to a form for which a relatively simple numerical scheme may be adopted. Several simulations are completed to examine the effect of varying physical parameters on the solution and in particular to test for situations of blockage or steady-state.
1984-01-01T00:00:00ZWood, A. S.The past three decades have seen a fast expanding interest in thermal problems exhibiting a change of phase, more commonly known as Stefan problems. With the rapid advance in computer technology the use and expansion of numerical simulation schemes has been responsible for large advances in this field. The increasing size of computers has led to more sophisticated and complex numerical solutions becoming feasible from a computational point of view. On the other hand, part of this interest has developed from industrial quarters where a knowledge of the location of a melting/freezing boundary may be of critical importance for certain processes. Much experimental work has been completed in this field. However, it is still useful to be able to obtain quick, accurate numerical solutions to such problems and it is with this in mind that this thesis is presented. Ideas from both of the above areas of interest are treated. In the first case a simple to program and computationally efficient numerical scheme is proposed for solving one dimensional Stefan problems and its merits are discussed in relation to several of the more common existing solution schemes. This scheme is then modified to cater for a two dimensional problem which crudely imitates a possible heating configuration in some industrial processes. The problem, with its attendant difficulties, is first approximated by a 'test' problem which is constructed so as to admit an analytic solution. This allows assessment of the numerical procedure in two dimensions. In the course of this work a pseudo-analytic solution was obtained for the original two dimensional problem. Finally, in collaboration with the British Gas Corporation, a complex industrial freezing problem is discussed concerning the flow of liquid through an enclosed channel. Some simplifying assumptions are proposed to reduce the problem to a form for which a relatively simple numerical scheme may be adopted. Several simulations are completed to examine the effect of varying physical parameters on the solution and in particular to test for situations of blockage or steady-state.Compressible boundary layers with sharp pressure gradientsReader-Harris, Michael Johnhttp://hdl.handle.net/10023/137952018-06-07T23:16:54Z1981-01-01T00:00:00ZThe work of this thesis was undertaken as a C.A.S.E. award project in collaboration with Rolls-Royce to examine compressible laminar boundary layers with sharp adverse pressure-gradients. Much of the work is devoted to the solution of two important particular problems. The first flow considered is that along a semi-infinite flat plate with uniform pressure when X < X₀ and with the pressure for X > X₀ being so chosen that the boundary layer is just on the point of separation for all X > X₀. Immediately downstream of X₀ there is a sharp pressure rise to which the flow reacts mainly in a thin inner sublayer; so inner and outer asymptotic expansions are derived and matched for the stream function and a function of the temperature. Throughout the thesis the ratio of the viscosity to the absolute temperature is taken to be a function of x, the distance along the wall, alone, and the Illingworth-Stewartson transformation is applied. The Prandtl number, σ, is taken to be of order unity and detailed results are presented for σ= 1 and 0.72. The second flow considered is that along a finite flat plate where the transformed external velocity U₁(X) is chosen such that
U₁(X) = u₀(-X/L)[super]ε, where O< ε <<1,
is the transformed length of the plate and X represents transformed distance downstream from the trailing edge. The skin friction, position of separation and heat transfer right up to separation are determined. On the basis of these two solutions, another solution which is not presented in detail, and a solution (due to Curie) to a fourth sharp pressure gradient problem, a general Stratford-type method for computing compressible boundary layers is derived, which may be used to predict the position of separation, skin friction, heat transfer, displacement and momentum thicknesses for a compressible boundary layer with an unfavourable pressure gradient. In all this work techniques of series analysis are used to good effect. This led us to look at another boundary-layer problem in which such techniques could be used, one in which two parallel infinite disks are initially rotating with angular velocity Ω about a common axis in incompressible fluid, the appropriate Reynolds number being very large. Suddenly the angular velocity of one of the disks is reversed. A new examination of this problem is presented in the appendix to the thesis.
1981-01-01T00:00:00ZReader-Harris, Michael JohnThe work of this thesis was undertaken as a C.A.S.E. award project in collaboration with Rolls-Royce to examine compressible laminar boundary layers with sharp adverse pressure-gradients. Much of the work is devoted to the solution of two important particular problems. The first flow considered is that along a semi-infinite flat plate with uniform pressure when X < X₀ and with the pressure for X > X₀ being so chosen that the boundary layer is just on the point of separation for all X > X₀. Immediately downstream of X₀ there is a sharp pressure rise to which the flow reacts mainly in a thin inner sublayer; so inner and outer asymptotic expansions are derived and matched for the stream function and a function of the temperature. Throughout the thesis the ratio of the viscosity to the absolute temperature is taken to be a function of x, the distance along the wall, alone, and the Illingworth-Stewartson transformation is applied. The Prandtl number, σ, is taken to be of order unity and detailed results are presented for σ= 1 and 0.72. The second flow considered is that along a finite flat plate where the transformed external velocity U₁(X) is chosen such that
U₁(X) = u₀(-X/L)[super]ε, where O< ε <<1,
is the transformed length of the plate and X represents transformed distance downstream from the trailing edge. The skin friction, position of separation and heat transfer right up to separation are determined. On the basis of these two solutions, another solution which is not presented in detail, and a solution (due to Curie) to a fourth sharp pressure gradient problem, a general Stratford-type method for computing compressible boundary layers is derived, which may be used to predict the position of separation, skin friction, heat transfer, displacement and momentum thicknesses for a compressible boundary layer with an unfavourable pressure gradient. In all this work techniques of series analysis are used to good effect. This led us to look at another boundary-layer problem in which such techniques could be used, one in which two parallel infinite disks are initially rotating with angular velocity Ω about a common axis in incompressible fluid, the appropriate Reynolds number being very large. Suddenly the angular velocity of one of the disks is reversed. A new examination of this problem is presented in the appendix to the thesis.The numerical solution of boundary value problems in partial differential equationsKeast, Patrickhttp://hdl.handle.net/10023/137932018-06-07T23:16:31Z1967-01-01T00:00:00Z1967-01-01T00:00:00ZKeast, PatrickOn the fast and accurate computer solution of partial differential systemsHill, Michael T.http://hdl.handle.net/10023/137912018-06-07T23:16:56Z1974-01-01T00:00:00ZTwo methods are presented for use on an electronic computer for the solution of partial differential systems. The first is concerned with accurate solutions of differential equations. It is equally applicable to ordinary differential equations and partial differential equations, and can be used for parabolic, hyperbolic or elliptic systems, and also for non-linear and mixed systems. It can be used in conjunction with existing schemes. Conversely, the method can be used as a very fast method of obtaining a rough solution of the system. It has an additional advantage over traditional higher order methods in that it does not require extra boundary conditions. The second method is concerned with the acceleration of the convergence rate in the solution of hyperbolic systems. The number of iterations has been reduced from tens of thousands with the traditional Lax-Wendroff methods to the order of twenty iterations. Analyses for both the differential and the difference systems are presented. Again the method is easily added to existing programs. The two methods may be used together to give one fast and accurate method.
1974-01-01T00:00:00ZHill, Michael T.Two methods are presented for use on an electronic computer for the solution of partial differential systems. The first is concerned with accurate solutions of differential equations. It is equally applicable to ordinary differential equations and partial differential equations, and can be used for parabolic, hyperbolic or elliptic systems, and also for non-linear and mixed systems. It can be used in conjunction with existing schemes. Conversely, the method can be used as a very fast method of obtaining a rough solution of the system. It has an additional advantage over traditional higher order methods in that it does not require extra boundary conditions. The second method is concerned with the acceleration of the convergence rate in the solution of hyperbolic systems. The number of iterations has been reduced from tens of thousands with the traditional Lax-Wendroff methods to the order of twenty iterations. Analyses for both the differential and the difference systems are presented. Again the method is easily added to existing programs. The two methods may be used together to give one fast and accurate method.Alternating direction methods for hyperbolic systemsGourlay, A. R.http://hdl.handle.net/10023/137882018-06-07T23:16:23Z1966-01-01T00:00:00Z1966-01-01T00:00:00ZGourlay, A. R.Alternating direction implicit methods for partial differential equationsFairweather, Graemehttp://hdl.handle.net/10023/137842018-06-07T23:16:27Z1966-01-01T00:00:00Z1966-01-01T00:00:00ZFairweather, GraemeThe use of non-polynomial interpolants in the numerical solution of ordinary differential equationsShaw, Brianhttp://hdl.handle.net/10023/137832018-06-07T23:17:28Z1966-01-01T00:00:00Z1966-01-01T00:00:00ZShaw, BrianFinite difference methods for non-linear hyperbolic systemsMorris, John Ll.http://hdl.handle.net/10023/137822018-07-18T23:16:49Z1968-01-01T00:00:00Z1968-01-01T00:00:00ZMorris, John Ll.Inner product quadrature formulasGribble, Julian de Gruchyhttp://hdl.handle.net/10023/137802018-06-07T23:16:56Z1979-01-01T00:00:00ZWe investigate an approach to approximating the integral
(0.1) ⨍[sub]R w(x)f(x)g(x)dx ≡ I (f;g),
where R is a region in one-dimensional Euclidean space, and w a weight function. Since (0.1) may be regarded as a continuous bi-linear functional in f and g we approximate it by a discrete bi-linear functional, which we term an Inner Product Quadrature Formula (I.P.Q.F.).
(0.2) Q(f;g) ≡ f̲ᵀAg̲,
Where f̲ᵀ = (Sₒ(f), . . . , s[sub]m(f))ᵀ
g̲ᵀ = (Tₒ(g), . . . , T[sub]n(g)) ᵀ
A = (aᵢ[sub]j)ᵐi=o,ⁿj=0,
And a[sub]i[sub]j are real numbers, 𝛴 ᵐi=0 𝛴ⁿj =0 |aᵢ [sub]j | > 0
The so-called elementary functionals {Sᵢ}ᵐi=0 and {T[sub]j}ⁿj=0 are two sets of linearly independent linear functionals, acting f and g respectively, defined over a certain subspace of functions to which f and g belong. The simplest example of these functionals is function evaluation at a given point.
The matrix A is determined by requiring (0.2) to be exact for certain classes of functions f and g, say
F𝜀𝛷𝛾 ≡ {𝛷₀, . . . , 𝛷ᵧ}, 𝛾≥0
G𝜀𝛹𝛿 ≡ {𝛹₀, . . . , 𝛹[sub] 𝛿} 𝛿≥0
In Chapter 1 we introduce the concept of I.P.Q.F. in more detail and make some general comments about approaches available when examining numerical integration. After explaining in some detail why we feel I.P.Q.F. are a useful tool in §2.1, we proceed in the remainder of Chapter 2 to investigate various conditions which may be placed on 𝛷ᵞ, 𝛹[super] 𝛿 {Sᵢ}ᵐi=0 and {T[sub]j}ⁿj=0 in order to guarantee the existence of I.P.Q.F. exact when F𝜀𝛷𝛾 and G𝜀𝛹𝛿.
In particular we investigate the question of maximizing 𝛾+ 𝛿. In the case where 𝛷ᵢ and 𝛹[sub]j are the standard monomials of degree i and j respectively, some results have already been published in B.I.T. (1977) p. 392-408. We investigate various choices of 𝛷ᵢ and 𝛹[sub]j :
(a) {𝛷ᵢ}ᵐ⁺¹ I = 0 (i.e. 𝛾 = m+1) and {𝛹[sub]j}ᵐ[sub]j = 0 (i.e. 𝛿 = m) being Tchebychev sets (§2.7),
(b) {𝛷ᵢ}²ᵐ⁺¹ I = 0 (i.e. 𝛾 = 2m+1) being a Tchebychev set and 𝛹[super]𝛿 contains only one function (i.e. 𝛿 = 0) (§2.6)
(c) 𝛷ᵢ ≡ (𝛷[sub]l)ⁱ, i=0,1, . . . and 𝛷ᵢ = 𝛹ᵢ, i= 0, 1, … (§2.8).
In Chapter 3 we consider the question of compounding I.P.Q.F. both in the classical sense, and, briefly, by examining spline functions, regarding them as providing a link between an I.P.Q.F on one hand and a compounded I.P.Q.F. on the other. Various methods of theoretically estimating the errors involved are considered in Chapter M-. In the fifth Chapter we examine various ways in which the concept of I.P.Q.F. might (or might not) be extended. Finally, we make some brief comments about the possible applications of I.P.Q.F., and give a few examples.
1979-01-01T00:00:00ZGribble, Julian de GruchyWe investigate an approach to approximating the integral
(0.1) ⨍[sub]R w(x)f(x)g(x)dx ≡ I (f;g),
where R is a region in one-dimensional Euclidean space, and w a weight function. Since (0.1) may be regarded as a continuous bi-linear functional in f and g we approximate it by a discrete bi-linear functional, which we term an Inner Product Quadrature Formula (I.P.Q.F.).
(0.2) Q(f;g) ≡ f̲ᵀAg̲,
Where f̲ᵀ = (Sₒ(f), . . . , s[sub]m(f))ᵀ
g̲ᵀ = (Tₒ(g), . . . , T[sub]n(g)) ᵀ
A = (aᵢ[sub]j)ᵐi=o,ⁿj=0,
And a[sub]i[sub]j are real numbers, 𝛴 ᵐi=0 𝛴ⁿj =0 |aᵢ [sub]j | > 0
The so-called elementary functionals {Sᵢ}ᵐi=0 and {T[sub]j}ⁿj=0 are two sets of linearly independent linear functionals, acting f and g respectively, defined over a certain subspace of functions to which f and g belong. The simplest example of these functionals is function evaluation at a given point.
The matrix A is determined by requiring (0.2) to be exact for certain classes of functions f and g, say
F𝜀𝛷𝛾 ≡ {𝛷₀, . . . , 𝛷ᵧ}, 𝛾≥0
G𝜀𝛹𝛿 ≡ {𝛹₀, . . . , 𝛹[sub] 𝛿} 𝛿≥0
In Chapter 1 we introduce the concept of I.P.Q.F. in more detail and make some general comments about approaches available when examining numerical integration. After explaining in some detail why we feel I.P.Q.F. are a useful tool in §2.1, we proceed in the remainder of Chapter 2 to investigate various conditions which may be placed on 𝛷ᵞ, 𝛹[super] 𝛿 {Sᵢ}ᵐi=0 and {T[sub]j}ⁿj=0 in order to guarantee the existence of I.P.Q.F. exact when F𝜀𝛷𝛾 and G𝜀𝛹𝛿.
In particular we investigate the question of maximizing 𝛾+ 𝛿. In the case where 𝛷ᵢ and 𝛹[sub]j are the standard monomials of degree i and j respectively, some results have already been published in B.I.T. (1977) p. 392-408. We investigate various choices of 𝛷ᵢ and 𝛹[sub]j :
(a) {𝛷ᵢ}ᵐ⁺¹ I = 0 (i.e. 𝛾 = m+1) and {𝛹[sub]j}ᵐ[sub]j = 0 (i.e. 𝛿 = m) being Tchebychev sets (§2.7),
(b) {𝛷ᵢ}²ᵐ⁺¹ I = 0 (i.e. 𝛾 = 2m+1) being a Tchebychev set and 𝛹[super]𝛿 contains only one function (i.e. 𝛿 = 0) (§2.6)
(c) 𝛷ᵢ ≡ (𝛷[sub]l)ⁱ, i=0,1, . . . and 𝛷ᵢ = 𝛹ᵢ, i= 0, 1, … (§2.8).
In Chapter 3 we consider the question of compounding I.P.Q.F. both in the classical sense, and, briefly, by examining spline functions, regarding them as providing a link between an I.P.Q.F on one hand and a compounded I.P.Q.F. on the other. Various methods of theoretically estimating the errors involved are considered in Chapter M-. In the fifth Chapter we examine various ways in which the concept of I.P.Q.F. might (or might not) be extended. Finally, we make some brief comments about the possible applications of I.P.Q.F., and give a few examples.Interval methods for non-linear systemsShearer, J. M.http://hdl.handle.net/10023/137792018-06-07T14:55:57Z1986-01-01T00:00:00ZIn numerical mathematics, there is a need for methods which provide a user with the solution to his problem without requiring him to understand the mathematics underlying the method of solution. Such a method involves computable tests to determine whether or not a solution exists in a given region, and whether, if it exists, such a solution may be found by using the given method. Two valuable tools for the implementation of such methods are interval mathematics and symbolic computation. In. practice all computers have memories of finite size and cannot perform exact arithmetic. Therefore, in addition to the error which is inherent in a given numerical method, namely truncation error, there is also the error due to rounding. Using interval arithmetic, computable tests which guarantee the existence of a solution to a given problem in a given region, and the convergence of a particular iterative method to this solution, become practically realizable. This is not possible using real arithmetic due to the accumulation of rounding error on a computer. The advent of packages which allow symbolic computations to be carried out on a given computer is an important advance for computational numerical mathematics. In particular, the ability to compute derivatives automatically removes the need for a user to supply them, thus eliminating a major source of error in the use of methods requiring first or higher derivatives. In this thesis some methods which use interval arithmetic and symbolic computation for the solution of systems of nonlinear algebraic equations are presented. Some algorithms based on the symmetric single-step algorithm are described. These methods however do not possess computable existence, uniqueness, and convergence tests. Algorithms which do possess such tests, based on the Krawczyk-Moore algorithm are also presented. A simple package which allows symbolic computations to be carried out is described. Several applications for such a package are given. In particular, an interval form of Brown's method is presented.
1986-01-01T00:00:00ZShearer, J. M.In numerical mathematics, there is a need for methods which provide a user with the solution to his problem without requiring him to understand the mathematics underlying the method of solution. Such a method involves computable tests to determine whether or not a solution exists in a given region, and whether, if it exists, such a solution may be found by using the given method. Two valuable tools for the implementation of such methods are interval mathematics and symbolic computation. In. practice all computers have memories of finite size and cannot perform exact arithmetic. Therefore, in addition to the error which is inherent in a given numerical method, namely truncation error, there is also the error due to rounding. Using interval arithmetic, computable tests which guarantee the existence of a solution to a given problem in a given region, and the convergence of a particular iterative method to this solution, become practically realizable. This is not possible using real arithmetic due to the accumulation of rounding error on a computer. The advent of packages which allow symbolic computations to be carried out on a given computer is an important advance for computational numerical mathematics. In particular, the ability to compute derivatives automatically removes the need for a user to supply them, thus eliminating a major source of error in the use of methods requiring first or higher derivatives. In this thesis some methods which use interval arithmetic and symbolic computation for the solution of systems of nonlinear algebraic equations are presented. Some algorithms based on the symmetric single-step algorithm are described. These methods however do not possess computable existence, uniqueness, and convergence tests. Algorithms which do possess such tests, based on the Krawczyk-Moore algorithm are also presented. A simple package which allows symbolic computations to be carried out is described. Several applications for such a package are given. In particular, an interval form of Brown's method is presented.Statistical problems in measuring surface ozone and modelling its patternsHutchison, Paul Stewarthttp://hdl.handle.net/10023/137732018-06-07T14:58:44Z1996-01-01T00:00:00ZThe thesis examines ground level air pollution data supplied by ITE Bush, Penicuik, Midlothian, Scotland. There is a brief examination of sulphur dioxide concentration data, but the Thesis is primarily concerned with ozone. The diurnal behaviour of ozone is the major topic, and a new methodology of classification of 'ozone days' is introduced and discussed. In chapter 2, the inverse Gaussian distribution is considered and rejected as a possible alternative to the standard approach of using the lognormal as a model for the frequency distribution of observed sulphur dioxide concentrations. In chapter 3, the behaviour of digital gas pollution analysers is investigated by making use of data obtained from two such machines operating side by side. A time series model of the differences between the readings obtained from the two machines is considered, and possible effects on modelling discussed. In chapter 4, the changes in the diurnal behaviour of ozone over a year are examined. A new approach involving a distortion of the time axis is shown to give diurnal ozone curves more homogeneous properties and have beneficial effects for modelling purposes. Chapter 5 extends the analysis of the diurnal behaviour of ozone begun in chapter 4 by considering individual 'ozone days' and attempting to classify them as one of several typical 'types' of day. The time distortion method introduced in chapter 4 is used, and a new classification methodology is introduced for considering data of this type. The statistical properties of this method are discussed in chapter 6.
1996-01-01T00:00:00ZHutchison, Paul StewartThe thesis examines ground level air pollution data supplied by ITE Bush, Penicuik, Midlothian, Scotland. There is a brief examination of sulphur dioxide concentration data, but the Thesis is primarily concerned with ozone. The diurnal behaviour of ozone is the major topic, and a new methodology of classification of 'ozone days' is introduced and discussed. In chapter 2, the inverse Gaussian distribution is considered and rejected as a possible alternative to the standard approach of using the lognormal as a model for the frequency distribution of observed sulphur dioxide concentrations. In chapter 3, the behaviour of digital gas pollution analysers is investigated by making use of data obtained from two such machines operating side by side. A time series model of the differences between the readings obtained from the two machines is considered, and possible effects on modelling discussed. In chapter 4, the changes in the diurnal behaviour of ozone over a year are examined. A new approach involving a distortion of the time axis is shown to give diurnal ozone curves more homogeneous properties and have beneficial effects for modelling purposes. Chapter 5 extends the analysis of the diurnal behaviour of ozone begun in chapter 4 by considering individual 'ozone days' and attempting to classify them as one of several typical 'types' of day. The time distortion method introduced in chapter 4 is used, and a new classification methodology is introduced for considering data of this type. The statistical properties of this method are discussed in chapter 6.A study of character recognition using geometric moments under conditions of simple and non-simple lossTucker, N. D.http://hdl.handle.net/10023/137682018-06-13T16:08:18Z1974-01-01T00:00:00ZThe theory of Loss Functions Is a fundamental part of Statistical Decision Theory and of Pattern Recognition. However It is a subject which few have studied In detail. This thesis is an attempt to develop a simple character recognition process In which losses may be Implemented when and where necessary. After a brief account of the history of Loss Functions and an Introduction to elementary Decision Theory, some examples have been constructed to demonstrate how various decision boundaries approximate to the optimal boundary and what Increase In loss would be associated with these sub-optimal boundaries. The results show that the Euclidean and Hamming distance discriminants can be sufficiently close approximations that the decision process may be legitimately simplified by the use of these linear boundaries. Geometric moments were adopted for the computer simulation of the recognition process because each moment is closely related to the symmetry and structure of a character, unlike many other features. The theory of Moments is discussed, in particular their geometrical properties. A brief description of the programs used in the simulation follows. Two different data sets were investigated, the first being hand-drawn capitals and the second machine-scanned lower case type script. This latter set was in the form of a message, which presented interesting programming problems in itself. The results from the application of different discriminants to these sets under conditions of simple loss are analysed and the recognition efficiencies are found to vary between about 30% and. 99% depending on the number of moments being used and the type of discriminant. Next certain theoretical problems are studied. The relations between the rejection rate, the error rate and the rejection threshold are discussed both theoretically and practically. Also an attempt is made to predict theoretically the variation of efficiency with the number of moments used in the discrimination. This hypothesis is then tested on the data already calculated and shown to be true within reasonable limits. A discussion of moment ordering by defining their re-solving powers is undertaken and it seems likely that the moments normally used unordered are among the most satisfactory. Finally, some time is devoted towards methods of improving recognition efficiency. Information content is discussed along with the possibilities inherent in the use of digraph and trigraph probabilities. A breakdown of the errors in the recognition system adopted here is presented along with suggestions to improve the technique. The execution time of the different decision mechanisms is then inspected and a refined 2-Stage method is produced. Lastly the various methods by which a decision mechanism might be improved are united under a common loss matrix, formed by a product of matrices each of which represents a particular facet of the recognition problem.
1974-01-01T00:00:00ZTucker, N. D.The theory of Loss Functions Is a fundamental part of Statistical Decision Theory and of Pattern Recognition. However It is a subject which few have studied In detail. This thesis is an attempt to develop a simple character recognition process In which losses may be Implemented when and where necessary. After a brief account of the history of Loss Functions and an Introduction to elementary Decision Theory, some examples have been constructed to demonstrate how various decision boundaries approximate to the optimal boundary and what Increase In loss would be associated with these sub-optimal boundaries. The results show that the Euclidean and Hamming distance discriminants can be sufficiently close approximations that the decision process may be legitimately simplified by the use of these linear boundaries. Geometric moments were adopted for the computer simulation of the recognition process because each moment is closely related to the symmetry and structure of a character, unlike many other features. The theory of Moments is discussed, in particular their geometrical properties. A brief description of the programs used in the simulation follows. Two different data sets were investigated, the first being hand-drawn capitals and the second machine-scanned lower case type script. This latter set was in the form of a message, which presented interesting programming problems in itself. The results from the application of different discriminants to these sets under conditions of simple loss are analysed and the recognition efficiencies are found to vary between about 30% and. 99% depending on the number of moments being used and the type of discriminant. Next certain theoretical problems are studied. The relations between the rejection rate, the error rate and the rejection threshold are discussed both theoretically and practically. Also an attempt is made to predict theoretically the variation of efficiency with the number of moments used in the discrimination. This hypothesis is then tested on the data already calculated and shown to be true within reasonable limits. A discussion of moment ordering by defining their re-solving powers is undertaken and it seems likely that the moments normally used unordered are among the most satisfactory. Finally, some time is devoted towards methods of improving recognition efficiency. Information content is discussed along with the possibilities inherent in the use of digraph and trigraph probabilities. A breakdown of the errors in the recognition system adopted here is presented along with suggestions to improve the technique. The execution time of the different decision mechanisms is then inspected and a refined 2-Stage method is produced. Lastly the various methods by which a decision mechanism might be improved are united under a common loss matrix, formed by a product of matrices each of which represents a particular facet of the recognition problem.A study of the work and methods of Henry Briggs, with special reference to the early history of interpolationWaterson, Andrewhttp://hdl.handle.net/10023/137602018-06-13T23:17:37Z1941-01-01T00:00:00Z1941-01-01T00:00:00ZWaterson, AndrewTransformations in regression, estimation, testing and modellingParker, Imeldahttp://hdl.handle.net/10023/137592018-06-06T23:16:31Z1988-01-01T00:00:00ZTransformation is a powerful tool for model building. In regression the response variable is transformed in order to achieve the usual assumptions of normality, constant variance and additivity of effects. Here the normality assumption is replaced by the Laplace distributional assumption, appropriate when more large errors occur than would be expected if the errors were normally distributed. The parametric model is enlarged to include a transformation parameter and a likelihood procedure is adopted for estimating this parameter simultaneously with other parameters of interest. Diagnostic methods are described for assessing the influence of individual observations on the choice of transformation. Examples are presented. In distribution methodology the independent responses are transformed in order that a distributional assumption is satisfied for the transformed data. Here the interest is in the family of distributions which are not dependent on an unknown shape parameter. The gamma distribution (known order), with special case the exponential distribution, is a member of this family. An information number approach is proposed for transforming a known distribution to the gamma distribution (known order). The approach provides an insight into the large-sample behaviour of the likelihood procedure considered by Draper and Guttman (1968) for investigating transformations of data which allow the transformed observations to follow a gamma distribution. The information number approach is illustrated for three examples end the improvement towards the gamma distribution introduced by transformation is measured numerically and graphically. A graphical procedure is proposed for the general case of investigating transformations of data which allow the transformed observations to follow a distribution dependent on unknown threshold and scale parameters. The procedure is extended to include model testing and estimation for any distribution which with the aid of a power transformation can be put in the simple form of a distribution that is not dependent on an unknown shape parameter. The procedure is based on a ratio, R(y), which is constructed from the power transformation. Also described is a ratio-based technique for estimating the threshold parameter in important parametric models, including the three-parameter Weibull and lognormal distributions. Ratio estimation for the weibull distribution is assessed and compared with the modified maximum likelihood estimation of Cohen and Whitten (1982) in terms of bias and root mean squared error, by means of a simulation study. The methods are illustrated with several examples and extend naturally to singly Type 1 and Type 2 censored data.
1988-01-01T00:00:00ZParker, ImeldaTransformation is a powerful tool for model building. In regression the response variable is transformed in order to achieve the usual assumptions of normality, constant variance and additivity of effects. Here the normality assumption is replaced by the Laplace distributional assumption, appropriate when more large errors occur than would be expected if the errors were normally distributed. The parametric model is enlarged to include a transformation parameter and a likelihood procedure is adopted for estimating this parameter simultaneously with other parameters of interest. Diagnostic methods are described for assessing the influence of individual observations on the choice of transformation. Examples are presented. In distribution methodology the independent responses are transformed in order that a distributional assumption is satisfied for the transformed data. Here the interest is in the family of distributions which are not dependent on an unknown shape parameter. The gamma distribution (known order), with special case the exponential distribution, is a member of this family. An information number approach is proposed for transforming a known distribution to the gamma distribution (known order). The approach provides an insight into the large-sample behaviour of the likelihood procedure considered by Draper and Guttman (1968) for investigating transformations of data which allow the transformed observations to follow a gamma distribution. The information number approach is illustrated for three examples end the improvement towards the gamma distribution introduced by transformation is measured numerically and graphically. A graphical procedure is proposed for the general case of investigating transformations of data which allow the transformed observations to follow a distribution dependent on unknown threshold and scale parameters. The procedure is extended to include model testing and estimation for any distribution which with the aid of a power transformation can be put in the simple form of a distribution that is not dependent on an unknown shape parameter. The procedure is based on a ratio, R(y), which is constructed from the power transformation. Also described is a ratio-based technique for estimating the threshold parameter in important parametric models, including the three-parameter Weibull and lognormal distributions. Ratio estimation for the weibull distribution is assessed and compared with the modified maximum likelihood estimation of Cohen and Whitten (1982) in terms of bias and root mean squared error, by means of a simulation study. The methods are illustrated with several examples and extend naturally to singly Type 1 and Type 2 censored data.Parameterisation-invariant versions of Wald testsLarsen, Pia Veldthttp://hdl.handle.net/10023/137502018-06-06T23:16:57Z1999-01-01T00:00:00ZAlthough Wald tests form one of the major classes of hypothesis tests, they suffer from the well-known major drawback that they are not invariant under reparameterisation. This thesis uses the differential-geometric concept of a yoke to introduce one-parameter families of geometric Wald statistics, which are parameterisation-invariant statistics in the spirit of the traditional Wald statistics. Both the geometric Wald statistics based on the expected likelihood yokes and those based on the observed likelihood yokes are investigated. Bartlett-type adjustments of the geometric Wald statistics are obtained, in order to improve the accuracy of the chi-squared approximations to their distributions under the null hypothesis.
1999-01-01T00:00:00ZLarsen, Pia VeldtAlthough Wald tests form one of the major classes of hypothesis tests, they suffer from the well-known major drawback that they are not invariant under reparameterisation. This thesis uses the differential-geometric concept of a yoke to introduce one-parameter families of geometric Wald statistics, which are parameterisation-invariant statistics in the spirit of the traditional Wald statistics. Both the geometric Wald statistics based on the expected likelihood yokes and those based on the observed likelihood yokes are investigated. Bartlett-type adjustments of the geometric Wald statistics are obtained, in order to improve the accuracy of the chi-squared approximations to their distributions under the null hypothesis.Estimating the parameters in mixtures of circular and spherical distributionsKoutbeiy, Majdi Aminehttp://hdl.handle.net/10023/137482018-06-06T23:16:21Z1990-01-01T00:00:00ZIn this thesis we compare various methods for estimating the unknown parameters in mixtures of circular and spherical distributions. We study the von Mises distribution on the circle and the Fisher distribution on the sphere. We propose a new method of estimation based on the characteristic function and compare it with the classical methods based on maximum likelihood and moments. Thus far these methods have only been successfully applied to distributions on the line. Here we show that the extension to circular and spherical distributions is reasonably straightforward and convergence to the final estimates is fairly rapid. We apply these methods to various simulated and real data sets and show that the results obtained for the mixture of two von Mises distributions are satisfactory but generally depend on the sample size and method of estimation used. However, results obtained for the mixture of two Fisher distributions show that maximum likelihood performs best overall.
1990-01-01T00:00:00ZKoutbeiy, Majdi AmineIn this thesis we compare various methods for estimating the unknown parameters in mixtures of circular and spherical distributions. We study the von Mises distribution on the circle and the Fisher distribution on the sphere. We propose a new method of estimation based on the characteristic function and compare it with the classical methods based on maximum likelihood and moments. Thus far these methods have only been successfully applied to distributions on the line. Here we show that the extension to circular and spherical distributions is reasonably straightforward and convergence to the final estimates is fairly rapid. We apply these methods to various simulated and real data sets and show that the results obtained for the mixture of two von Mises distributions are satisfactory but generally depend on the sample size and method of estimation used. However, results obtained for the mixture of two Fisher distributions show that maximum likelihood performs best overall.Statistical aspects of the population regulation of migrating brown trout "Salmo trutta" in a Lake District streamFryer, Robert Johnhttp://hdl.handle.net/10023/137462018-06-25T16:12:54Z1990-01-01T00:00:00ZStatistical aspects of the population regulation of a migratory brown trout population are investigated. The life cycle of the trout, the study area and the sampling routine are described in Chapter 1. Models of numerical changes in fish populations are reviewed in Chapter 2. Measures that assess the nonlinear behaviour of nonlinear regression models are described in Chapter 3. The additive error Ricker model describes the relationship between the number of 0+ parr in May/June and the number of eggs. The nonlinear behaviour of the model is investigated in Chapter 4. The parameter effects nonlinearity of the model is reduced by a reparameterisation. Chapter 5 investigates the effect of errors in the egg variable on the distributions of the least squares estimators of the additive error and the multiplicative error Ricker models. The errors-in-variables considerably increase the variances of the least squares estimators. Models of the relationships between the numbers of 0+ parr in August/September, the number of 1+ parr, the egg production of a year class and the number of eggs are developed in Chapter 6. These models account for the effect of summer drought on survival. Survival is density dependent during the first summer of the life cycle and density independent thereafter. Standard measures of nonlinearity can seriously underestimate the nonlinear behaviour of piecewise linear change-point models. New measures of nonlinearity appropriate for piecewise linear change-point models are developed in Chapter 7. Chapter 8 develops a model of the growth of brown trout fed on maximum rations as a function of time, body weight and water temperature. Chapter 9 develops a model that relates the survival rate of 0+ parr between May/June and August/September to the length distribution of the trout in May/June. The results of the Thesis are discussed in Chapter 10.
1990-01-01T00:00:00ZFryer, Robert JohnStatistical aspects of the population regulation of a migratory brown trout population are investigated. The life cycle of the trout, the study area and the sampling routine are described in Chapter 1. Models of numerical changes in fish populations are reviewed in Chapter 2. Measures that assess the nonlinear behaviour of nonlinear regression models are described in Chapter 3. The additive error Ricker model describes the relationship between the number of 0+ parr in May/June and the number of eggs. The nonlinear behaviour of the model is investigated in Chapter 4. The parameter effects nonlinearity of the model is reduced by a reparameterisation. Chapter 5 investigates the effect of errors in the egg variable on the distributions of the least squares estimators of the additive error and the multiplicative error Ricker models. The errors-in-variables considerably increase the variances of the least squares estimators. Models of the relationships between the numbers of 0+ parr in August/September, the number of 1+ parr, the egg production of a year class and the number of eggs are developed in Chapter 6. These models account for the effect of summer drought on survival. Survival is density dependent during the first summer of the life cycle and density independent thereafter. Standard measures of nonlinearity can seriously underestimate the nonlinear behaviour of piecewise linear change-point models. New measures of nonlinearity appropriate for piecewise linear change-point models are developed in Chapter 7. Chapter 8 develops a model of the growth of brown trout fed on maximum rations as a function of time, body weight and water temperature. Chapter 9 develops a model that relates the survival rate of 0+ parr between May/June and August/September to the length distribution of the trout in May/June. The results of the Thesis are discussed in Chapter 10.Reliability theory in operational researchAl-Baidhani, Fadil Ajabhttp://hdl.handle.net/10023/137452018-06-25T16:09:17Z1991-01-01T00:00:00ZThis thesis is concerned principally with the problem of estimating the parameters of the Weibull and Beta distributions using several different techniques. These distributions are used in the area of reliability testing and it is important to achieve the best estimates possible of the parameters involved. After considering several accepted methods of estimating the relevant parameters, it is considered that the best method depends on the aim of the analysis, and on the value of the shape parameter 𝛽. For estimating the two-parameter Weibull distribution, it is recommended that Generalized Least Squares (GLS) is the best method to use for values of 𝛽 between 0.5 and 30. However, Maximum Likelihood Estimator (MLE) is a good method for estimating quantiles.
On this basis, the three-parameter Weibull distribution is investigated. The traditional parametrization is compared with a new parametrization developed in this work. By considering parameter effects and intrinsic curvature it is shown that the new parametrization results in a linear effect of the shape parameter. Also it has advantages in quantile estimation because of its ability to provide estimates for a wider range of data sets.
A less frequently used distribution in the field of reliability is the Beta distribution. The lack of frequency of its use is partly due to the difficulty in estimating its parameters. A simple, applicable method is developed here of estimating these parameters. This 'group method' involves estimating the two ends of the distribution. It is shown that this procedure can be used, together with other methods of estimating the two- parameter Beta distribution successfully to estimate the four-parameter Beta distribution.
1991-01-01T00:00:00ZAl-Baidhani, Fadil AjabThis thesis is concerned principally with the problem of estimating the parameters of the Weibull and Beta distributions using several different techniques. These distributions are used in the area of reliability testing and it is important to achieve the best estimates possible of the parameters involved. After considering several accepted methods of estimating the relevant parameters, it is considered that the best method depends on the aim of the analysis, and on the value of the shape parameter 𝛽. For estimating the two-parameter Weibull distribution, it is recommended that Generalized Least Squares (GLS) is the best method to use for values of 𝛽 between 0.5 and 30. However, Maximum Likelihood Estimator (MLE) is a good method for estimating quantiles.
On this basis, the three-parameter Weibull distribution is investigated. The traditional parametrization is compared with a new parametrization developed in this work. By considering parameter effects and intrinsic curvature it is shown that the new parametrization results in a linear effect of the shape parameter. Also it has advantages in quantile estimation because of its ability to provide estimates for a wider range of data sets.
A less frequently used distribution in the field of reliability is the Beta distribution. The lack of frequency of its use is partly due to the difficulty in estimating its parameters. A simple, applicable method is developed here of estimating these parameters. This 'group method' involves estimating the two ends of the distribution. It is shown that this procedure can be used, together with other methods of estimating the two- parameter Beta distribution successfully to estimate the four-parameter Beta distribution.Inference for plant-captureAshbridge, Jonathanhttp://hdl.handle.net/10023/137412018-07-26T09:21:24Z1998-01-01T00:00:00ZWhen investigating the dynamics of an animal population, a primary objective is to obtain reasonable estimates of abundance or population size. This thesis concentrates on the problem of obtaining point estimates of abundance from capture-recapture data and on how such estimation can be improved by using the method of plant-capture. Plant-capture constitutes a natural generalisation of capture-recapture. In a plant-capture study a pre-marked population of known size is added to the target population of unknown size. The capture-recapture experiment is then carried out on the augmented population. Chapter 1 considers the addition of planted individuals to target populations which behave according to the standard capture-recapture model M₀. Chapter 2 investigates an analogous model based on sampling in continuous time. In each of these chapters, distributional results are derived under the assumption that the behaviour of the plants is indistinguishable from that of members of the target population. Maximum likelihood estimators and other new estimators are proposed for each model. The results suggest that the use of plants is beneficial, and furthermore that the new estimators perform more satisfactorily than the maximum likelihood estimators. Chapter 3 introduces, initially in the absence of plants, a new class of estimators, described as coverage adjusted estimators, for the standard capture-recapture model M[sub]h. These new estimators are shown, through simulation and real life data, to compare favourably with estimators that have previously been proposed. Plant-capture versions of these new estimators are then derived and the usefulness of the plants is demonstrated through simulation. Chapter 4 describes how the approach taken in chapter 3 can be modified to produce a new estimator for the analogous continuous time model. This estimator is then shown through simulation to be preferable to estimators that have previously been proposed.
1998-01-01T00:00:00ZAshbridge, JonathanWhen investigating the dynamics of an animal population, a primary objective is to obtain reasonable estimates of abundance or population size. This thesis concentrates on the problem of obtaining point estimates of abundance from capture-recapture data and on how such estimation can be improved by using the method of plant-capture. Plant-capture constitutes a natural generalisation of capture-recapture. In a plant-capture study a pre-marked population of known size is added to the target population of unknown size. The capture-recapture experiment is then carried out on the augmented population. Chapter 1 considers the addition of planted individuals to target populations which behave according to the standard capture-recapture model M₀. Chapter 2 investigates an analogous model based on sampling in continuous time. In each of these chapters, distributional results are derived under the assumption that the behaviour of the plants is indistinguishable from that of members of the target population. Maximum likelihood estimators and other new estimators are proposed for each model. The results suggest that the use of plants is beneficial, and furthermore that the new estimators perform more satisfactorily than the maximum likelihood estimators. Chapter 3 introduces, initially in the absence of plants, a new class of estimators, described as coverage adjusted estimators, for the standard capture-recapture model M[sub]h. These new estimators are shown, through simulation and real life data, to compare favourably with estimators that have previously been proposed. Plant-capture versions of these new estimators are then derived and the usefulness of the plants is demonstrated through simulation. Chapter 4 describes how the approach taken in chapter 3 can be modified to produce a new estimator for the analogous continuous time model. This estimator is then shown through simulation to be preferable to estimators that have previously been proposed.The asymptotic distribution and robustness of the likelihood ratio and score test statisticsEmberson, E. A.http://hdl.handle.net/10023/137382018-06-13T16:14:04Z1995-01-01T00:00:00ZCordeiro & Ferrari (1991) use the asymptotic expansion of Harris (1985) for the moment generating function of the score statistic to produce a generalization of Bartlett adjustment for application to the score statistic. It is shown here that Harris's expansion is not invariant under reparameterization and an invariant expansion is derived using a method based on the expected likelihood yoke. A necessary and sufficient condition for the existence of a generalized Bartlett adjustment for an arbitrary statistic is given in terms of its moment generating function. Generalized Bartlett adjustments to the likelihood ratio and score test statistics are derived in the case where the interest parameter is one-dimensional under the assumption of a mis-specified model, where the true distribution is not assumed to be that under the null hypothesis.
1995-01-01T00:00:00ZEmberson, E. A.Cordeiro & Ferrari (1991) use the asymptotic expansion of Harris (1985) for the moment generating function of the score statistic to produce a generalization of Bartlett adjustment for application to the score statistic. It is shown here that Harris's expansion is not invariant under reparameterization and an invariant expansion is derived using a method based on the expected likelihood yoke. A necessary and sufficient condition for the existence of a generalized Bartlett adjustment for an arbitrary statistic is given in terms of its moment generating function. Generalized Bartlett adjustments to the likelihood ratio and score test statistics are derived in the case where the interest parameter is one-dimensional under the assumption of a mis-specified model, where the true distribution is not assumed to be that under the null hypothesis.On the equivalence of Markov Algorithms and Turing machines and some consequent resultsPapathanassiou, Eleftherioshttp://hdl.handle.net/10023/137362018-06-13T11:57:24Z1979-01-01T00:00:00ZTuring Machines and Markov Algorithms are, and were designed to be, the most powerful devices possible in the field of abstract automata: by their means any and every computable function can be computed.
Because of their equal, indeed maximal, strength, it was naturally assumed that these devices should be equivalent. Nonetheless a formal, exact proof of this universally presumed equivalence was lacking.
The present dissertation rectifies that omission by developing the desired complete, rigorous proof of the equivalence between Turing Machines and Markov Algorithms. The demonstration is being conducted in a constructionist way: for any given Markov Algorithm it is shown that a Turing Machine can be constructed capable of performing exactly what the Algorithm can do and nothing more, and vice versa.
The proof consists in the theoretical construction, given an arbitrary Markov Algorithm, of a Turing Machine behaving in exactly the same way as the Algorithm for all possible inputs; and conversely. Furthermore, the proof is given concrete shape by designing a computer program which can actually carry out the said theoretical constructions.
The equivalence between TM and MA as proven in the first part of our thesis, is being used in the second part for establishing some important consequent results: Thus the equivalence of Deterministic and Nondeterministic MA, of TM and Type 0 Grammars, and of Labelled and Unlabelled MA is concisely shown, and the use of TM as recognizers for type 1 and 3 grammars exclusively is exhibited. It is interesting that, by utilizing the equivalence of TM and MA, it was made possible that the proofs of these latter results be based on primitive principles.
1979-01-01T00:00:00ZPapathanassiou, EleftheriosTuring Machines and Markov Algorithms are, and were designed to be, the most powerful devices possible in the field of abstract automata: by their means any and every computable function can be computed.
Because of their equal, indeed maximal, strength, it was naturally assumed that these devices should be equivalent. Nonetheless a formal, exact proof of this universally presumed equivalence was lacking.
The present dissertation rectifies that omission by developing the desired complete, rigorous proof of the equivalence between Turing Machines and Markov Algorithms. The demonstration is being conducted in a constructionist way: for any given Markov Algorithm it is shown that a Turing Machine can be constructed capable of performing exactly what the Algorithm can do and nothing more, and vice versa.
The proof consists in the theoretical construction, given an arbitrary Markov Algorithm, of a Turing Machine behaving in exactly the same way as the Algorithm for all possible inputs; and conversely. Furthermore, the proof is given concrete shape by designing a computer program which can actually carry out the said theoretical constructions.
The equivalence between TM and MA as proven in the first part of our thesis, is being used in the second part for establishing some important consequent results: Thus the equivalence of Deterministic and Nondeterministic MA, of TM and Type 0 Grammars, and of Labelled and Unlabelled MA is concisely shown, and the use of TM as recognizers for type 1 and 3 grammars exclusively is exhibited. It is interesting that, by utilizing the equivalence of TM and MA, it was made possible that the proofs of these latter results be based on primitive principles.Some contributions to the theory of mathematical programmingSaksena, Chandra P.http://hdl.handle.net/10023/137342018-06-13T11:50:27Z1970-01-01T00:00:00ZAs stated earlier the Simplex Method (or its variations e.g. Dual Simplex Method) has thus far been the most effective and widely used general method for the solution of linear programming problems. The Simplex Method in its various forms starts initially with a basic feasible solution and continues its moves in different iterations within the feasible region till it finds the optimal solution. The only other notable variation of the Simplex Method, namely the Dual Simplex Method, on the other hand, by virtue of the special formulation of the linear programming problem, starts with an in-feasible solution and continues to move in the in-feasible region till it finds the optimal solution at which it enters the feasible region. In other respects both the Simplex and the Dual Simplex Methods follow essentially the same principle for obtaining the optimal solution. The rigorous mathematical features have been widely discussed in the literature [12, 16, 34, 35, 38, 68, 77] and only those formal aspects of this topic which are closely related to the subject of this thesis will be outlined.
The Multiplex Method, though reported in the literature [30, 15, 69, 71, 29, 32], is not so well known and has also not been widely coded on electronic computers. It had earlier been programmed for the English Electric’s Computer ‘DEUCE’ by the author [72] and Ferranti’s ‘MERCURY’ by Ole-John Dahl in 1960 [15]. Later both the above mentioned computers were obsolete and the efforts presently concentrate on coding it for UNIVAC 1100 and IBM 360. The Multiplex Method, as such, has been included in the present thesis and discussed in some detail in chapter 2. The flow diagram and the algorithm for the method is given in section 2.4, chapter 2.
The main body of the thesis consists of developing a new linear programming method which has been called the Bounding Hyperplane Method – Part I. This is explained in detail in chapter 3. The method could initially start with either a basic feasible or in-feasible point and in its subsequent moves it may either alternate between the feasible and the in-feasible regions or get restricted to either of them depending upon the problem. It is applicable as a new phase which we call phase 0 to the Simple Method, particularly in situations where an initial basic feasible point is not available. In such cases it either results in a feasible point at the end of phase 0 or else yields a ‘better’ in-feasible point for phase 1 operations of the Simplex Method. Moreover, it is found that the number of iterations required to reach either the former by the application of phase 0 or the latter by the application of first phase 0 and then phase 1 are, in general, less than those required by following phase 1 alone. This is explained with illustrations in Chapter 6. Even when applied alone the method, in general, yields the optimal solution in fewer iterations as compared with the Simplex Method. This is illustrated with examples in chapter 3.
We also develop and illustrate a powerful but straight-forward method whereby we first find the solution to the equality constraints and (if the former does not yield an inconsistent solution point) then the transformations to the latter are obtained from the equality solution tableau corresponding to the former. This results in reducing the iteration time appreciably for each iteration of the method. It has been called the B.H.P.M. – part II and is discussed in chapter 4.
To estimate the time taken by the B.H.P and the Simplex Method, the two codes (written in Fortran) have been run on a number of problems taken from the literature. The results have been summarised in chapter 7.
Finally, the suggestions for further research towards i. the extensions of the B.H.P.M. to the quadratic programming problem where the function in (1.1.1) is positive semi-definite, and (ii) the accuracy of computations in linear programming, in general, are discussed in sections 8.1 and 8.2 respectively of chapter 8.
1970-01-01T00:00:00ZSaksena, Chandra P.As stated earlier the Simplex Method (or its variations e.g. Dual Simplex Method) has thus far been the most effective and widely used general method for the solution of linear programming problems. The Simplex Method in its various forms starts initially with a basic feasible solution and continues its moves in different iterations within the feasible region till it finds the optimal solution. The only other notable variation of the Simplex Method, namely the Dual Simplex Method, on the other hand, by virtue of the special formulation of the linear programming problem, starts with an in-feasible solution and continues to move in the in-feasible region till it finds the optimal solution at which it enters the feasible region. In other respects both the Simplex and the Dual Simplex Methods follow essentially the same principle for obtaining the optimal solution. The rigorous mathematical features have been widely discussed in the literature [12, 16, 34, 35, 38, 68, 77] and only those formal aspects of this topic which are closely related to the subject of this thesis will be outlined.
The Multiplex Method, though reported in the literature [30, 15, 69, 71, 29, 32], is not so well known and has also not been widely coded on electronic computers. It had earlier been programmed for the English Electric’s Computer ‘DEUCE’ by the author [72] and Ferranti’s ‘MERCURY’ by Ole-John Dahl in 1960 [15]. Later both the above mentioned computers were obsolete and the efforts presently concentrate on coding it for UNIVAC 1100 and IBM 360. The Multiplex Method, as such, has been included in the present thesis and discussed in some detail in chapter 2. The flow diagram and the algorithm for the method is given in section 2.4, chapter 2.
The main body of the thesis consists of developing a new linear programming method which has been called the Bounding Hyperplane Method – Part I. This is explained in detail in chapter 3. The method could initially start with either a basic feasible or in-feasible point and in its subsequent moves it may either alternate between the feasible and the in-feasible regions or get restricted to either of them depending upon the problem. It is applicable as a new phase which we call phase 0 to the Simple Method, particularly in situations where an initial basic feasible point is not available. In such cases it either results in a feasible point at the end of phase 0 or else yields a ‘better’ in-feasible point for phase 1 operations of the Simplex Method. Moreover, it is found that the number of iterations required to reach either the former by the application of phase 0 or the latter by the application of first phase 0 and then phase 1 are, in general, less than those required by following phase 1 alone. This is explained with illustrations in Chapter 6. Even when applied alone the method, in general, yields the optimal solution in fewer iterations as compared with the Simplex Method. This is illustrated with examples in chapter 3.
We also develop and illustrate a powerful but straight-forward method whereby we first find the solution to the equality constraints and (if the former does not yield an inconsistent solution point) then the transformations to the latter are obtained from the equality solution tableau corresponding to the former. This results in reducing the iteration time appreciably for each iteration of the method. It has been called the B.H.P.M. – part II and is discussed in chapter 4.
To estimate the time taken by the B.H.P and the Simplex Method, the two codes (written in Fortran) have been run on a number of problems taken from the literature. The results have been summarised in chapter 7.
Finally, the suggestions for further research towards i. the extensions of the B.H.P.M. to the quadratic programming problem where the function in (1.1.1) is positive semi-definite, and (ii) the accuracy of computations in linear programming, in general, are discussed in sections 8.1 and 8.2 respectively of chapter 8.Involutive automorphisms and real forms of Kac-Moody algebrasClarke, Stefanhttp://hdl.handle.net/10023/137312018-07-26T09:21:56Z1996-01-01T00:00:00ZInvolutive automorphisms of complex affine Kac-Moody algebras (in particular, their conjugacy classes within the group of all automorphisms) and their compact real forms are studied, using the matrix formulation which was developed by Cornwell. The initial study of the a⁽¹⁾ series of affine untwisted Kac-Moody algebras is extended to include the complex affine untwisted Kac-Moody algebras B⁽¹⁾, C⁽¹⁾ and D⁽¹⁾. From the information obtained, explicit bases for real forms of these Kac-Moody algebras are then constructed. A scheme for naming some real forms is suggested. Further work is included which examines the involutive automorphisms and the real forms of A₂⁽²⁾and the algebra G⁽¹⁾₂ (which is based upon an exceptional simple Lie algebra). The work involving the algebra A₂⁽²⁾is part of work towards extending the matrix formulation to twisted Kac-Moody algebras. The analysis also acts as a practical test of this method, and from it we may infer different ways of using the formulation to eventually obtain a complete picture of the conjugacy classes of the involutive automorphisms of all the affine Kac-Moody algebras.
1996-01-01T00:00:00ZClarke, StefanInvolutive automorphisms of complex affine Kac-Moody algebras (in particular, their conjugacy classes within the group of all automorphisms) and their compact real forms are studied, using the matrix formulation which was developed by Cornwell. The initial study of the a⁽¹⁾ series of affine untwisted Kac-Moody algebras is extended to include the complex affine untwisted Kac-Moody algebras B⁽¹⁾, C⁽¹⁾ and D⁽¹⁾. From the information obtained, explicit bases for real forms of these Kac-Moody algebras are then constructed. A scheme for naming some real forms is suggested. Further work is included which examines the involutive automorphisms and the real forms of A₂⁽²⁾and the algebra G⁽¹⁾₂ (which is based upon an exceptional simple Lie algebra). The work involving the algebra A₂⁽²⁾is part of work towards extending the matrix formulation to twisted Kac-Moody algebras. The analysis also acts as a practical test of this method, and from it we may infer different ways of using the formulation to eventually obtain a complete picture of the conjugacy classes of the involutive automorphisms of all the affine Kac-Moody algebras.Subalgebras of free nilpotent and polynilpotent lie algebrasBoral, Melihhttp://hdl.handle.net/10023/137292018-06-11T16:09:40Z1977-01-01T00:00:00ZIn this thesis we study subalgebras in free nilpotent and polynilpotent Lie algebras. Chapter 1 sets up the notation and includes definitions and elementary properties of free and certain reduced free Lie algebras that we use throughout this thesis. We also describe a Hall basis of a free Lie algebra as in [4] and a basis for a free polynilpotent Lie algebra which was developed in [24].
In Chapter 2 we first consider the class of nilpotency of subalbebras of free nilpotent Lie algebras starting with two-generator subalgebras. Then we study those subalgebras in a free nilpotent Lie algebra which, are themselves free nilpotent. We give necessary and sufficient conditions in the case of two-generator subalgebras.
Chapter 3 extends the results obtained in Chapter 2 to the polynilpotent case. First we look at two-generator subalgebras of a free polynilpotent Lie algebra. Then we consider more general subalgebras. Finally we study those subalgebras which are themselves free polynilpotent and give necessary and sufficient conditions for two-generator subalgebras to be free polynilpotent.
In Chapter 4 we first study certain properties of ideals in free, free nilpotent and free polynilpotent Lie algebras and establish the fact that in a free polynilpotent Lie algebra a nonzero ideal which is finitely-generated as a subalgebra must be equal to the whole algebra. Then we consider the quotient Lie algebra of a lower central term of a free Lie algebra by a term of the lower central series of an ideal. We then generalize the results to cover the free nilpotent and free polynilpotent cases. In the last section of Chapter 4 we consider ideals of free nilpotent (and later polynilpotent) Lie algebras as free nilpotent (polynilpotent) subalgebras and establish the fact that in most non-trivial cases such an ideal cannot be free nilpotent (polynilpotent).
In the last chapter we consider the m+k-th term of the lower central series of a free Lie algebra as a subalgebra of the m-th term for m ⩽ k and generalize the results proved in [25]. We give reasons for the failure of these results in the case m > k.
1977-01-01T00:00:00ZBoral, MelihIn this thesis we study subalgebras in free nilpotent and polynilpotent Lie algebras. Chapter 1 sets up the notation and includes definitions and elementary properties of free and certain reduced free Lie algebras that we use throughout this thesis. We also describe a Hall basis of a free Lie algebra as in [4] and a basis for a free polynilpotent Lie algebra which was developed in [24].
In Chapter 2 we first consider the class of nilpotency of subalbebras of free nilpotent Lie algebras starting with two-generator subalgebras. Then we study those subalgebras in a free nilpotent Lie algebra which, are themselves free nilpotent. We give necessary and sufficient conditions in the case of two-generator subalgebras.
Chapter 3 extends the results obtained in Chapter 2 to the polynilpotent case. First we look at two-generator subalgebras of a free polynilpotent Lie algebra. Then we consider more general subalgebras. Finally we study those subalgebras which are themselves free polynilpotent and give necessary and sufficient conditions for two-generator subalgebras to be free polynilpotent.
In Chapter 4 we first study certain properties of ideals in free, free nilpotent and free polynilpotent Lie algebras and establish the fact that in a free polynilpotent Lie algebra a nonzero ideal which is finitely-generated as a subalgebra must be equal to the whole algebra. Then we consider the quotient Lie algebra of a lower central term of a free Lie algebra by a term of the lower central series of an ideal. We then generalize the results to cover the free nilpotent and free polynilpotent cases. In the last section of Chapter 4 we consider ideals of free nilpotent (and later polynilpotent) Lie algebras as free nilpotent (polynilpotent) subalgebras and establish the fact that in most non-trivial cases such an ideal cannot be free nilpotent (polynilpotent).
In the last chapter we consider the m+k-th term of the lower central series of a free Lie algebra as a subalgebra of the m-th term for m ⩽ k and generalize the results proved in [25]. We give reasons for the failure of these results in the case m > k.Finite difference solutions of the Von Mises equationThomson, John Younghttp://hdl.handle.net/10023/137272018-07-18T23:16:41Z1958-01-01T00:00:00ZPrandtl in 1904 discovered that the flow of a fluid over a thin obstacle can be adequately represented by an approximate set of equations, much simpler than the complex Navier-Stokes equations which govern the motion of fluid.
A particularly simple for of these equations, for the two-dimensional steady flow of a fluid past a flat plate, are the Von Mises Boundary layer equations. Unfortunately the Von Mises transformation introduces a singularity at the plate and this discouraged the use of the equations as a means for obtaining numerical solutions of boundary layer problems in incompressible and compressible flow.
In this thesis, we show that this difficulty can be overcome and the Von Mises equations are used as a basis for a finite difference evaluation of the velocity and temperature in the boundary layer adjacent to a flat plate, particular attention being given to conditions near the plate and more especially to the separation point.
In the section on compressible flow, the calculations also yield a check on certain common simplifying assumptions.
1958-01-01T00:00:00ZThomson, John YoungPrandtl in 1904 discovered that the flow of a fluid over a thin obstacle can be adequately represented by an approximate set of equations, much simpler than the complex Navier-Stokes equations which govern the motion of fluid.
A particularly simple for of these equations, for the two-dimensional steady flow of a fluid past a flat plate, are the Von Mises Boundary layer equations. Unfortunately the Von Mises transformation introduces a singularity at the plate and this discouraged the use of the equations as a means for obtaining numerical solutions of boundary layer problems in incompressible and compressible flow.
In this thesis, we show that this difficulty can be overcome and the Von Mises equations are used as a basis for a finite difference evaluation of the velocity and temperature in the boundary layer adjacent to a flat plate, particular attention being given to conditions near the plate and more especially to the separation point.
In the section on compressible flow, the calculations also yield a check on certain common simplifying assumptions.Semigroups of singular endomorphisms of vector spaceDawlings, Robert J. H.http://hdl.handle.net/10023/137252018-06-07T16:14:28Z1980-01-01T00:00:00ZIn 1967, J. A. Erdős showed, using a matrix theory approach that the semigroup Sing[sub]n of singular endomorphisms of an n-dimensional vector space is generated by the set E of idempotent endomorphisms of rank n - 1. This thesis gives an alternative proof using a linear algebra and semigroup theory approach. It is also shown that not all the elements of E are needed to generate Sing[sub]n. Necessary conditions for a subset of E to generate found; these conditions are shown to be sufficient if the vector space is defined over a finite field. In this case, the minimum order of all subsets of E that generate Sing[sub]n is found. The problem of determining the number of subsets of E that generate Sing[sub]n and have this minimum order is considered; it is completely solved when the vector space is two-dimensional. From the proof given by Erdős, it could be deduced that any element of Sing[sub]n could be expressed as the product of, at most, 2n elements of E. It is shown here that this bound may be reduced to n, and that this is best possible. It is also shown that, if E⁺ is the set of all idempotent of Singn, then (E⁺)ⁿ⁻¹ is strictly contained in Sing[sub]n. Finally, it is shown that Erdős's result cannot be extended to the semigroup Sing of continuous singular endomorphisms of a separable Hilbert space. The sub semigroup of Sing generated by the idempotent of Sing is determined and is, clearly, strictly contained in Sing.
1980-01-01T00:00:00ZDawlings, Robert J. H.In 1967, J. A. Erdős showed, using a matrix theory approach that the semigroup Sing[sub]n of singular endomorphisms of an n-dimensional vector space is generated by the set E of idempotent endomorphisms of rank n - 1. This thesis gives an alternative proof using a linear algebra and semigroup theory approach. It is also shown that not all the elements of E are needed to generate Sing[sub]n. Necessary conditions for a subset of E to generate found; these conditions are shown to be sufficient if the vector space is defined over a finite field. In this case, the minimum order of all subsets of E that generate Sing[sub]n is found. The problem of determining the number of subsets of E that generate Sing[sub]n and have this minimum order is considered; it is completely solved when the vector space is two-dimensional. From the proof given by Erdős, it could be deduced that any element of Sing[sub]n could be expressed as the product of, at most, 2n elements of E. It is shown here that this bound may be reduced to n, and that this is best possible. It is also shown that, if E⁺ is the set of all idempotent of Singn, then (E⁺)ⁿ⁻¹ is strictly contained in Sing[sub]n. Finally, it is shown that Erdős's result cannot be extended to the semigroup Sing of continuous singular endomorphisms of a separable Hilbert space. The sub semigroup of Sing generated by the idempotent of Sing is determined and is, clearly, strictly contained in Sing.Formal languages and idempotent semigroupsSezinando, Helena Maria da Encarnaçãohttp://hdl.handle.net/10023/137242018-06-07T16:12:32Z1991-01-01T00:00:00ZThe structure of the lattice 𝗟𝗕 of varieties of idempotent semigroups or bands (as universal algebras) was determined by Birjukov, Fennemore and Gerhard. Wis- math determined the structure of a related lattice: the lattice LBM of varieties of band monoids. In the first two parts we study several questions about these varieties.
In Part I we compute the cardinalities of the Green classes of the free objects in each variety of 𝗟𝗕 [𝗟𝗕𝗠]. These cardinalities constitute a useful piece of information in the study of several questions about these varieties and some of the conclusions obtained here are used in parts II and III.
Part II concerns expansions of bands [band monoids]. More precisely, we compute here the cut-down to generators of the Rhodes expansions of the free objects in the varieties of 𝗟𝗕. We define Rhodes expansion of a monoid, its cut-down to generators and we compute the cut-down to generators of the Rhodes expansions of the free objects in the varieties of 𝗟𝗕𝗠.
In Part III we deal with Eilenberg varieties of band monoids. The last chapter is particularly concerned with the description of the varieties of languages corresponding to these varieties.
1991-01-01T00:00:00ZSezinando, Helena Maria da EncarnaçãoThe structure of the lattice 𝗟𝗕 of varieties of idempotent semigroups or bands (as universal algebras) was determined by Birjukov, Fennemore and Gerhard. Wis- math determined the structure of a related lattice: the lattice LBM of varieties of band monoids. In the first two parts we study several questions about these varieties.
In Part I we compute the cardinalities of the Green classes of the free objects in each variety of 𝗟𝗕 [𝗟𝗕𝗠]. These cardinalities constitute a useful piece of information in the study of several questions about these varieties and some of the conclusions obtained here are used in parts II and III.
Part II concerns expansions of bands [band monoids]. More precisely, we compute here the cut-down to generators of the Rhodes expansions of the free objects in the varieties of 𝗟𝗕. We define Rhodes expansion of a monoid, its cut-down to generators and we compute the cut-down to generators of the Rhodes expansions of the free objects in the varieties of 𝗟𝗕𝗠.
In Part III we deal with Eilenberg varieties of band monoids. The last chapter is particularly concerned with the description of the varieties of languages corresponding to these varieties.Random-walk theory and statistical mechanics of lattice systemsNieto, Alberto Robledohttp://hdl.handle.net/10023/137222018-06-07T16:10:35Z1974-01-01T00:00:00ZIt has been found elsewhere that when approximate relations for the two-particle correlation functions of classical statistical mechanics, such as the Percus-Yevick and the mean-spherical approximations, are applied to the lattice gas models with nearest-neighbour interactions simple expressions are obtained for the total correlation function in terms of the lattice Green's function. Since many of the properties of random walks on a lattice can be described by the lattice Green's function, it follows that these systems, at least when treated under these approximations, may be analysed in terms of the language of random walks.
Here the theory of random walks on lattices is appropriately extended to show that the relationship between the correlation functions and the lattice Green's function is not dependent upon the employment of these approximations, nor to a particular range or form of the potential function. Instead, this relationship is shown to be an alternative form of the Ornstein-Zernike relation between the direct and total correlation functions. The direct correlation function is directly related to the probability of a single step, whereas the total correlation function is given by the first-passage- time probabilities of the random walks. Thermodynamic properties, such as the isothermal compressibility, are also interpreted in terms of random-walk concepts.
The random-walk formulation is then extended to include the study of ordered phases in lattice-gas models and hence order-disorder transitions in these systems. Also, an asymptotic form for the lattice Green's function is derived to illustrate how the form of decay of the total correlation function at large distances depends on the properties of the potential function.
To show that the random-walk interpretation of the Ornstein-Zernike relation is not restricted to lattice systems, we define analogous random-walk functions for continuous space. These lead to a straight-forward generalization of most expressions for discrete space-; the relationship between the continuous-space total correlation and Green's functions has the same form as that for the lattice systems. We also explore the possibility of obtaining random-walk properties of a (lattice or continuous-space) system, not from the existing approximate expressions for the direct correlation function, but instead from a generalised Ornstein-Zernike relation based on a maximum principle of classical statistical mechanics.
Finally, we choose a few specific lattice-gas models to show how the method describes their different properties, such as the behaviour of the total correlation function or that of an order- disorder phase transition.
1974-01-01T00:00:00ZNieto, Alberto RobledoIt has been found elsewhere that when approximate relations for the two-particle correlation functions of classical statistical mechanics, such as the Percus-Yevick and the mean-spherical approximations, are applied to the lattice gas models with nearest-neighbour interactions simple expressions are obtained for the total correlation function in terms of the lattice Green's function. Since many of the properties of random walks on a lattice can be described by the lattice Green's function, it follows that these systems, at least when treated under these approximations, may be analysed in terms of the language of random walks.
Here the theory of random walks on lattices is appropriately extended to show that the relationship between the correlation functions and the lattice Green's function is not dependent upon the employment of these approximations, nor to a particular range or form of the potential function. Instead, this relationship is shown to be an alternative form of the Ornstein-Zernike relation between the direct and total correlation functions. The direct correlation function is directly related to the probability of a single step, whereas the total correlation function is given by the first-passage- time probabilities of the random walks. Thermodynamic properties, such as the isothermal compressibility, are also interpreted in terms of random-walk concepts.
The random-walk formulation is then extended to include the study of ordered phases in lattice-gas models and hence order-disorder transitions in these systems. Also, an asymptotic form for the lattice Green's function is derived to illustrate how the form of decay of the total correlation function at large distances depends on the properties of the potential function.
To show that the random-walk interpretation of the Ornstein-Zernike relation is not restricted to lattice systems, we define analogous random-walk functions for continuous space. These lead to a straight-forward generalization of most expressions for discrete space-; the relationship between the continuous-space total correlation and Green's functions has the same form as that for the lattice systems. We also explore the possibility of obtaining random-walk properties of a (lattice or continuous-space) system, not from the existing approximate expressions for the direct correlation function, but instead from a generalised Ornstein-Zernike relation based on a maximum principle of classical statistical mechanics.
Finally, we choose a few specific lattice-gas models to show how the method describes their different properties, such as the behaviour of the total correlation function or that of an order- disorder phase transition.Contributions to the theory of Ockham algebrasFang, Jiehttp://hdl.handle.net/10023/137202018-06-07T15:06:25Z1997-01-01T00:00:00ZIn the first part of this thesis we consider particular ordered sets (connected and of small height) and determine the cardinality of the corresponding dual MS - algebra and of its set of fixed points.
The remainder of the thesis is devoted to a study of congruences of Ockham algebras and a generalised variety K𝜔 of Ockham algebras that contains all of the Berman varieties K[sub]p,[sub]q. In particular we consider the congruences [sub]i(i = 1, 2,...) defined on an Ockham algebra (L; f) by
(x, y) ∊ [sub]i ⇔ fⁱ(x)= fⁱ(y)
and show that (L; f) ∊ K𝜔 is subdirectly irreducible if and only if the lattice of congruences of L reduces to the chain
𝜔 = 𝝫₀ ≤ 𝝫₁≤ 𝝫₂≤ … ≤𝝫𝜔<𝞲
Where 𝝫𝜔 = ⌵ [sub]i≥0𝝫i. Finally we obtain a characterisation of the finite simple Ockham algebras.
1997-01-01T00:00:00ZFang, JieIn the first part of this thesis we consider particular ordered sets (connected and of small height) and determine the cardinality of the corresponding dual MS - algebra and of its set of fixed points.
The remainder of the thesis is devoted to a study of congruences of Ockham algebras and a generalised variety K𝜔 of Ockham algebras that contains all of the Berman varieties K[sub]p,[sub]q. In particular we consider the congruences [sub]i(i = 1, 2,...) defined on an Ockham algebra (L; f) by
(x, y) ∊ [sub]i ⇔ fⁱ(x)= fⁱ(y)
and show that (L; f) ∊ K𝜔 is subdirectly irreducible if and only if the lattice of congruences of L reduces to the chain
𝜔 = 𝝫₀ ≤ 𝝫₁≤ 𝝫₂≤ … ≤𝝫𝜔<𝞲
Where 𝝫𝜔 = ⌵ [sub]i≥0𝝫i. Finally we obtain a characterisation of the finite simple Ockham algebras.The descent algebras of Coxeter groupsVan WIlligenburg, Stephaniehttp://hdl.handle.net/10023/137132018-06-07T15:04:40Z1997-01-01T00:00:00ZA descent algebra is a subalgebra of the group algebra of a Coxeter group. They were first defined over a field of characteristic zero. In this thesis, the main areas of research to be addressed are;
1. The formulation of a rule for multiplying two elements of descent algebra of the Coxeter groups of type D.
2. The identification of properties exhibited by descent algebras over a field of prime characteristic.
In addressing the first, a framework which exploits the specific properties of Coxeter groups is set up. With this framework, a new justification is given for existing rules for multiplying together two elements in the descent algebras of the Coxeter groups of type A and B. This framework is then used to derive a new multiplication rule for the descent algebra of the Coxeter groups of type D.
To address the second, a descent algebra over a field of prime characteristic, p, is defined. A homomorphism into the algebra of generalised p-modular characters is then described. This homomorphism is then used to obtain the radical, and allows the irreducible modules of the descent algebra to be determined.
Results from the two areas addressed are then exploited to give an explicit description of the radical of the descent algebra of the symmetric groups, over a finite field. In this instance, the nilpotency index of the radical and the irreducible representations are also described. Similarly, the descent algebra of the hyper-octahedral groups, over a finite field, has its radical, nilpotency index, and irreducible representations explicitly determined.
1997-01-01T00:00:00ZVan WIlligenburg, StephanieA descent algebra is a subalgebra of the group algebra of a Coxeter group. They were first defined over a field of characteristic zero. In this thesis, the main areas of research to be addressed are;
1. The formulation of a rule for multiplying two elements of descent algebra of the Coxeter groups of type D.
2. The identification of properties exhibited by descent algebras over a field of prime characteristic.
In addressing the first, a framework which exploits the specific properties of Coxeter groups is set up. With this framework, a new justification is given for existing rules for multiplying together two elements in the descent algebras of the Coxeter groups of type A and B. This framework is then used to derive a new multiplication rule for the descent algebra of the Coxeter groups of type D.
To address the second, a descent algebra over a field of prime characteristic, p, is defined. A homomorphism into the algebra of generalised p-modular characters is then described. This homomorphism is then used to obtain the radical, and allows the irreducible modules of the descent algebra to be determined.
Results from the two areas addressed are then exploited to give an explicit description of the radical of the descent algebra of the symmetric groups, over a finite field. In this instance, the nilpotency index of the radical and the irreducible representations are also described. Similarly, the descent algebra of the hyper-octahedral groups, over a finite field, has its radical, nilpotency index, and irreducible representations explicitly determined.Certain classes of group presentationsVatansever, Bilalhttp://hdl.handle.net/10023/137092018-06-07T15:02:35Z1993-01-01T00:00:00ZIn Chapter two we look at the class
F(n) = <R, S | Rⁿ = Sⁿ = (Rᵃ¹Sᵇ¹)ˣ¹(Rᶜ¹Sᵈ¹)ʸ¹(Rᵃ²Sᵇ²)ˣ² (Rᶜ²Sᵈ²)ʸ² …(RᵃᵐSᵇᵐ)ˣᵐ (RᶜᵐSᵈᵐ)ʸᵐ = 1 >.
For some values of n, a[sub]i , b[sub]i, d[sub]i, x[sub]i, y[sub]i we give results on these groups where we have been able to determine their order, either finite or infinite. In the last section in Chapter two we study two classes of groups generated by A and B and subject to the following relations:
Relations for class 1:
A⁴ = 1, B⁴ = 1, (B(AB)²)⁴ = 1, (B(BA)⁶)⁴ = 1, (B(BA)¹⁴)⁴ = 1, …,
B(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-2)⁴ = 1
A⁻¹B⁻¹)²⁽ⁿ⁻³⁾ᐟ²B(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-2)B(BA)⁽²⁽ⁿ⁻³⁾ᐟ²B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁻¹⁾ᐟ²-2) B⁻¹
A⁻¹B⁻¹)²⁽ⁿ⁺¹⁾ᐟ²-3) A(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-1)B⁻¹= 1
(BA)²⁽ⁿ⁻¹⁾ᐟ² B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁻¹⁾ᐟ²-2) B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁺¹⁾ᐟ²-3) A² =1
Relations for class 2:
A⁴ = 1, B⁴ = 1, (B(AB)²)⁴ = 1, (B(BA)⁶)⁴ = 1, (B(BA)¹⁴)⁴ = 1, …, B(BA)⁽²⁽ⁿᐟ²⁻²⁾)⁴ = 1 , B⁻¹(BA)² ⁽ⁿ⁻²⁾ᐟ²B(BA) ⁽²ⁿᐟ²⁻²⁾ B(A⁻¹B⁻¹)²⁽ⁿ⁻²⁾ᐟ²-1) = 1, (BA) ⁽²ⁿᐟ²+2⁽ⁿ⁻²⁾ᐟ²+2)B(BA) ⁽²ⁿᐟ²-2)B(A⁻¹B⁻¹)²⁽ⁿ⁻²⁾ᐟ²-1)A² =1.
The groups in the first class turn out to be the cyclic group of order 2 and the groups in the second class turn out to be metabelian groups of order 4. (2ⁿᐟ²-1)² . Moreover the derived group of the groups in the second class is the direct product of two copies of a cyclic group of order (2ⁿᐟ²-1)². In Chapter three we study the groups with a presentation of the form:
<A,B|A⁴ = 1, Bⁿ = 1, AⁱBʲAᵏBᵗ =1
and determine all possibilities with conditions: j+t = 0 and i,k ∊ { + 1, 2 }.
Also in the second section of Chapter three we study the groups with a presentation of the form:
<A,B | A⁴ = 1, Bⁿ =1, AⁱBʲAᵏBᵗA ᵐBᵖ =1>
and determine some of the possibilities with conditions: j = l,t = l,p = -2 and i,k,m ∊ ℤ. In Chapter four we give new efficient presentations for the groups PSL(2,p), where p is an odd prime, p ∊ { 5,7,11,13,17,19,23,29,31,37, 41,43,53,59,79,83,89,109,139,229 }. We give permutation generators for these groups which satisfy our efficient presentation. Also we give new efficient presentations for PSL(2,p), where p is a prime power and p ∊ { 9,25,27,49,169}. Also in Chapter four, permutation generators are given for these groups which satisfy our presentations. In Chapter five we give new efficient presentations for the groups SL(2,p), where p is an odd prime and p ∊ { 5,7,11,13,17,19,23,29,31,41, 43,53,79,89,109,139,229 }. Also we give new efficient presentations for the groups SL(2,p), where p is an prime power and p ∊ { 8,16,25,27,49,169 }. In Chapter six we study the class of groups with the presentation
<a,b |aᵖ =1, bᵐ⁺ᵖa⁻ᵐbᵐa⁻ᵐ =1, (ab)² = 1>
,p an odd number and m ∊ ℤ. For some values of p and m these groups have connections with the groups PSL(2,p). In Chapter 7 we attempt to show the efficiency of PSL(2, ℤ[sub]n ) x PSL(2, ℤ[sub]m). For some values of n and m we give efficient presentation for these groups. In the same chapter we also attempt to show the efficiency of PSL(2, ℤ [sub]p) x PSL(2,32). For some values of p we give an efficient presentation for these groups. In the last section of the thesis we give efficient presentations for the following direct products
(i) PSL(2,5) X PSL(2,3²)
(ii) PSL(2,7) X PSL(2,3²)
(iii) PSL(2,5) X PSL(2,3³)
Also in the last section of the thesis the structure of a perfect group of order 161280 is investigated.
1993-01-01T00:00:00ZVatansever, BilalIn Chapter two we look at the class
F(n) = <R, S | Rⁿ = Sⁿ = (Rᵃ¹Sᵇ¹)ˣ¹(Rᶜ¹Sᵈ¹)ʸ¹(Rᵃ²Sᵇ²)ˣ² (Rᶜ²Sᵈ²)ʸ² …(RᵃᵐSᵇᵐ)ˣᵐ (RᶜᵐSᵈᵐ)ʸᵐ = 1 >.
For some values of n, a[sub]i , b[sub]i, d[sub]i, x[sub]i, y[sub]i we give results on these groups where we have been able to determine their order, either finite or infinite. In the last section in Chapter two we study two classes of groups generated by A and B and subject to the following relations:
Relations for class 1:
A⁴ = 1, B⁴ = 1, (B(AB)²)⁴ = 1, (B(BA)⁶)⁴ = 1, (B(BA)¹⁴)⁴ = 1, …,
B(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-2)⁴ = 1
A⁻¹B⁻¹)²⁽ⁿ⁻³⁾ᐟ²B(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-2)B(BA)⁽²⁽ⁿ⁻³⁾ᐟ²B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁻¹⁾ᐟ²-2) B⁻¹
A⁻¹B⁻¹)²⁽ⁿ⁺¹⁾ᐟ²-3) A(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-1)B⁻¹= 1
(BA)²⁽ⁿ⁻¹⁾ᐟ² B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁻¹⁾ᐟ²-2) B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁺¹⁾ᐟ²-3) A² =1
Relations for class 2:
A⁴ = 1, B⁴ = 1, (B(AB)²)⁴ = 1, (B(BA)⁶)⁴ = 1, (B(BA)¹⁴)⁴ = 1, …, B(BA)⁽²⁽ⁿᐟ²⁻²⁾)⁴ = 1 , B⁻¹(BA)² ⁽ⁿ⁻²⁾ᐟ²B(BA) ⁽²ⁿᐟ²⁻²⁾ B(A⁻¹B⁻¹)²⁽ⁿ⁻²⁾ᐟ²-1) = 1, (BA) ⁽²ⁿᐟ²+2⁽ⁿ⁻²⁾ᐟ²+2)B(BA) ⁽²ⁿᐟ²-2)B(A⁻¹B⁻¹)²⁽ⁿ⁻²⁾ᐟ²-1)A² =1.
The groups in the first class turn out to be the cyclic group of order 2 and the groups in the second class turn out to be metabelian groups of order 4. (2ⁿᐟ²-1)² . Moreover the derived group of the groups in the second class is the direct product of two copies of a cyclic group of order (2ⁿᐟ²-1)². In Chapter three we study the groups with a presentation of the form:
<A,B|A⁴ = 1, Bⁿ = 1, AⁱBʲAᵏBᵗ =1
and determine all possibilities with conditions: j+t = 0 and i,k ∊ { + 1, 2 }.
Also in the second section of Chapter three we study the groups with a presentation of the form:
<A,B | A⁴ = 1, Bⁿ =1, AⁱBʲAᵏBᵗA ᵐBᵖ =1>
and determine some of the possibilities with conditions: j = l,t = l,p = -2 and i,k,m ∊ ℤ. In Chapter four we give new efficient presentations for the groups PSL(2,p), where p is an odd prime, p ∊ { 5,7,11,13,17,19,23,29,31,37, 41,43,53,59,79,83,89,109,139,229 }. We give permutation generators for these groups which satisfy our efficient presentation. Also we give new efficient presentations for PSL(2,p), where p is a prime power and p ∊ { 9,25,27,49,169}. Also in Chapter four, permutation generators are given for these groups which satisfy our presentations. In Chapter five we give new efficient presentations for the groups SL(2,p), where p is an odd prime and p ∊ { 5,7,11,13,17,19,23,29,31,41, 43,53,79,89,109,139,229 }. Also we give new efficient presentations for the groups SL(2,p), where p is an prime power and p ∊ { 8,16,25,27,49,169 }. In Chapter six we study the class of groups with the presentation
<a,b |aᵖ =1, bᵐ⁺ᵖa⁻ᵐbᵐa⁻ᵐ =1, (ab)² = 1>
,p an odd number and m ∊ ℤ. For some values of p and m these groups have connections with the groups PSL(2,p). In Chapter 7 we attempt to show the efficiency of PSL(2, ℤ[sub]n ) x PSL(2, ℤ[sub]m). For some values of n and m we give efficient presentation for these groups. In the same chapter we also attempt to show the efficiency of PSL(2, ℤ [sub]p) x PSL(2,32). For some values of p we give an efficient presentation for these groups. In the last section of the thesis we give efficient presentations for the following direct products
(i) PSL(2,5) X PSL(2,3²)
(ii) PSL(2,7) X PSL(2,3²)
(iii) PSL(2,5) X PSL(2,3³)
Also in the last section of the thesis the structure of a perfect group of order 161280 is investigated.Semigroups with length morphismsSaunders, Bryan Jameshttp://hdl.handle.net/10023/137062018-06-04T23:16:41Z1998-01-01T00:00:00ZThe class of metrical semigroups is defined as the set consisting of those semigroups which can be homomorphically mapped into the semigroup of natural numbers (without zero) under addition.
The finitely generated members of this class are characterised and the infinitely generated case is discussed. A semigroup is called locally metrical if every finitely generated subsemigroup is metrical.
The classical Green's relations are trivial on any metrical semigroup. Generalisations 𝓗+, 𝓛+ and 𝓡+ of the Green's relations are defined and it is shown that for any cancellative metrical semigroup, S, 𝓗 + is " as big as possible " if and only if S is isomorphic to a special type of semidirect product of 𝗡 and a group. Lyndon's characterisation of free groups by length functions is discussed andalink between length functions, metrical semigroups and semigroups embeddable into free semigroups is investigated. Next the maximal locally metrical ideal of a semigroup is discussed, and the class of t-compressible semigroups is defined as the set consisting of those semigroups that have no locally metrical ideal. The class of t-compressible semigroups is seen to contain the classes of regular and simple semigroups. Finally it is shown that a large class of semigroups can be decomposed into a chain of locally metrical ideals together with a t-compressible semigroup.
1998-01-01T00:00:00ZSaunders, Bryan JamesThe class of metrical semigroups is defined as the set consisting of those semigroups which can be homomorphically mapped into the semigroup of natural numbers (without zero) under addition.
The finitely generated members of this class are characterised and the infinitely generated case is discussed. A semigroup is called locally metrical if every finitely generated subsemigroup is metrical.
The classical Green's relations are trivial on any metrical semigroup. Generalisations 𝓗+, 𝓛+ and 𝓡+ of the Green's relations are defined and it is shown that for any cancellative metrical semigroup, S, 𝓗 + is " as big as possible " if and only if S is isomorphic to a special type of semidirect product of 𝗡 and a group. Lyndon's characterisation of free groups by length functions is discussed andalink between length functions, metrical semigroups and semigroups embeddable into free semigroups is investigated. Next the maximal locally metrical ideal of a semigroup is discussed, and the class of t-compressible semigroups is defined as the set consisting of those semigroups that have no locally metrical ideal. The class of t-compressible semigroups is seen to contain the classes of regular and simple semigroups. Finally it is shown that a large class of semigroups can be decomposed into a chain of locally metrical ideals together with a t-compressible semigroup.Infinite transformation semigroupsMarques, Maria Paulahttp://hdl.handle.net/10023/137052018-06-04T23:17:09Z1983-01-01T00:00:00ZIn this thesis some topics in the field of Infinite Transformation Semigroups are investigated.
In 1966 Howie considered the full transformation semigroup 𝓣 (x) on an infinite set x of cardinality m. For each 𝝰 in 𝓣 (x) he defined defect of 𝝰 = def 𝝰 and collapse of 𝝰= C(a) to be the sets X \ X 𝝰 and { 𝓍 ∊ x : (∃∊ x, y ≠ 𝓍) X𝝰 = Y𝝰 }, respectively. Later, in 1981 he introduced the set
S[sub]m̱ = {𝝰 ∊ 𝓣(x): |def 𝝰 | = | c(𝝰) | = | ran 𝝰 | = m, |y 𝝰 [super]-1 | <m,
(∀ y ∊ ran 𝝰) }
which is a subsemigroup of 𝓣 (x) provided the cardinal m is regular. Taking m to be a regular cardinal number, Howie proved that S[sub]m̱ is then a bisimple, idempotent-generated semigroup of depth 4. Next he considered the congruence defined in S[sub]m̱ by
△[sub]m̱ = {(𝝰, β) ∊ S[sub]m̱ x S[sub]m̱ : max (|D(𝝰, β) 𝝰| , | D((𝝰, β) β | ) < m̱ }
where D(𝝰, β) = { 𝓍 ∊ X : 𝓍 𝝰 ≠ 𝓍β } and showed that S[sub]m̱* = S[sub]m̱/ △[sub]m̱ is a bisimple, congruence-free and idempotent-generated semigroup of depth 4.
In this thesis comparable results are obtained for the semigroup P[sub]m̱ which is the top principal factor of the semigroup
𝓠[sub]m̱ = {𝝰 ∊ 𝓣(x): |def 𝝰 | = | c(𝝰) | = m̱}
Here it is no longer necessary to restrict to a regular cardinal m̱. The set S[sub]m̱ considered by Howie fails to be a subsemigroup of 𝓣 (𝓍) if m̱ is not regular. It is shown that in this case <S[sub]m̱ > = O[sub]m̱ . In the case where m̱ = 𝓍₀ (a regular cardinal) it is shown that △[sub]𝓍₀ is the only proper congruence on S[sub]m̱.
Within the symmetric inverse semigroup 𝓣(𝓍), the Baer-Levi semigroup B of type (m̱, m̱) on X is considered and a dual B* found. The products BB* and B*B are investigated and the semigroup Km̱ = <B*B> is described. The top principal factor of Km̱ is denoted by Lm̱ and it is shown that Lm̱ = B*B ⋃ {O}. On the set Lm̱ a congruence δ[sub]m̱, closely analogous to the congruence △[sub]m̱ defined above, is considered, and it is shown that Lm̱ / δ[sub]m̱ is a o-bisimple, inverse and nilpotent-generated semigroup.
Finally, two embedding theorems for inverse semigroups and semigroups in general are presented. The cardinalities of some of the semigroups introduced in this thesis are studied.
1983-01-01T00:00:00ZMarques, Maria PaulaIn this thesis some topics in the field of Infinite Transformation Semigroups are investigated.
In 1966 Howie considered the full transformation semigroup 𝓣 (x) on an infinite set x of cardinality m. For each 𝝰 in 𝓣 (x) he defined defect of 𝝰 = def 𝝰 and collapse of 𝝰= C(a) to be the sets X \ X 𝝰 and { 𝓍 ∊ x : (∃∊ x, y ≠ 𝓍) X𝝰 = Y𝝰 }, respectively. Later, in 1981 he introduced the set
S[sub]m̱ = {𝝰 ∊ 𝓣(x): |def 𝝰 | = | c(𝝰) | = | ran 𝝰 | = m, |y 𝝰 [super]-1 | <m,
(∀ y ∊ ran 𝝰) }
which is a subsemigroup of 𝓣 (x) provided the cardinal m is regular. Taking m to be a regular cardinal number, Howie proved that S[sub]m̱ is then a bisimple, idempotent-generated semigroup of depth 4. Next he considered the congruence defined in S[sub]m̱ by
△[sub]m̱ = {(𝝰, β) ∊ S[sub]m̱ x S[sub]m̱ : max (|D(𝝰, β) 𝝰| , | D((𝝰, β) β | ) < m̱ }
where D(𝝰, β) = { 𝓍 ∊ X : 𝓍 𝝰 ≠ 𝓍β } and showed that S[sub]m̱* = S[sub]m̱/ △[sub]m̱ is a bisimple, congruence-free and idempotent-generated semigroup of depth 4.
In this thesis comparable results are obtained for the semigroup P[sub]m̱ which is the top principal factor of the semigroup
𝓠[sub]m̱ = {𝝰 ∊ 𝓣(x): |def 𝝰 | = | c(𝝰) | = m̱}
Here it is no longer necessary to restrict to a regular cardinal m̱. The set S[sub]m̱ considered by Howie fails to be a subsemigroup of 𝓣 (𝓍) if m̱ is not regular. It is shown that in this case <S[sub]m̱ > = O[sub]m̱ . In the case where m̱ = 𝓍₀ (a regular cardinal) it is shown that △[sub]𝓍₀ is the only proper congruence on S[sub]m̱.
Within the symmetric inverse semigroup 𝓣(𝓍), the Baer-Levi semigroup B of type (m̱, m̱) on X is considered and a dual B* found. The products BB* and B*B are investigated and the semigroup Km̱ = <B*B> is described. The top principal factor of Km̱ is denoted by Lm̱ and it is shown that Lm̱ = B*B ⋃ {O}. On the set Lm̱ a congruence δ[sub]m̱, closely analogous to the congruence △[sub]m̱ defined above, is considered, and it is shown that Lm̱ / δ[sub]m̱ is a o-bisimple, inverse and nilpotent-generated semigroup.
Finally, two embedding theorems for inverse semigroups and semigroups in general are presented. The cardinalities of some of the semigroups introduced in this thesis are studied.Idempotents, nilpotents, rank and order in finite transformation semigroupsGarba, Goje Ubahttp://hdl.handle.net/10023/137032018-06-04T23:16:44Z1992-01-01T00:00:00ZLet E, E₁ denote, respectively, the set of singular idempotents in T[sub]n (the semigroup of all full transformations on a finite set X[sub]n = {1,..., n}) and the set of idempotents of defect 1. For a singular element 𝑎 in Tn, let k(𝑎),k₁ (𝑎) be defined by the properties
𝑎 ∈ Eᵏ⁽ᵃ⁾, 𝑎 ∉ Eᵏ⁽ᵃ⁾⁻¹,
𝑎 ∈ E₁ᵏ¹⁽ᵃ⁾, 𝑎 ∉ E₁ᵏ¹⁽ᵃ⁾⁻¹.
In this Thesis, we obtain results analogous to those of Iwahori (1977), Howie (1980), Saito (1989) and Howie, Lusk and McFadden (1990) concerning the values of k(𝑎) and k₁(𝑎) for the partial transformation semigroup P[sub]n. The analogue of Howie and McFadden's (1990) result on the rank of the semigroup K(n,r) = { 𝑎 ∈ T [sub]n: |im 𝑎 | ≤ r,2 ≤ r ≤ n-1} is also obtained.
The nilpotent-generated subsemigroup of P[sub]n was characterised by Sullivan in 1987. In this work, we have obtained its depth and rank.
Nilpotents in IO[sub]n and PO[sub]n (the semigroup of all partial one-one order-preserving maps, and all partial order-preserving maps) are studied. A characterisation of their nilpotent-generated subsemigroups is obtained. So also are their depth and rank. We have also characterised their nilpotent-generated subsemigroup for the infinite set X = {1,2,...}. The rank of the semigroup L(n,r) = {a ∈ S : |im 𝑎 | ≤r, 1 ≤ r ≤ n - 2} is investigated for S = O[sub]n,PO[sub]n,SPO[sub]n and I[sub]n (where O[sub]n is the semigroup of all order-preserving full transformations, SPO[sub]n the semigroup of all strictly partial order- preserving maps, and In the semigroup of one-one partial transformation).
1992-01-01T00:00:00ZGarba, Goje UbaLet E, E₁ denote, respectively, the set of singular idempotents in T[sub]n (the semigroup of all full transformations on a finite set X[sub]n = {1,..., n}) and the set of idempotents of defect 1. For a singular element 𝑎 in Tn, let k(𝑎),k₁ (𝑎) be defined by the properties
𝑎 ∈ Eᵏ⁽ᵃ⁾, 𝑎 ∉ Eᵏ⁽ᵃ⁾⁻¹,
𝑎 ∈ E₁ᵏ¹⁽ᵃ⁾, 𝑎 ∉ E₁ᵏ¹⁽ᵃ⁾⁻¹.
In this Thesis, we obtain results analogous to those of Iwahori (1977), Howie (1980), Saito (1989) and Howie, Lusk and McFadden (1990) concerning the values of k(𝑎) and k₁(𝑎) for the partial transformation semigroup P[sub]n. The analogue of Howie and McFadden's (1990) result on the rank of the semigroup K(n,r) = { 𝑎 ∈ T [sub]n: |im 𝑎 | ≤ r,2 ≤ r ≤ n-1} is also obtained.
The nilpotent-generated subsemigroup of P[sub]n was characterised by Sullivan in 1987. In this work, we have obtained its depth and rank.
Nilpotents in IO[sub]n and PO[sub]n (the semigroup of all partial one-one order-preserving maps, and all partial order-preserving maps) are studied. A characterisation of their nilpotent-generated subsemigroups is obtained. So also are their depth and rank. We have also characterised their nilpotent-generated subsemigroup for the infinite set X = {1,2,...}. The rank of the semigroup L(n,r) = {a ∈ S : |im 𝑎 | ≤r, 1 ≤ r ≤ n - 2} is investigated for S = O[sub]n,PO[sub]n,SPO[sub]n and I[sub]n (where O[sub]n is the semigroup of all order-preserving full transformations, SPO[sub]n the semigroup of all strictly partial order- preserving maps, and In the semigroup of one-one partial transformation).On a family of semigroup congruencesKopamu, Samuel Joseph Lyambianhttp://hdl.handle.net/10023/136992018-06-04T23:16:46Z1996-01-01T00:00:00ZWe introduce in this thesis a new family of semigroup congruences, and we set out to prove that it is worth studying them for the following very important reasons:
(a) that it provides an alternative way of studying algebraic structures of semigroups, thus shedding new light over semigroup structures already known, and it also provides new information about other structures not formerly understood;
(b) that it is useful for constructing new semigroups, hence producing new and interesting classes of semigroups from known classes; and
(c) that it is useful for classifying semigroups, particularly in describing lattices formed by semigroup species such as varieties, pseudovarieties, existence varieties etc.
This interesting family of congruences is described as follows: for any semigroup S, and any ordered pair (n,m) of non-negative integers, define ⦵(n,m) = {(a,b): uav = ubv, for all ⋿Sn and v ⋿Sm}, and we make the convention that S¹ = S and that S0 denotes the set containing only the empty word. The particular cases ⦵(0,1), ⦵(1,0) and ⦵(0,0) were considered by the author in his M.Sc. thesis (1991). In fact, one can recognise ⦵(1,0) to be the well known kernel of the right regular representation of S. It turns out that if S is reductive (for example, if S is a monoid), then ⦵(i,j) is equal to ⦵(0,0) - the identity relation on S, for every (i,j).
After developing the tools required for the latter part of the thesis in Chapters 0-2, in Chapter 3 we introduce a new class of semigroups - the class of all structurally regular semigroups. Making use of a new Mal'tsev-type product, in Chapters 4,5,6 and 7, we describe the lattices formed by certain varieties of structurally regular semigroups.
Many interesting open problems are posed throughout the thesis, and brief literature reviews are inserted in the text where appropriate.
1996-01-01T00:00:00ZKopamu, Samuel Joseph LyambianWe introduce in this thesis a new family of semigroup congruences, and we set out to prove that it is worth studying them for the following very important reasons:
(a) that it provides an alternative way of studying algebraic structures of semigroups, thus shedding new light over semigroup structures already known, and it also provides new information about other structures not formerly understood;
(b) that it is useful for constructing new semigroups, hence producing new and interesting classes of semigroups from known classes; and
(c) that it is useful for classifying semigroups, particularly in describing lattices formed by semigroup species such as varieties, pseudovarieties, existence varieties etc.
This interesting family of congruences is described as follows: for any semigroup S, and any ordered pair (n,m) of non-negative integers, define ⦵(n,m) = {(a,b): uav = ubv, for all ⋿Sn and v ⋿Sm}, and we make the convention that S¹ = S and that S0 denotes the set containing only the empty word. The particular cases ⦵(0,1), ⦵(1,0) and ⦵(0,0) were considered by the author in his M.Sc. thesis (1991). In fact, one can recognise ⦵(1,0) to be the well known kernel of the right regular representation of S. It turns out that if S is reductive (for example, if S is a monoid), then ⦵(i,j) is equal to ⦵(0,0) - the identity relation on S, for every (i,j).
After developing the tools required for the latter part of the thesis in Chapters 0-2, in Chapter 3 we introduce a new class of semigroups - the class of all structurally regular semigroups. Making use of a new Mal'tsev-type product, in Chapters 4,5,6 and 7, we describe the lattices formed by certain varieties of structurally regular semigroups.
Many interesting open problems are posed throughout the thesis, and brief literature reviews are inserted in the text where appropriate.Numerical preservation of velocity induced invariant regions for reaction-diffusion systems on evolving surfacesFrittelli, MassimoMadzvamuse, AnotidoSgura, IvonneVenkataraman, Chandrasekharhttp://hdl.handle.net/10023/136982019-01-06T03:36:29Z2018-06-01T00:00:00ZWe propose and analyse a finite element method with mass lumping (LESFEM) for the numerical approximation of reaction–diffusion systems (RDSs) on surfaces in ℝ3 that evolve under a given velocity field. A fully-discrete method based on the implicit–explicit (IMEX) Euler time-discretisation is formulated and dilation rates which act as indicators of the surface evolution are introduced. Under the assumption that the mesh preserves the Delaunay regularity under evolution, we prove a sufficient condition, that depends on the dilation rates, for the existence of invariant regions (i) at the spatially discrete level with no restriction on the mesh size and (ii) at the fully-discrete level under a timestep restriction that depends on the kinetics, only. In the specific case of the linear heat equation, we prove a semi- and a fully-discrete maximum principle. For the well-known activator-depleted and Thomas reaction–diffusion models we prove the existence of a family of rectangles in the phase space that are invariant only under specific growth laws. Two numerical examples are provided to computationally demonstrate (i) the discrete maximum principle and optimal convergence for the heat equation on a linearly growing sphere and (ii) the existence of an invariant region for the LESFEM–IMEX Euler discretisation of a RDS on a logistically growing surface.
The authors (MF, AM, IS CV) would like to thank the Isaac Newton Institute for Mathematical Sciences for its hospitality during the programme [Coupling Geometric PDEs with Physics for Cell Morphology, Motility and Pattern Formation] supported by EPSRC Grant Number EP/K032208/1. MF’s and IS’s research work has been performed under the auspices of the Italian National Group for Scientific Calculus (GNCS-INdAM). This work (AM, CV) is partly supported by the EPSRC grant number EP/J016780/1 and the Leverhulme Trust Research Project Grant (RPG-2014-149). AM acknowledges funding from the European Union Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 642866.
2018-06-01T00:00:00ZFrittelli, MassimoMadzvamuse, AnotidoSgura, IvonneVenkataraman, ChandrasekharWe propose and analyse a finite element method with mass lumping (LESFEM) for the numerical approximation of reaction–diffusion systems (RDSs) on surfaces in ℝ3 that evolve under a given velocity field. A fully-discrete method based on the implicit–explicit (IMEX) Euler time-discretisation is formulated and dilation rates which act as indicators of the surface evolution are introduced. Under the assumption that the mesh preserves the Delaunay regularity under evolution, we prove a sufficient condition, that depends on the dilation rates, for the existence of invariant regions (i) at the spatially discrete level with no restriction on the mesh size and (ii) at the fully-discrete level under a timestep restriction that depends on the kinetics, only. In the specific case of the linear heat equation, we prove a semi- and a fully-discrete maximum principle. For the well-known activator-depleted and Thomas reaction–diffusion models we prove the existence of a family of rectangles in the phase space that are invariant only under specific growth laws. Two numerical examples are provided to computationally demonstrate (i) the discrete maximum principle and optimal convergence for the heat equation on a linearly growing sphere and (ii) the existence of an invariant region for the LESFEM–IMEX Euler discretisation of a RDS on a logistically growing surface.Nonstandard quantum groups: twisting constructions and noncommutative differential geometryJacobs, Andrew D.http://hdl.handle.net/10023/136932018-06-04T23:16:55Z1998-01-01T00:00:00ZThe general subject of this thesis is quantum groups. The major original results are obtained in the particular areas of twisting constructions and noncommutative differential geometry.
Chapters 1 and 2 are intended to explain to the reader what are quantum groups. They are written in the form of a series of linked results and definitions. Chapter 1 reviews the theory of Lie algebras and Lie groups, focusing attention in particular on the classical Lie algebras and groups. Though none of the quoted results are due to the author, such a review, aimed specifically at setting up the paradigm which provides essential guidance in the theory of quantum groups, does not seem to have appeared already. In Chapter 2 the elements of the quantum group theory are recalled. Once again, almost none of the results are due to the author, though in Section 2.10, some results concerning the nonstandard Jordanian group are presented, by way of a worked example, which have not been published.
Chapter 3 concerns twisting constructions. We introduce a new class of 2-cocycles defined explicitly on the generators of certain multiparameter standard quantum groups. These allow us, through the process of twisting the familiar standard quantum groups, to generate new as well as previously known examples of non-standard quantum groups. In particular we are able to construct generalisations of both the Cremmer-Gervais deformation of SL(3) and the so called esoteric quantum groups of Fronsdal and Galindo in an explicit and straightforward manner.
In Chapter 4 we consider the differential calculus on Hopf algebras as introduced by Woronowicz. We classify all 4-dimensional first order bicovariant calculi on the Jordanian quantum group GL[sub]h,[sub]g(2) and all 3-dimensional first order bicovariant calculi on the Jordanian quantum group SL[sub]h(2). In both cases we assume that the bicovariant bimodules are generated as left modules by the differentials of the quantum group generators. It is found that there are 3 1-parameter families of 4-dimensional bicovariant first order calculi on GL[sub]h,[sub]g(2) and that there is a single, unique, 3-dimensional bicovariant calculus on SL[sub]h(2). This 3-dimensional calculus may be obtained through a classical-like reduction from any one of the three families of 4-dimensional calculi on GL[sub]h,[sub]g(2). Details of the higher order calculi and also the quantum Lie algebras are presented for all calculi. The quantum Lie algebra obtained from the bicovariant calculus on SL[sub]h(2) is shown to be isomorphic to the quantum Lie algebra we obtain as an ad-submodule within the Jordanian universal enveloping algebra U[sub]h(sl[sub]2(C)) and also through a consideration of the decomposition of the tensor product of two copies of the deformed adjoint module. We also obtain the quantum Killing form for this quantum Lie algebra.
1998-01-01T00:00:00ZJacobs, Andrew D.The general subject of this thesis is quantum groups. The major original results are obtained in the particular areas of twisting constructions and noncommutative differential geometry.
Chapters 1 and 2 are intended to explain to the reader what are quantum groups. They are written in the form of a series of linked results and definitions. Chapter 1 reviews the theory of Lie algebras and Lie groups, focusing attention in particular on the classical Lie algebras and groups. Though none of the quoted results are due to the author, such a review, aimed specifically at setting up the paradigm which provides essential guidance in the theory of quantum groups, does not seem to have appeared already. In Chapter 2 the elements of the quantum group theory are recalled. Once again, almost none of the results are due to the author, though in Section 2.10, some results concerning the nonstandard Jordanian group are presented, by way of a worked example, which have not been published.
Chapter 3 concerns twisting constructions. We introduce a new class of 2-cocycles defined explicitly on the generators of certain multiparameter standard quantum groups. These allow us, through the process of twisting the familiar standard quantum groups, to generate new as well as previously known examples of non-standard quantum groups. In particular we are able to construct generalisations of both the Cremmer-Gervais deformation of SL(3) and the so called esoteric quantum groups of Fronsdal and Galindo in an explicit and straightforward manner.
In Chapter 4 we consider the differential calculus on Hopf algebras as introduced by Woronowicz. We classify all 4-dimensional first order bicovariant calculi on the Jordanian quantum group GL[sub]h,[sub]g(2) and all 3-dimensional first order bicovariant calculi on the Jordanian quantum group SL[sub]h(2). In both cases we assume that the bicovariant bimodules are generated as left modules by the differentials of the quantum group generators. It is found that there are 3 1-parameter families of 4-dimensional bicovariant first order calculi on GL[sub]h,[sub]g(2) and that there is a single, unique, 3-dimensional bicovariant calculus on SL[sub]h(2). This 3-dimensional calculus may be obtained through a classical-like reduction from any one of the three families of 4-dimensional calculi on GL[sub]h,[sub]g(2). Details of the higher order calculi and also the quantum Lie algebras are presented for all calculi. The quantum Lie algebra obtained from the bicovariant calculus on SL[sub]h(2) is shown to be isomorphic to the quantum Lie algebra we obtain as an ad-submodule within the Jordanian universal enveloping algebra U[sub]h(sl[sub]2(C)) and also through a consideration of the decomposition of the tensor product of two copies of the deformed adjoint module. We also obtain the quantum Killing form for this quantum Lie algebra.Computing with simple groups: maximal subgroups and presentationsJamali, Ali-Rezahttp://hdl.handle.net/10023/136922018-06-04T23:17:16Z1989-01-01T00:00:00ZFor the non-abelian simple groups G of order up to 10⁶ , excluding the groups PSL(2,q), q > 9, the presentations in terms of an involution a and an element b of minimal order (with respect to a) such that G=<a,b> are well known. The presentations are complete in the sense that any pair (x,y) of generators of G satisfying x²=yᵐ=1, with m minimal, will satisfy the defining relations of just one presentation in the list. There are 106 such presentations.
Using a computer, we give generators for each maximal subgroup of the groups G. For each presentation of G, the generators of maximal subgroups are given as words in the group generators. Similarly generators for a Sylow p-subgroup of G, for each p, are given. For each group G, we give a representative for each conjugacy class of the group as a word in the group generators.
Minimal presentations for each Sylow p-subgroup of the groups G, and for most of the maximal subgroups of G are constructed. To obtain such presentations, the Schur multipliers of the underlying groups are calculated.
The same tasks are carried out for those groups PSL(2,q) of order less than 10⁶ which are included in the "ATLAS of finite groups". For these groups we consider a presentation on two generators x, y with x²=y³=1.
A finite group G is said to be efficient if it has a presentation on d generators and d+rank(M(G)) relations (for some d) where M(G) is the Schur multiplier of G. We show that the simple groups J₁, PSU(3,5) and M₂₂ are efficient. We also give efficient presentations for the direct products A₅xA₆, A₅xA₆,A₆xA₇ where Ĥ denotes the covering group of H.
1989-01-01T00:00:00ZJamali, Ali-RezaFor the non-abelian simple groups G of order up to 10⁶ , excluding the groups PSL(2,q), q > 9, the presentations in terms of an involution a and an element b of minimal order (with respect to a) such that G=<a,b> are well known. The presentations are complete in the sense that any pair (x,y) of generators of G satisfying x²=yᵐ=1, with m minimal, will satisfy the defining relations of just one presentation in the list. There are 106 such presentations.
Using a computer, we give generators for each maximal subgroup of the groups G. For each presentation of G, the generators of maximal subgroups are given as words in the group generators. Similarly generators for a Sylow p-subgroup of G, for each p, are given. For each group G, we give a representative for each conjugacy class of the group as a word in the group generators.
Minimal presentations for each Sylow p-subgroup of the groups G, and for most of the maximal subgroups of G are constructed. To obtain such presentations, the Schur multipliers of the underlying groups are calculated.
The same tasks are carried out for those groups PSL(2,q) of order less than 10⁶ which are included in the "ATLAS of finite groups". For these groups we consider a presentation on two generators x, y with x²=y³=1.
A finite group G is said to be efficient if it has a presentation on d generators and d+rank(M(G)) relations (for some d) where M(G) is the Schur multiplier of G. We show that the simple groups J₁, PSU(3,5) and M₂₂ are efficient. We also give efficient presentations for the direct products A₅xA₆, A₅xA₆,A₆xA₇ where Ĥ denotes the covering group of H.Semigroup presentationsIbrahim, Mohammed Ali Fayahttp://hdl.handle.net/10023/136892018-09-04T13:33:53Z1997-01-01T00:00:00ZIn this thesis we consider the following two fundamental problems for semigroup presentations:
1. Given a semigroup find a presentation defining it.
2. Given a presentation describe the semigroup defined by it.
We also establish other related results.
After an introduction in Chapter 1, we consider the first problem in Chapter 2, and establish a presentation for the commutative semigroup of integers Zpt. Dually, in Chapter 3 we consider the second problem and study presentations of semigroups related to the direct product of cyclic groups. In Chapter 4 we study presentations of semigroups related to dihedral groups and establish their V-classes structure in Chapter 5. In Chapter 6 we establish some results related to the Schutzenberger group which were suggested by our studies of the semigroup presentations in Chapters 3 and 4. Finally, in Chapter 7 we define and study new classes of semigroups which we call R, L-semi-commutative and semi-commutative semigroups and they were also suggested by our studies of the semigroup presentations in Chapters 3 and 4.
1997-01-01T00:00:00ZIbrahim, Mohammed Ali FayaIn this thesis we consider the following two fundamental problems for semigroup presentations:
1. Given a semigroup find a presentation defining it.
2. Given a presentation describe the semigroup defined by it.
We also establish other related results.
After an introduction in Chapter 1, we consider the first problem in Chapter 2, and establish a presentation for the commutative semigroup of integers Zpt. Dually, in Chapter 3 we consider the second problem and study presentations of semigroups related to the direct product of cyclic groups. In Chapter 4 we study presentations of semigroups related to dihedral groups and establish their V-classes structure in Chapter 5. In Chapter 6 we establish some results related to the Schutzenberger group which were suggested by our studies of the semigroup presentations in Chapters 3 and 4. Finally, in Chapter 7 we define and study new classes of semigroups which we call R, L-semi-commutative and semi-commutative semigroups and they were also suggested by our studies of the semigroup presentations in Chapters 3 and 4.Algorithms for subgroup presentations: computer implementation and applicationsHeggie, Patricia, M.http://hdl.handle.net/10023/136842018-06-04T23:17:03Z1991-01-01T00:00:00ZOne of the main algorithms of computational group theory is the Todd-Coxeter coset enumeration algorithm, which provides a systematic method for finding the index of a subgroup of a finitely presented group. This has been extended in various ways to provide not only the index of a subgroup, but also a presentation for the subgroup. These methods tie in with a technique introduced by Reidemeister in the 1920's and later improved by Schreier, now known as the Reidemeister-Schreier algorithm.
In this thesis we discuss some of these variants of the Todd-Coxeter algorithm and their inter-relation, and also look at existing computer implementations of these different techniques. We then go on to describe a new package for coset methods which incorporates various types of coset enumeration, including modified Todd- Coxeter methods and the Reidemeister-Schreier process. This also has the capability of carrying out Tietze transformation simplification. Statistics obtained from running the new package on a collection of test examples are given, and the various techniques compared.
Finally, we use these algorithms, both theoretically and as computer implementations, to investigate a particular class of finitely presented groups defined by the presentation: < a, b | aⁿ = b² = (ab-1) ß =1, ab² = ba²>. Some interesting results have been discovered about these groups for various values of β and n. For example, if n is odd, the groups turn out to be finite and metabelian, and if β= 3 or β= 4 the derived group has an order which is dependent on the values of n (mod 8) and n (mod 12) respectively.
1991-01-01T00:00:00ZHeggie, Patricia, M.One of the main algorithms of computational group theory is the Todd-Coxeter coset enumeration algorithm, which provides a systematic method for finding the index of a subgroup of a finitely presented group. This has been extended in various ways to provide not only the index of a subgroup, but also a presentation for the subgroup. These methods tie in with a technique introduced by Reidemeister in the 1920's and later improved by Schreier, now known as the Reidemeister-Schreier algorithm.
In this thesis we discuss some of these variants of the Todd-Coxeter algorithm and their inter-relation, and also look at existing computer implementations of these different techniques. We then go on to describe a new package for coset methods which incorporates various types of coset enumeration, including modified Todd- Coxeter methods and the Reidemeister-Schreier process. This also has the capability of carrying out Tietze transformation simplification. Statistics obtained from running the new package on a collection of test examples are given, and the various techniques compared.
Finally, we use these algorithms, both theoretically and as computer implementations, to investigate a particular class of finitely presented groups defined by the presentation: < a, b | aⁿ = b² = (ab-1) ß =1, ab² = ba²>. Some interesting results have been discovered about these groups for various values of β and n. For example, if n is odd, the groups turn out to be finite and metabelian, and if β= 3 or β= 4 the derived group has an order which is dependent on the values of n (mod 8) and n (mod 12) respectively.On the efficiency of finite groupsBrookes, Melaniehttp://hdl.handle.net/10023/136822018-06-04T23:16:49Z1996-01-01T00:00:00ZIn Chapter 2 of this thesis we look at methods for finding efficient presentations of the transitive permutation groups of degree ≤ 12. Chapter 3 gives efficient presentations for certain direct products of groups including PSL(2, P)2 SL(2, p) X SL(2, 8), PSL(2, p) x C2, for prime p ≥ 5 and PSL(2, 25)3. Chapter 4 introduces a new class of inefficient groups and Chapter 5 gives a brief survey of some of the open problems relating to the efficiency of finite groups.
1996-01-01T00:00:00ZBrookes, MelanieIn Chapter 2 of this thesis we look at methods for finding efficient presentations of the transitive permutation groups of degree ≤ 12. Chapter 3 gives efficient presentations for certain direct products of groups including PSL(2, P)2 SL(2, p) X SL(2, 8), PSL(2, p) x C2, for prime p ≥ 5 and PSL(2, 25)3. Chapter 4 introduces a new class of inefficient groups and Chapter 5 gives a brief survey of some of the open problems relating to the efficiency of finite groups.Last call: Passive acoustic monitoring shows continued rapid decline of critically endangered vaquitaThomas, LenJaramillo-legorreta, ArmandoCardenas-Hinojosa, GustavoNieto-Garcia, EdwynaRojas-Bracho, LorenzoVer Hoef, Jay M.Moore, JeffreyTaylor, BarbaraBarlow, JayTregenza, Nicholashttp://hdl.handle.net/10023/135762019-01-20T01:35:54Z2017-11-01T00:00:00ZThe vaquita is a critically endangered species of porpoise. It produces echolocation clicks, making it a good candidate for passive acoustic monitoring. A systematic grid of sensors has been deployed for 3 months annually since 2011; results from 2016 are reported here. Statistical models (to compensate for non-uniform data loss) show an overall decline in the acoustic detection rate between 2015 and 2016 of 49% (95% credible interval 82% decline to 8% increase), and total decline between 2011 and 2016 of over 90%. Assuming the acoustic detection rate is proportional to population size, approximately 30 vaquita (95% credible interval 8–96) remained in November 2016.
Funding: the Mexican Government (through the Mexican Secretaría de Medio Ambiente y Recursos Naturales), especially Minister R. Pacchiano and A. Michel; U.S. Marine Mammal Commission, in particular T. Ragen, R. Lent, and P. Thomas; the World Wildlife Fund (WWF) Mexico, in particular O. Vidal and E. Sanjurjo; Le Equipe Cousteau; The Ocean Foundation; Fonds de Dotation pour la Biodiversité; MAAF Assurances (Save Your Logo); WWF-US; Opel Project Earth; Fideicomiso Fondo para la Biodiversidad; Instituto Nacional de Ecología y Cambio Climático; Comisión Nacional de Áreas Naturales Protegidas; and Directorate of the Reserva de la Biósfera Alto Golfo de California y Delta del Río Colorado.
2017-11-01T00:00:00ZThomas, LenJaramillo-legorreta, ArmandoCardenas-Hinojosa, GustavoNieto-Garcia, EdwynaRojas-Bracho, LorenzoVer Hoef, Jay M.Moore, JeffreyTaylor, BarbaraBarlow, JayTregenza, NicholasThe vaquita is a critically endangered species of porpoise. It produces echolocation clicks, making it a good candidate for passive acoustic monitoring. A systematic grid of sensors has been deployed for 3 months annually since 2011; results from 2016 are reported here. Statistical models (to compensate for non-uniform data loss) show an overall decline in the acoustic detection rate between 2015 and 2016 of 49% (95% credible interval 82% decline to 8% increase), and total decline between 2011 and 2016 of over 90%. Assuming the acoustic detection rate is proportional to population size, approximately 30 vaquita (95% credible interval 8–96) remained in November 2016.Impact of type II spicules in the corona : simulations and synthetic observablesMartínez-Sykora, JuanDe Pontieu, BartDe Moortel, InekeHansteen, ViggoCarlsson, Matshttp://hdl.handle.net/10023/135692019-01-06T03:36:37Z2018-06-19T00:00:00ZThe role of type II spicules in the corona has been a much debated topic in recent years. This paper aims to shed light on the impact of type II spicules in the corona using novel 2.5D radiative MHD simulations including ion-neutral interaction effects with the Bifrost code. We find that the formation of simulated type II spicules, driven by the release of magnetic tension, impacts the corona in various manners. Associated with the formation of spicules, the corona exhibits 1) magneto-acoustic shocks and flows which supply mass to coronal loops, and 2) transversal magnetic waves and electric currents that propagate at Alfvén speeds. The transversal waves and electric currents, generated by the spicule's driver and lasting for many minutes, are dissipated and heat the associated loop. These complex interactions in the corona can be connected with blue shifted secondary components in coronal spectral lines (Red-Blue asymmetries) observed with Hinode/EIS and SOHO/SUMER, as well as the EUV counterpart of type II spicules and propagating coronal disturbances (PCDs) observed with the 171 Å and 193 Å SDO/AIA channels.
We gratefully acknowledge support by NASA grants, NNX16AG90G, NNH15ZDA001N, NNX17AD33G, and NNG09FA40C (IRIS), NSF grant AST1714955. This research has received funding from the UK Science and Technology Facilities Council (Consolidated Grant ST/K000950/1) and the European Union Horizon 2020 research and innovation programme (grant agreement No. 647214). This research was supported by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC Grant agreement nr. 291058. We thankfully acknowledge the support of the Research Council of Norway through grant 230938/F50, through its Center of Excellence scheme, project number 262622, and through grants of computing time from the Programme for Supercomputing.
2018-06-19T00:00:00ZMartínez-Sykora, JuanDe Pontieu, BartDe Moortel, InekeHansteen, ViggoCarlsson, MatsThe role of type II spicules in the corona has been a much debated topic in recent years. This paper aims to shed light on the impact of type II spicules in the corona using novel 2.5D radiative MHD simulations including ion-neutral interaction effects with the Bifrost code. We find that the formation of simulated type II spicules, driven by the release of magnetic tension, impacts the corona in various manners. Associated with the formation of spicules, the corona exhibits 1) magneto-acoustic shocks and flows which supply mass to coronal loops, and 2) transversal magnetic waves and electric currents that propagate at Alfvén speeds. The transversal waves and electric currents, generated by the spicule's driver and lasting for many minutes, are dissipated and heat the associated loop. These complex interactions in the corona can be connected with blue shifted secondary components in coronal spectral lines (Red-Blue asymmetries) observed with Hinode/EIS and SOHO/SUMER, as well as the EUV counterpart of type II spicules and propagating coronal disturbances (PCDs) observed with the 171 Å and 193 Å SDO/AIA channels.Application of the Todd-Coxeter coset enumeration algorithmCampbell, C. M. (Colin Matthew)http://hdl.handle.net/10023/135082018-05-23T23:15:58Z1975-01-01T00:00:00ZThis thesis is concerned with a topic in combinatorial group theory and, in particular, with a study of some groups with finite presentations. After preliminary definitions and theorems we describe the Todd-Coxeter coset enumeration algorithm and the modified Todd-Coxeter algorithm which shows that, given a finitely generated subgroup H of finite index in a finitely presented group G, we can find a presentation for H. We then give elementary examples illustrating the algorithms and include a discussion on the computer programmes that are to be used. In the main part of the thesis we investigate two classes of cyclically presented groups. Supposewhere w1 = w is a word in a1,a2,...,an, and wi+1 is obtained from wi by applying the permutation (1 2 ... n) to the suffices of the a's. The first class we investigate are the groups that is the groups G(l,m,n) are groups of type G2 (w). Secondly we investigate the Fibonacci-type groups H(r,n,k,s,h) obtained when, for some integers r,s,h > 1, k > O, the word w is given by Fibonacci groups being the special case given by k = s = h = 1. For both classes we begin by giving some homomorphisms and isomorphisms that may be obtained. We show, using the Todd-Coxeter algorithm when appropriate, that the six groups G(2,2,3), G(2,2,-3), G(-l,-l,4), G(2,3,-2), G(-2,2,-1) and G(-2,3,l) are finite non-metacyclic groups of deficiency zero, having orders 215.33, 28.33, 29.3.5, 23.33.7, 23.3.5.11 amd 23.36 respectively. We also show that the groups G(1-n, 6, n) where n = 1 mod 5 give an infinite series of non-metacyclic groups. We consider the structure of the non-metacyclic groups H(3,6,1,1,1) and H(3,6,5,l,2) both of order 1512, showing that neither is isomorphic to G(2, 3, -2) another non-metacyclic group of order 1512. In a paper on the Fibonacci groups D.L. Johnson, J.W. Wamsley and D. Wright pose two questions relating to the Fibonacci groups for the case r = 1 mod n, namely to find 2-generator 2-relation presentations for them and also their orders. We answer these questions and generalise the results to the class H(r,n,k,s,1) where it is shown that H(r,n,k,s,1) is metacyclic if (i) r = s mod n, (ii) (r,n) = 1, (iii) (r + k - 1, n) - 1, and a 2-generator 2-relation presentation is found for these groups. Further if (iv) (r,s) = 1, then we show that H(r,n,k,s,1) is a finite metacyclic group of order rn - sn. A possible generalisation to the groups H(r,n,k,s,h) is considered. Finally the metacyclic groups H(r,4,1,2,1), r odd are discussed.
1975-01-01T00:00:00ZCampbell, C. M. (Colin Matthew)This thesis is concerned with a topic in combinatorial group theory and, in particular, with a study of some groups with finite presentations. After preliminary definitions and theorems we describe the Todd-Coxeter coset enumeration algorithm and the modified Todd-Coxeter algorithm which shows that, given a finitely generated subgroup H of finite index in a finitely presented group G, we can find a presentation for H. We then give elementary examples illustrating the algorithms and include a discussion on the computer programmes that are to be used. In the main part of the thesis we investigate two classes of cyclically presented groups. Supposewhere w1 = w is a word in a1,a2,...,an, and wi+1 is obtained from wi by applying the permutation (1 2 ... n) to the suffices of the a's. The first class we investigate are the groups that is the groups G(l,m,n) are groups of type G2 (w). Secondly we investigate the Fibonacci-type groups H(r,n,k,s,h) obtained when, for some integers r,s,h > 1, k > O, the word w is given by Fibonacci groups being the special case given by k = s = h = 1. For both classes we begin by giving some homomorphisms and isomorphisms that may be obtained. We show, using the Todd-Coxeter algorithm when appropriate, that the six groups G(2,2,3), G(2,2,-3), G(-l,-l,4), G(2,3,-2), G(-2,2,-1) and G(-2,3,l) are finite non-metacyclic groups of deficiency zero, having orders 215.33, 28.33, 29.3.5, 23.33.7, 23.3.5.11 amd 23.36 respectively. We also show that the groups G(1-n, 6, n) where n = 1 mod 5 give an infinite series of non-metacyclic groups. We consider the structure of the non-metacyclic groups H(3,6,1,1,1) and H(3,6,5,l,2) both of order 1512, showing that neither is isomorphic to G(2, 3, -2) another non-metacyclic group of order 1512. In a paper on the Fibonacci groups D.L. Johnson, J.W. Wamsley and D. Wright pose two questions relating to the Fibonacci groups for the case r = 1 mod n, namely to find 2-generator 2-relation presentations for them and also their orders. We answer these questions and generalise the results to the class H(r,n,k,s,1) where it is shown that H(r,n,k,s,1) is metacyclic if (i) r = s mod n, (ii) (r,n) = 1, (iii) (r + k - 1, n) - 1, and a 2-generator 2-relation presentation is found for these groups. Further if (iv) (r,s) = 1, then we show that H(r,n,k,s,1) is a finite metacyclic group of order rn - sn. A possible generalisation to the groups H(r,n,k,s,h) is considered. Finally the metacyclic groups H(r,4,1,2,1), r odd are discussed.Centralisers and normalisers in symmetric and alternating groupsBilgiç, Huseyinhttp://hdl.handle.net/10023/135072018-05-23T23:15:57Z1998-01-01T00:00:00ZIn this thesis, we analyse the structure of the centraliser of an element and of the normaliser of a cyclic subgroup in both Sn and An. We show that the centraliser in Sn of a permutation can be written as a direct product of centralisers of regular permutations and that the centraliser of a regular permutation is a wreath product. In certain cases we prove that this wreath product splits as a direct product and we analyse the centre of the subgroup. We calculate the centraliser of a general permutation in An and show how this is related to the centralisers of regular permutations. We investigate the normaliser of the cyclic subgroup generated by an element of Sn and show how this is related to the centraliser of the permutation. We calculate the centre of the normaliser and investigate when the normaliser splits as a direct product. We carry out a similar investigation for normalisers of cyclic subgroups of An and investigate the relationship between normalisers in An and Sn. We give presentations for both centralisers and normalisers.
1998-01-01T00:00:00ZBilgiç, HuseyinIn this thesis, we analyse the structure of the centraliser of an element and of the normaliser of a cyclic subgroup in both Sn and An. We show that the centraliser in Sn of a permutation can be written as a direct product of centralisers of regular permutations and that the centraliser of a regular permutation is a wreath product. In certain cases we prove that this wreath product splits as a direct product and we analyse the centre of the subgroup. We calculate the centraliser of a general permutation in An and show how this is related to the centralisers of regular permutations. We investigate the normaliser of the cyclic subgroup generated by an element of Sn and show how this is related to the centraliser of the permutation. We calculate the centre of the normaliser and investigate when the normaliser splits as a direct product. We carry out a similar investigation for normalisers of cyclic subgroups of An and investigate the relationship between normalisers in An and Sn. We give presentations for both centralisers and normalisers.Some applications of computer algebra and interval mathematicsMonsi, Mansor Binhttp://hdl.handle.net/10023/135022018-05-23T23:15:43Z1988-01-01T00:00:00ZThis thesis contains some applications of Computer Algebra to unconstrained optimization and some applications of Interval Mathematics to the problem of simultaneously bounding the simple zeros of polynomials. Chapter 1 contains a brief introduction to Computer Algebra and Interval Mathematics, and several of the fundamental results from Interval Mathematics which are used in Chapters 4 and 5. Chapter 2 contains a survey of those features of the symbol manipulation package ALgLIB[Shew-85] which it is necessary to understand in order to use ALgLIB as explained in Chapter 3. Chapter 3 contains a description of Sisser's method [Sis-82a] for unconstrained minimization and several modifications thereof which are implemented using the pseudo-code of Dennis and Schnabel [DenS-83], and ALgLIB, Chapter 3 also contains numerical results corresponding to Sisser's method and its modifications for 7 examples. Chapter 4 contains a new algorithm PRSS for the simultaneous estimation of polynomial zeros and the corresponding interval form IRSS for simultaneously bounding real polynomial zeros. Comparisons are made with some related existing algorithms. Numerical results of the comparisons are also given in this chapter. Chapter 5 contains an application of an idea due to Neumaier [Neu-85] to the problem of constructing interval versions of point iterative procedures for the estimation of simple zeros of analytic functions. In particular, interval versions of some point iterative procedures for the simultaneous estimation of simple (complex) polynomial zeros are described. Finally, numerical results are given to show the efficiency of the new algorithm.
1988-01-01T00:00:00ZMonsi, Mansor BinThis thesis contains some applications of Computer Algebra to unconstrained optimization and some applications of Interval Mathematics to the problem of simultaneously bounding the simple zeros of polynomials. Chapter 1 contains a brief introduction to Computer Algebra and Interval Mathematics, and several of the fundamental results from Interval Mathematics which are used in Chapters 4 and 5. Chapter 2 contains a survey of those features of the symbol manipulation package ALgLIB[Shew-85] which it is necessary to understand in order to use ALgLIB as explained in Chapter 3. Chapter 3 contains a description of Sisser's method [Sis-82a] for unconstrained minimization and several modifications thereof which are implemented using the pseudo-code of Dennis and Schnabel [DenS-83], and ALgLIB, Chapter 3 also contains numerical results corresponding to Sisser's method and its modifications for 7 examples. Chapter 4 contains a new algorithm PRSS for the simultaneous estimation of polynomial zeros and the corresponding interval form IRSS for simultaneously bounding real polynomial zeros. Comparisons are made with some related existing algorithms. Numerical results of the comparisons are also given in this chapter. Chapter 5 contains an application of an idea due to Neumaier [Neu-85] to the problem of constructing interval versions of point iterative procedures for the estimation of simple zeros of analytic functions. In particular, interval versions of some point iterative procedures for the simultaneous estimation of simple (complex) polynomial zeros are described. Finally, numerical results are given to show the efficiency of the new algorithm.Proof diagrams and term rewriting with applications to computational algebraShand, Duncanhttp://hdl.handle.net/10023/134982018-05-23T23:15:31Z1997-01-01T00:00:00ZIn this thesis lessons learned from the use of computer algebra systems and machine assisted theorem provers are developed in order to give an insight into both the problems and their solutions. Many algorithms in computational algebra and automated deduction (for example Grobner basis computations and Knuth-Bendix completion) tend to produce redundant facts and can contain more than one proof of any particular fact. This thesis introduces proof diagrams in order to compare and contrast the proofs of facts which such procedures generate. Proof diagrams make it possible to analyse the effect of heuristics which can be used to guide implementations of such algorithms. An extended version of an inference system for Knuth-Bendix completion is introduced. It is possible to see that this extension characterises the applicability of critical pair criteria, which are heuristics used in completion. We investigate a number of executions of a completion procedure by analysing the associated proof diagrams. This leads to a better understanding of the heuristics used to control these examples. Derived rales of inference are also investigated in this thesis. This is done in the formalism of proof diagrams. Rewrite rules for proof diagrams are defined: this is motivated by the notion of a transformation tactic in the Nuprl proof development system. A method to automatically extract 'useful' derived inference rales is also discussed. 'Off the shelf' theorem provers, such as the Larch Prover and Otter, are compared to specialised programs from computational group theory. This analysis makes it possible to see where methods from automated deduction can improve on the tools which group theorists currently use. Problems which can be attacked with theorem provers but not with currently used specialised programs are also indicated. Tietze transformations, from group theory, are discussed. This makes it possible to link ideas used in Knuth-Bendix completion programs and group presentation simplification programs. Tietze transformations provide heuristics for more efficient and effective implementations of these programs.
1997-01-01T00:00:00ZShand, DuncanIn this thesis lessons learned from the use of computer algebra systems and machine assisted theorem provers are developed in order to give an insight into both the problems and their solutions. Many algorithms in computational algebra and automated deduction (for example Grobner basis computations and Knuth-Bendix completion) tend to produce redundant facts and can contain more than one proof of any particular fact. This thesis introduces proof diagrams in order to compare and contrast the proofs of facts which such procedures generate. Proof diagrams make it possible to analyse the effect of heuristics which can be used to guide implementations of such algorithms. An extended version of an inference system for Knuth-Bendix completion is introduced. It is possible to see that this extension characterises the applicability of critical pair criteria, which are heuristics used in completion. We investigate a number of executions of a completion procedure by analysing the associated proof diagrams. This leads to a better understanding of the heuristics used to control these examples. Derived rales of inference are also investigated in this thesis. This is done in the formalism of proof diagrams. Rewrite rules for proof diagrams are defined: this is motivated by the notion of a transformation tactic in the Nuprl proof development system. A method to automatically extract 'useful' derived inference rales is also discussed. 'Off the shelf' theorem provers, such as the Larch Prover and Otter, are compared to specialised programs from computational group theory. This analysis makes it possible to see where methods from automated deduction can improve on the tools which group theorists currently use. Problems which can be attacked with theorem provers but not with currently used specialised programs are also indicated. Tietze transformations, from group theory, are discussed. This makes it possible to link ideas used in Knuth-Bendix completion programs and group presentation simplification programs. Tietze transformations provide heuristics for more efficient and effective implementations of these programs.Tools and techniques for machine-assisted meta-theoryAdams, Andrew, 1969-http://hdl.handle.net/10023/133822018-05-17T23:17:00Z1997-01-01T00:00:00ZMachine-assisted formal proofs are becoming commonplace in certain fields of mathematics and theoretical computer science. New formal systems and variations on old ones are constantly invented. The meta-theory of such systems, i.e. proofs about the system as opposed to proofs within the system, are mostly done informally with a pen and paper. Yet the meta-theory of deductive systems is an area which would obviously benefit from machine support for formal proof. Is the software currently available sufficiently powerful yet easy enough to use to make machine assistance for formal meta-theory a viable proposition? This thesis presents work done by the author on formalizing proof theory from [DP97a] in various formal systems: SEQUEL [Tar93, Tar97], Isabelle [Pau94] and Coq [BB+96]. SEQUEL and Isabelle were found to be difficult to use for this type of work. In particular, the lack of automated production of induction principles in SEQUEL and Isabelle undermined confidence in the resulting formal proofs. Coq was found to be suitable for the formalisation methodology first chosen: the use of nameless dummy variables (de Bruijn indices) as pioneered in [dB72]. A second approach (inspired by the work of McKinna and Pollack [vBJMR94, MP97]) formalising named variables was also the subject of some initial work, and a comparison of these two approaches is presented. The formalisation was restricted to the implicational fragment of propositional logic. The informal theory has been extended to cover full propositional logic by Dyckhoff and Pinto, and extension of the formalisation using de Bruijn indices would appear to present few difficulties. An overview of other work in this area, in terms of both the tools and formalisation methods, is also presented. The theory formalised differs from other such work in that other formalisations have involved only one calculus. [DP97a] involves the relationships between three different calculi. There is consequently a much greater requirement for equality reasoning in the formalisation. It is concluded that a formalisation of any significance is still difficult, particularly one involving multiple calculi. No tools currently exist that allow for the easy representation of even quite simple systems in a way that fits human intuitions while still allowing for automatic derivation of induction principles. New work on integrating higher order abstract syntax and induction may be the way forward, although such work is still in the early stages.
1997-01-01T00:00:00ZAdams, Andrew, 1969-Machine-assisted formal proofs are becoming commonplace in certain fields of mathematics and theoretical computer science. New formal systems and variations on old ones are constantly invented. The meta-theory of such systems, i.e. proofs about the system as opposed to proofs within the system, are mostly done informally with a pen and paper. Yet the meta-theory of deductive systems is an area which would obviously benefit from machine support for formal proof. Is the software currently available sufficiently powerful yet easy enough to use to make machine assistance for formal meta-theory a viable proposition? This thesis presents work done by the author on formalizing proof theory from [DP97a] in various formal systems: SEQUEL [Tar93, Tar97], Isabelle [Pau94] and Coq [BB+96]. SEQUEL and Isabelle were found to be difficult to use for this type of work. In particular, the lack of automated production of induction principles in SEQUEL and Isabelle undermined confidence in the resulting formal proofs. Coq was found to be suitable for the formalisation methodology first chosen: the use of nameless dummy variables (de Bruijn indices) as pioneered in [dB72]. A second approach (inspired by the work of McKinna and Pollack [vBJMR94, MP97]) formalising named variables was also the subject of some initial work, and a comparison of these two approaches is presented. The formalisation was restricted to the implicational fragment of propositional logic. The informal theory has been extended to cover full propositional logic by Dyckhoff and Pinto, and extension of the formalisation using de Bruijn indices would appear to present few difficulties. An overview of other work in this area, in terms of both the tools and formalisation methods, is also presented. The theory formalised differs from other such work in that other formalisations have involved only one calculus. [DP97a] involves the relationships between three different calculi. There is consequently a much greater requirement for equality reasoning in the formalisation. It is concluded that a formalisation of any significance is still difficult, particularly one involving multiple calculi. No tools currently exist that allow for the easy representation of even quite simple systems in a way that fits human intuitions while still allowing for automatic derivation of induction principles. New work on integrating higher order abstract syntax and induction may be the way forward, although such work is still in the early stages.The Arabic translation of Theodosius's SphaericaMartin, Thomas J.http://hdl.handle.net/10023/133802018-05-17T23:16:41Z1975-01-01T00:00:00ZThe thesis "The Arabic Translation of Theodosius's Sphaerica" is an edition of the Istanbul manuscript Topkapi Seray Ahmet III 3464.2. Included is a comparative apparatus of the Greek and Arabic texts showing possible correspondence between the posited Greek exemplar of the translator and the various Greek manuscript traditions reported by J.L. Heiberg in his critical edition of the text. Further differences are pointed out in the English Trajislation. There is also a glossary of terminology- giving listings from Greek to Arabic and Arabic to Greek. An appendix discussing the execution of the drawings in the Arabic manuscript and their relation to the Greek drawings as reported by Heiberg is also given. Other appendices include a chart representing the convention seemingly adopted by the translator for lettering the drawings, a listing of inconsistent grammatical usage found in the manuscript, parallel passages from the Greek text, the text of the present edition, the versions of al-Maghribi and al-Tusi, and a privately owned manuscript, and finally a list of interlinear sigla found on the first few folios of the manuscript the purpose of which is unclear.
1975-01-01T00:00:00ZMartin, Thomas J.The thesis "The Arabic Translation of Theodosius's Sphaerica" is an edition of the Istanbul manuscript Topkapi Seray Ahmet III 3464.2. Included is a comparative apparatus of the Greek and Arabic texts showing possible correspondence between the posited Greek exemplar of the translator and the various Greek manuscript traditions reported by J.L. Heiberg in his critical edition of the text. Further differences are pointed out in the English Trajislation. There is also a glossary of terminology- giving listings from Greek to Arabic and Arabic to Greek. An appendix discussing the execution of the drawings in the Arabic manuscript and their relation to the Greek drawings as reported by Heiberg is also given. Other appendices include a chart representing the convention seemingly adopted by the translator for lettering the drawings, a listing of inconsistent grammatical usage found in the manuscript, parallel passages from the Greek text, the text of the present edition, the versions of al-Maghribi and al-Tusi, and a privately owned manuscript, and finally a list of interlinear sigla found on the first few folios of the manuscript the purpose of which is unclear.The life and work of Prof. George Chrystal (1851-1911)Yousuf, Mohammadhttp://hdl.handle.net/10023/133792018-05-17T23:16:23Z1990-01-01T00:00:00ZThis thesis is principally concerned with George Chrystal's life and his work, mainly in three directions viz., as an experimentalist, a mathematician, and an educationist. The main object is to bring to light the work of a personality who is representative of many more who are always forgotten. The majority of historians of science consider the works of the giants in science, ignoring totally the contributions made by the less prominent people like Prof. George Chrystal. In fact their contributions serve as one of the most important factors in propagation of scientific knowledge. His main contributions: verification of Ohm's Law experimentally; Non-Euclidean geometry; differential equations; text books on algebra; theory of seiches; institution of leaving certificate examination in Scottish education and many more have been discussed in detail. A survey of Chrystal's general thought is given in so far as it may be gathered from his scattered remarks. The references are mentioned by numerals in the superscript, details of which are given at the end of each chapter. The main text consists of six chapters. There are three appendices at the end,' Appendix 'A' consists of his correspondence with different scientists, most of which is still unpublished. Appendix 'B' contains a bibliography of his contributions in chronological order, and Appendix 'C contains his three Promoter's addresses. Tables and figures are attached at their proper places, including some rarely available photographs.
1990-01-01T00:00:00ZYousuf, MohammadThis thesis is principally concerned with George Chrystal's life and his work, mainly in three directions viz., as an experimentalist, a mathematician, and an educationist. The main object is to bring to light the work of a personality who is representative of many more who are always forgotten. The majority of historians of science consider the works of the giants in science, ignoring totally the contributions made by the less prominent people like Prof. George Chrystal. In fact their contributions serve as one of the most important factors in propagation of scientific knowledge. His main contributions: verification of Ohm's Law experimentally; Non-Euclidean geometry; differential equations; text books on algebra; theory of seiches; institution of leaving certificate examination in Scottish education and many more have been discussed in detail. A survey of Chrystal's general thought is given in so far as it may be gathered from his scattered remarks. The references are mentioned by numerals in the superscript, details of which are given at the end of each chapter. The main text consists of six chapters. There are three appendices at the end,' Appendix 'A' consists of his correspondence with different scientists, most of which is still unpublished. Appendix 'B' contains a bibliography of his contributions in chronological order, and Appendix 'C contains his three Promoter's addresses. Tables and figures are attached at their proper places, including some rarely available photographs.Normalisation techniques in proof theory and category theoryHamza, Taher Tawfik Ahmedhttp://hdl.handle.net/10023/133712018-05-17T23:16:45Z1986-01-01T00:00:00ZThe word problem for the free categories with some structure generated by a category X can be solved using proof-theoretical means. These free categories give a semantics in which derivations of GENTZEN's propositional sequent calculus can be interpreted by means of arrows of those categories. In this thesis we describe, implement and document the cut-elimination and the normalization techniques in proof theory as outlined in SZABO [1978]: we show how these are used in order to solve, mechanically, the word problem for the free categories with structure of : cartesian, bicartesian, distributive bicartesian, cartesian closed, and bicartesian closed. This implementation is extended by a procedure to interpret intuitionistic propositional sequent derivations as arrows of the above categories. Implementation of those techniques has forced us to modify the techniques in various inessential ways. The description and the representation in the syntax of our implementation of the above categories is contained in chapters 1 - 5, where each chapter describes one theory and concludes with examples of the system In use to represent concepts and solve simple word problems from category theory ( of various typos ). Appendix 1 contains some apparent printing errors we have observed in the work done by SZABO. The algorithms used in the proof of the cut-elimination theorems and normalization through chapters 1 - 5 are collected in appendices 2 - 4. Appendices 5 - 8 concern the implementation and its user manual.
1986-01-01T00:00:00ZHamza, Taher Tawfik AhmedThe word problem for the free categories with some structure generated by a category X can be solved using proof-theoretical means. These free categories give a semantics in which derivations of GENTZEN's propositional sequent calculus can be interpreted by means of arrows of those categories. In this thesis we describe, implement and document the cut-elimination and the normalization techniques in proof theory as outlined in SZABO [1978]: we show how these are used in order to solve, mechanically, the word problem for the free categories with structure of : cartesian, bicartesian, distributive bicartesian, cartesian closed, and bicartesian closed. This implementation is extended by a procedure to interpret intuitionistic propositional sequent derivations as arrows of the above categories. Implementation of those techniques has forced us to modify the techniques in various inessential ways. The description and the representation in the syntax of our implementation of the above categories is contained in chapters 1 - 5, where each chapter describes one theory and concludes with examples of the system In use to represent concepts and solve simple word problems from category theory ( of various typos ). Appendix 1 contains some apparent printing errors we have observed in the work done by SZABO. The algorithms used in the proof of the cut-elimination theorems and normalization through chapters 1 - 5 are collected in appendices 2 - 4. Appendices 5 - 8 concern the implementation and its user manual.Parameter identification through mode isolation for reaction-diffusion systems on arbitrary geometriesMurphy, LauraVenkataraman, ChandrasekharMadzvamuse, Anotidahttp://hdl.handle.net/10023/133632019-01-06T03:35:47Z2018-05-03T00:00:00ZWe present a computational framework for isolating spatial patterns arising in the steady states of reaction-diffusion systems. Such systems have been used to model many natural phenomena in areas such as developmental and cancer biology, cell motility and material science. In many of these applications, often one is interested in identifying parameters which will lead to a particular pattern for a given reaction-diffusion model. To attempt to answer this, we compute eigenpairs of the Laplacian on a variety of domains and use linear stability analysis to determine parameter values for the system that will lead to spatially inhomogeneous steady states whose patterns correspond to particular eigenfunctions. This method has previously been used on domains and surfaces where the eigenvalues and eigenfunctions are found analytically in closed form. Our contribution to this methodology is that we numerically compute eigenpairs on arbitrary domains and surfaces. Here we present examples and demonstrate that mode isolation is straightforward especially for low eigenvalues. Additionally we see that the inhomogeneous steady state can be a linear combination of eigenfunctions. Finally we show an example suggesting that pattern formation is robust on similar surfaces in cases that the surface either has or does not have a boundary.
This work (LM) was supported by an EPSRC Doctoral Training Centre Studentship through the University of Sussex. CV and AM acknowledge support from the Leverhulme Trust Research Project Grant (RPG-2014-149) and the EPSRC grant (EP/J016780/1). This research was partly undertaken whilst LM, CV and AM were participants in the Isaac Newton Institute Program, Coupling Geometric PDEs with Physics for Cell Morphology, Motility and Pattern Formation. This work (AM) has received funding from the European Union Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement (No 642866). AM was partially supported by a grant from the Simons Foundation. AM is a Royal Society Wolfson Research Merit Award Holder, generously supported by the Wolfson Foundation. LM acknowledges the support from the University of Sussex ITS for computational purposes.
2018-05-03T00:00:00ZMurphy, LauraVenkataraman, ChandrasekharMadzvamuse, AnotidaWe present a computational framework for isolating spatial patterns arising in the steady states of reaction-diffusion systems. Such systems have been used to model many natural phenomena in areas such as developmental and cancer biology, cell motility and material science. In many of these applications, often one is interested in identifying parameters which will lead to a particular pattern for a given reaction-diffusion model. To attempt to answer this, we compute eigenpairs of the Laplacian on a variety of domains and use linear stability analysis to determine parameter values for the system that will lead to spatially inhomogeneous steady states whose patterns correspond to particular eigenfunctions. This method has previously been used on domains and surfaces where the eigenvalues and eigenfunctions are found analytically in closed form. Our contribution to this methodology is that we numerically compute eigenpairs on arbitrary domains and surfaces. Here we present examples and demonstrate that mode isolation is straightforward especially for low eigenvalues. Additionally we see that the inhomogeneous steady state can be a linear combination of eigenfunctions. Finally we show an example suggesting that pattern formation is robust on similar surfaces in cases that the surface either has or does not have a boundary.Proof search issues in some non-classical logicsHowe, Jacob M.http://hdl.handle.net/10023/133622018-05-17T23:16:19Z1999-01-01T00:00:00ZThis thesis develops techniques and ideas on proof search. Proof search is used with one of two meanings. Proof search can be thought of either as the search for a yes/no answer to a query (theorem proving), or as the search for all proofs of a formula (proof enumeration). This thesis is an investigation into issues in proof search in both these senses for some non-classical logics. Gentzen systems are well suited for use in proof search in both senses. The rules of Gentzen sequent calculi are such that implementations can be directed by the top level syntax of sequents, unlike other logical calculi such as natural deduction. All the calculi for proof search in this thesis are Gentzen sequent calculi. In Chapter 2, permutation of inference rules for Intuitionistic Linear Logic is studied. A focusing calculus, ILLF, in the style of Andreoli ([And92]) is developed. This calculus allows only one proof in each equivalence class of proofs equivalent up to permutations of inferences. The issue here is both theorem proving and proof enumeration. For certain logics, normal natural deductions provide a proof-theoretic semantics. Proof enumeration is then the enumeration of all these deductions. Herbelin's cut- free LJT ([Her95], here called MJ) is a Gentzen system for intuitionistic logic allowing derivations that correspond in a 1-1 way to the normal natural deductions of intuitionistic logic. This calculus is therefore well suited to proof enumeration. Such calculi are called 'permutation-free' calculi. In Chapter 3, MJ is extended to a calculus for an intuitionistic modal logic (due to Curry) called Lax Logic. We call this calculus PFLAX. The proof theory of MJ is extended to PFLAX. Chapter 4 presents work on theorem proving for propositional logics using a history mechanism for loop-checking. This mechanism is a refinement of one developed by Heuerding et al ([HSZ96]). It is applied to two calculi for intuitionistic logic and also to two modal logics; Lax Logic and intuitionistic S4. The calculi for intuitionistic logic are compared both theoretically and experimentally with other decision procedures for the logic. Chapter 5 is a short investigation of embedding intuitionistic logic in Intuitionistic Linear Logic. A new embedding of intuitionistic logic in Intuitionistic Linear Logic is given. For the hereditary Harrop fragment of intuitionistic logic, this embedding induces the calculus MJ for intuitionistic logic. In Chapter 6 a 'permutation-free' calculus is given for Intuitionistic Linear Logic. Again, its proof-theoretic properties are investigated. The calculus is proved to be sound and complete with respect to a proof-theoretic semantics and (weak) cut- elimination is proved. Logic programming can be thought of as proof enumeration in constructive logics. All the proof enumeration calculi in this thesis have been developed with logic programming in mind. We discuss at the appropriate points the relationship between the calculi developed here and logic programming. Appendix A contains presentations of the logical calculi used and Appendix B contains the sets of benchmark formulae used in Chapter 4.
1999-01-01T00:00:00ZHowe, Jacob M.This thesis develops techniques and ideas on proof search. Proof search is used with one of two meanings. Proof search can be thought of either as the search for a yes/no answer to a query (theorem proving), or as the search for all proofs of a formula (proof enumeration). This thesis is an investigation into issues in proof search in both these senses for some non-classical logics. Gentzen systems are well suited for use in proof search in both senses. The rules of Gentzen sequent calculi are such that implementations can be directed by the top level syntax of sequents, unlike other logical calculi such as natural deduction. All the calculi for proof search in this thesis are Gentzen sequent calculi. In Chapter 2, permutation of inference rules for Intuitionistic Linear Logic is studied. A focusing calculus, ILLF, in the style of Andreoli ([And92]) is developed. This calculus allows only one proof in each equivalence class of proofs equivalent up to permutations of inferences. The issue here is both theorem proving and proof enumeration. For certain logics, normal natural deductions provide a proof-theoretic semantics. Proof enumeration is then the enumeration of all these deductions. Herbelin's cut- free LJT ([Her95], here called MJ) is a Gentzen system for intuitionistic logic allowing derivations that correspond in a 1-1 way to the normal natural deductions of intuitionistic logic. This calculus is therefore well suited to proof enumeration. Such calculi are called 'permutation-free' calculi. In Chapter 3, MJ is extended to a calculus for an intuitionistic modal logic (due to Curry) called Lax Logic. We call this calculus PFLAX. The proof theory of MJ is extended to PFLAX. Chapter 4 presents work on theorem proving for propositional logics using a history mechanism for loop-checking. This mechanism is a refinement of one developed by Heuerding et al ([HSZ96]). It is applied to two calculi for intuitionistic logic and also to two modal logics; Lax Logic and intuitionistic S4. The calculi for intuitionistic logic are compared both theoretically and experimentally with other decision procedures for the logic. Chapter 5 is a short investigation of embedding intuitionistic logic in Intuitionistic Linear Logic. A new embedding of intuitionistic logic in Intuitionistic Linear Logic is given. For the hereditary Harrop fragment of intuitionistic logic, this embedding induces the calculus MJ for intuitionistic logic. In Chapter 6 a 'permutation-free' calculus is given for Intuitionistic Linear Logic. Again, its proof-theoretic properties are investigated. The calculus is proved to be sound and complete with respect to a proof-theoretic semantics and (weak) cut- elimination is proved. Logic programming can be thought of as proof enumeration in constructive logics. All the proof enumeration calculi in this thesis have been developed with logic programming in mind. We discuss at the appropriate points the relationship between the calculi developed here and logic programming. Appendix A contains presentations of the logical calculi used and Appendix B contains the sets of benchmark formulae used in Chapter 4.Distance sampling with camera trapsHowe, Eric J.Buckland, Stephen T.Després-Einspenner, Marie-LyneKühl, Hjalmarhttp://hdl.handle.net/10023/133282019-01-06T03:15:24Z2017-11-01T00:00:00Z1. Reliable estimates of animal density and abundance are essential for effective wildlife conservation and management. Camera trapping has proven efficient for sampling multiple species, but statistical estimators of density from camera trapping data for species that cannot be individually identified are still in development. 2. We extend point-transect methods for estimating animal density to accommodate data from camera traps, allowing researchers to exploit existing distance sampling theory and software for designing studies and analyzing data. We tested it by simulation, and used it to estimate densities of Maxwell’s duikers (Philantomba maxwellii) in Taï National Park, Côte d’Ivoire. 3. Densities estimated from simulated data were unbiased when we assumed animals were not available for detection during long periods of rest. Estimated duiker densities were higher than recent estimates from line transect surveys, which are believed to underestimate densities of forest ungulates. 4. We expect these methods to provide an effective means to estimate animal density from camera trapping data and to be applicable in a variety of settings.
We thank the Robert Bosch Foundation, the Max Planck Society, and the University of St Andrews for funding. The data files from which densities of Maxwell's duikers were estimated using Distance software, and data describing start times of videos of Maxwell's duikers, have been archived at the Dryad data repository (https://doi.org/10.5061/dryad.b4c70) (Howe et al. 2017).
2017-11-01T00:00:00ZHowe, Eric J.Buckland, Stephen T.Després-Einspenner, Marie-LyneKühl, Hjalmar1. Reliable estimates of animal density and abundance are essential for effective wildlife conservation and management. Camera trapping has proven efficient for sampling multiple species, but statistical estimators of density from camera trapping data for species that cannot be individually identified are still in development. 2. We extend point-transect methods for estimating animal density to accommodate data from camera traps, allowing researchers to exploit existing distance sampling theory and software for designing studies and analyzing data. We tested it by simulation, and used it to estimate densities of Maxwell’s duikers (Philantomba maxwellii) in Taï National Park, Côte d’Ivoire. 3. Densities estimated from simulated data were unbiased when we assumed animals were not available for detection during long periods of rest. Estimated duiker densities were higher than recent estimates from line transect surveys, which are believed to underestimate densities of forest ungulates. 4. We expect these methods to provide an effective means to estimate animal density from camera trapping data and to be applicable in a variety of settings.Chains of subsemigroupsCameron, Peter J.Gadouleau, MaximilienMitchell, James D.Peresse, Yannhttp://hdl.handle.net/10023/133132019-01-06T03:03:15Z2017-06-01T00:00:00ZWe investigate the maximum length of a chain of subsemigroups in various classes of semigroups, such as the full transformation semigroups, the general linear semigroups, and the semigroups of order-preserving transformations of finite chains. In some cases, we give lower bounds for the total number of subsemigroups of these semigroups. We give general results for finite completely regular and finite inverse semigroups. Wherever possible, we state our results in the greatest generality; in particular, we include infinite semigroups where the result is true for these. The length of a subgroup chain in a group is bounded by the logarithm of the group order. This fails for semigroups, but it is perhaps surprising that there is a lower bound for the length of a subsemigroup chain in the full transformation semigroup which is a constant multiple of the semigroup order.
2017-06-01T00:00:00ZCameron, Peter J.Gadouleau, MaximilienMitchell, James D.Peresse, YannWe investigate the maximum length of a chain of subsemigroups in various classes of semigroups, such as the full transformation semigroups, the general linear semigroups, and the semigroups of order-preserving transformations of finite chains. In some cases, we give lower bounds for the total number of subsemigroups of these semigroups. We give general results for finite completely regular and finite inverse semigroups. Wherever possible, we state our results in the greatest generality; in particular, we include infinite semigroups where the result is true for these. The length of a subgroup chain in a group is bounded by the logarithm of the group order. This fails for semigroups, but it is perhaps surprising that there is a lower bound for the length of a subsemigroup chain in the full transformation semigroup which is a constant multiple of the semigroup order.Imprimitive permutations in primitive groupsAraújo, JoaoAraújo, Joao PedroCameron, Peter JephsonDobson, TedHulpke, AlexanderLopes, Pedrohttp://hdl.handle.net/10023/132822019-01-06T03:15:26Z2017-09-15T00:00:00ZThe goal of this paper is to study primitive groups that are contained in the union of maximal (in the symmetric group) imprimitive groups. The study of types of permutations that appear inside primitive groups goes back to the origins of the theory of permutation groups. However, this is another instance of a situation common in mathematics in which a very natural problem turns out to be extremely difficult. Fortunately, the enormous progresses of the last few decades seem to allow a new momentum on the attack to this problem. In this paper we prove that there are infinite families of primitive groups contained in the union of imprimitive groups and propose a new hierarchy for primitive groups based on that fact. In addition we introduce some algorithms to handle permutations, provide the corresponding GAP implementation, solve some open problems, and propose a large list of open problems.
2017-09-15T00:00:00ZAraújo, JoaoAraújo, Joao PedroCameron, Peter JephsonDobson, TedHulpke, AlexanderLopes, PedroThe goal of this paper is to study primitive groups that are contained in the union of maximal (in the symmetric group) imprimitive groups. The study of types of permutations that appear inside primitive groups goes back to the origins of the theory of permutation groups. However, this is another instance of a situation common in mathematics in which a very natural problem turns out to be extremely difficult. Fortunately, the enormous progresses of the last few decades seem to allow a new momentum on the attack to this problem. In this paper we prove that there are infinite families of primitive groups contained in the union of imprimitive groups and propose a new hierarchy for primitive groups based on that fact. In addition we introduce some algorithms to handle permutations, provide the corresponding GAP implementation, solve some open problems, and propose a large list of open problems.A new technique for observationally derived boundary conditions for space weatherPagano, PaoloMackay, Duncan HendryYeates, Anthony Robinsonhttp://hdl.handle.net/10023/132712018-12-02T00:48:50Z2018-04-17T00:00:00ZContext. In recent years, space weather research has focused on developing modelling techniques to predict the arrival time and properties of coronal mass ejections (CMEs) at the Earth. The aim of this paper is to propose a new modelling technique suitable for the next generation of Space Weather predictive tools that is both efficient and accurate. The aim of the new approach is to provide interplanetary space weather forecasting models with accurate time dependent boundary conditions of erupting magnetic flux ropes in the upper solar corona. Methods. To produce boundary conditions, we couple two different modelling techniques, MHD simulations and a quasi-static non-potential evolution model. Both are applied on a spatial domain that covers the entire solar surface, although they extend over a different radial distance. The non-potential model uses a time series of observed synoptic magnetograms to drive the non-potential quasi-static evolution of the coronal magnetic field. This allows us to follow the formation and loss of equilibrium of magnetic flux ropes. Following this a MHD simulation captures the dynamic evolution of the erupting flux rope, when it is ejected into interplanetary space. Results. The present paper focuses on the MHD simulations that follow the ejection of magnetic flux ropes to 4R⊙. We first propose a technique for specifying the pre-eruptive plasma properties in the corona. Next, time dependent MHD simulations describe the ejection of two magnetic flux ropes, that produce time dependent boundary conditions for the magnetic field and plasma at 4R⊙ that in future may be applied to interplanetary space weather prediction models. Conclusions. In the present paper, we show that the dual use of quasi-static non-potential magnetic field simulations and full time dependent MHD simulations can produce realistic inhomogeneous boundary conditions for space weather forecasting tools. Before a fully operational model can be produced there are a number of technical and scientific challenges that still need to be addressed. Nevertheless, we illustrate that coupling quasi-static and MHD simulations in this way can significantly reduce the computational time required to produce realistic space weather boundary conditions.
This research has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 647214). D.H.M. would like to thank STFC and the Leverhulme Trust for their financial support. ARY was supported by STFC consortium grant ST/N000781/1 to the universities of Dundee and Durham.
2018-04-17T00:00:00ZPagano, PaoloMackay, Duncan HendryYeates, Anthony RobinsonContext. In recent years, space weather research has focused on developing modelling techniques to predict the arrival time and properties of coronal mass ejections (CMEs) at the Earth. The aim of this paper is to propose a new modelling technique suitable for the next generation of Space Weather predictive tools that is both efficient and accurate. The aim of the new approach is to provide interplanetary space weather forecasting models with accurate time dependent boundary conditions of erupting magnetic flux ropes in the upper solar corona. Methods. To produce boundary conditions, we couple two different modelling techniques, MHD simulations and a quasi-static non-potential evolution model. Both are applied on a spatial domain that covers the entire solar surface, although they extend over a different radial distance. The non-potential model uses a time series of observed synoptic magnetograms to drive the non-potential quasi-static evolution of the coronal magnetic field. This allows us to follow the formation and loss of equilibrium of magnetic flux ropes. Following this a MHD simulation captures the dynamic evolution of the erupting flux rope, when it is ejected into interplanetary space. Results. The present paper focuses on the MHD simulations that follow the ejection of magnetic flux ropes to 4R⊙. We first propose a technique for specifying the pre-eruptive plasma properties in the corona. Next, time dependent MHD simulations describe the ejection of two magnetic flux ropes, that produce time dependent boundary conditions for the magnetic field and plasma at 4R⊙ that in future may be applied to interplanetary space weather prediction models. Conclusions. In the present paper, we show that the dual use of quasi-static non-potential magnetic field simulations and full time dependent MHD simulations can produce realistic inhomogeneous boundary conditions for space weather forecasting tools. Before a fully operational model can be produced there are a number of technical and scientific challenges that still need to be addressed. Nevertheless, we illustrate that coupling quasi-static and MHD simulations in this way can significantly reduce the computational time required to produce realistic space weather boundary conditions.On the Hausdorff and packing measures of typical compact metric spacesJurina, S.MacGregor, N.Mitchell, A.Olsen, L.Stylianou, A.http://hdl.handle.net/10023/132682019-01-06T03:34:32Z2018-08-01T00:00:00ZWe study the Hausdorff and packing measures of typical compact metric spaces belonging to the Gromov–Hausdorff space (of all compact metric spaces) equipped with the Gromov–Hausdorff metric.
2018-08-01T00:00:00ZJurina, S.MacGregor, N.Mitchell, A.Olsen, L.Stylianou, A.We study the Hausdorff and packing measures of typical compact metric spaces belonging to the Gromov–Hausdorff space (of all compact metric spaces) equipped with the Gromov–Hausdorff metric.Contribution of phase-mixing of Alfvén waves to coronal heating in multi-harmonic loop oscillationsPagano, PaoloPascoe, David JamesDe Moortel, Inekehttp://hdl.handle.net/10023/132632018-12-16T00:38:12Z2018-04-26T00:00:00ZContext. Kink oscillations of a coronal loop are observed and studied in detail because they provide a unique probe into the structure of coronal loops through MHD seismology and a potential test of coronal heating through the phase-mixing of Alfvén waves . In particular, recent observations show that standing oscill ations of loops often involve also higher harmonics, beside the fundamental mode. The damping of these kink oscillations is explained by mode coupling with Alfvén waves. Aims. We investigate the consequences for wave-based coronal hea ting of higher harmonics and what coronal heating observational signatures we may use to infer the presence of higher harmonic kink oscillations. Methods. We perform a set of non-ideal MHD simulations where the damping of the kink oscillation of a flux tube via mode coupling is modelled. Our MHD simulation parameters are based on the seismological inversion of an observation for which the first three harmonics are detected. We study the phase-mixing of Alfvén waves that leads to the deposition of heat in the system, and we apply the seismological inversion techniques to the MHD simulation output. Results. We find that the heating due to phase-mixing of the Alfvén wave s triggered by the damping of the kink oscillation is relatively small, however we can illustrate i) how the heating location drifts due to the subsequent damping of lower order harmonics. We also address the role of the higher order harmonics and the width of the boundary shell in the energy deposition. Conclusions. We conclude that the coronal heating due to phase-mixing see ms not to provide enough energy to maintain the thermal structure of the solar corona even when multi-harmonics oscillations are included, and these oscillations play an inhibiting role in the development of smaller scale structures.
This research has received funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1 and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program ( grant agreement No. 647214). This work is supported by the European Research Council under the SeismoSun Research Project No. 321141 (DJP). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 724326). This work used the DiRAC Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment was funded by a BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grant ST/K00087X/1, DiRAC Operations grant ST/K003267/1 and Durham University. DiRAC is part of the National E-Infrastructure.
2018-04-26T00:00:00ZPagano, PaoloPascoe, David JamesDe Moortel, InekeContext. Kink oscillations of a coronal loop are observed and studied in detail because they provide a unique probe into the structure of coronal loops through MHD seismology and a potential test of coronal heating through the phase-mixing of Alfvén waves . In particular, recent observations show that standing oscill ations of loops often involve also higher harmonics, beside the fundamental mode. The damping of these kink oscillations is explained by mode coupling with Alfvén waves. Aims. We investigate the consequences for wave-based coronal hea ting of higher harmonics and what coronal heating observational signatures we may use to infer the presence of higher harmonic kink oscillations. Methods. We perform a set of non-ideal MHD simulations where the damping of the kink oscillation of a flux tube via mode coupling is modelled. Our MHD simulation parameters are based on the seismological inversion of an observation for which the first three harmonics are detected. We study the phase-mixing of Alfvén waves that leads to the deposition of heat in the system, and we apply the seismological inversion techniques to the MHD simulation output. Results. We find that the heating due to phase-mixing of the Alfvén wave s triggered by the damping of the kink oscillation is relatively small, however we can illustrate i) how the heating location drifts due to the subsequent damping of lower order harmonics. We also address the role of the higher order harmonics and the width of the boundary shell in the energy deposition. Conclusions. We conclude that the coronal heating due to phase-mixing see ms not to provide enough energy to maintain the thermal structure of the solar corona even when multi-harmonics oscillations are included, and these oscillations play an inhibiting role in the development of smaller scale structures.Hetonic quartets in a two-layer quasi-geostrophic flow : V-states and stabilityReinaud, J. N.Sokolovskiy, MikhailCarton, Xavierhttp://hdl.handle.net/10023/132472019-01-06T03:36:16Z2018-05-11T00:00:00ZWe investigate families of finite core vortex quartets in mutual equilibrium in a two- layer quasi-geostrophic flow. The finite core solutions stem from known solutions for discrete (singular) vortex quartets. Two vortices lie in the top layer and two vortices lie in the bottom layer. Two vortices have a positive potential vorticity anomaly while the two others have negative potential vorticity anomaly. The vortex configurations are therefore related to the baroclinic dipoles known in the literature as hetons. Two main branches of solutions exist depending on the arrangement of the vortices: the translating zigzag-shaped hetonic quartets and the rotating zigzag- shaped hetonic quartets. By addressing their linear stability, we show that while the rotating quartets can be unstable over a large range of the parameter space, most translating quartets are stable. This has implications on the longevity of such vortex equilibria in the oceans.
M.A.S. and X.C. were supported by RFBR/CNRS (PRC Grant No. 16-55-150001/1069). M.A.S. was supported also by RFBR (Grant No. 16-05-00121), RSF (Grant No. 14-50-00095, geophysical applications) and MESRF (Grant No. 14.W.03.31.0006, numerical simulation, vortex dynamics).
2018-05-11T00:00:00ZReinaud, J. N.Sokolovskiy, MikhailCarton, XavierWe investigate families of finite core vortex quartets in mutual equilibrium in a two- layer quasi-geostrophic flow. The finite core solutions stem from known solutions for discrete (singular) vortex quartets. Two vortices lie in the top layer and two vortices lie in the bottom layer. Two vortices have a positive potential vorticity anomaly while the two others have negative potential vorticity anomaly. The vortex configurations are therefore related to the baroclinic dipoles known in the literature as hetons. Two main branches of solutions exist depending on the arrangement of the vortices: the translating zigzag-shaped hetonic quartets and the rotating zigzag- shaped hetonic quartets. By addressing their linear stability, we show that while the rotating quartets can be unstable over a large range of the parameter space, most translating quartets are stable. This has implications on the longevity of such vortex equilibria in the oceans.Timing the landmark events in the evolution of clear cell renal cell cancer : TRACERx renalMitchell, Thomas J.Turajlic, SamraRowan, AndrewNicol, DavidFarmery, James H.R.O’Brien, TimMartincorena, InigoTarpey, PatrickAngelopoulos, NicosYates, Lucy R.Butler, Adam P.Raine, KeiranStewart, Grant D.Challacombe, BenFernando, ArchanaLopez, Jose I.Hazell, SteveChandra, AshishChowdhury, SimonRudman, SarahSoultati, AspasiaStamp, GordonFotiadis, NicosPickering, LisaAu, LewisSpain, LaviniaLynch, JoannaStares, MarkTeague, JonMaura, FrancescoWedge, David C.Horswell, StuartChambers, TimLitchfield, KevinXu, HangStewart, AengusElaidi, RezaOudard, StéphaneMcGranahan, NicholasCsabai, IstvanGore, MartinFutreal, P. AndrewLarkin, JamesLynch, Andy G.Szallasi, ZoltanSwanton, CharlesCampbell, Peter J.http://hdl.handle.net/10023/131312019-01-20T01:36:25Z2018-04-19T00:00:00ZClear cell renal cell carcinoma (ccRCC) is characterized by near-universal loss of the short arm of chromosome 3, deleting several tumor suppressor genes. We analyzed whole genomes from 95 biopsies across 33 patients with clear cell renal cell carcinoma. We find hotspots of point mutations in the 5′ UTR of TERT, targeting a MYC-MAX-MAD1 repressor associated with telomere lengthening. The most common structural abnormality generates simultaneous 3p loss and 5q gain (36% patients), typically through chromothripsis. This event occurs in childhood or adolescence, generally as the initiating event that precedes emergence of the tumor’s most recent common ancestor by years to decades. Similar genomic changes drive inherited ccRCC. Modeling differences in age incidence between inherited and sporadic cancers suggests that the number of cells with 3p loss capable of initiating sporadic tumors is no more than a few hundred. Early development of ccRCC follows well-defined evolutionary trajectories, offering opportunity for early intervention.
The work presented in this manuscript was funded by EU FP7 (project PREDICT ID number 259303) and the Wellcome Trust and Cancer Research UK. S.T. is funded by Cancer Research UK (C50947/A18176). S.T., J.L., and M.G. receive funding from the National Institute for Health Research (NIHR) Biomedical Research Centre at the Royal Marsden Hospital and Institute of Cancer Research (A109). J.H.R.F. and A.G.L. were supported by the University of Cambridge, Cancer Research UK (C14303/A17197), and Hutchison Whampoa. K.L. is supported by a UK Medical Research Council Skills Development Fellowship Award. C.S. is funded by Cancer Research UK (TRACERx), the Rosetrees Trust, NovoNordisk Foundation (16584), EU FP7 (projects PREDICT and RESPONSIFY, ID number 259303), the Prostate Cancer Foundation, the Breast Cancer Research Foundation, the European Research Council (THESEUS), and National Institute for Health Research University College London Hospitals Biomedical Research Centre. P.J.C. has a Wellcome Trust Senior Clinical Research Fellowship (WT088340MA).
2018-04-19T00:00:00ZMitchell, Thomas J.Turajlic, SamraRowan, AndrewNicol, DavidFarmery, James H.R.O’Brien, TimMartincorena, InigoTarpey, PatrickAngelopoulos, NicosYates, Lucy R.Butler, Adam P.Raine, KeiranStewart, Grant D.Challacombe, BenFernando, ArchanaLopez, Jose I.Hazell, SteveChandra, AshishChowdhury, SimonRudman, SarahSoultati, AspasiaStamp, GordonFotiadis, NicosPickering, LisaAu, LewisSpain, LaviniaLynch, JoannaStares, MarkTeague, JonMaura, FrancescoWedge, David C.Horswell, StuartChambers, TimLitchfield, KevinXu, HangStewart, AengusElaidi, RezaOudard, StéphaneMcGranahan, NicholasCsabai, IstvanGore, MartinFutreal, P. AndrewLarkin, JamesLynch, Andy G.Szallasi, ZoltanSwanton, CharlesCampbell, Peter J.Clear cell renal cell carcinoma (ccRCC) is characterized by near-universal loss of the short arm of chromosome 3, deleting several tumor suppressor genes. We analyzed whole genomes from 95 biopsies across 33 patients with clear cell renal cell carcinoma. We find hotspots of point mutations in the 5′ UTR of TERT, targeting a MYC-MAX-MAD1 repressor associated with telomere lengthening. The most common structural abnormality generates simultaneous 3p loss and 5q gain (36% patients), typically through chromothripsis. This event occurs in childhood or adolescence, generally as the initiating event that precedes emergence of the tumor’s most recent common ancestor by years to decades. Similar genomic changes drive inherited ccRCC. Modeling differences in age incidence between inherited and sporadic cancers suggests that the number of cells with 3p loss capable of initiating sporadic tumors is no more than a few hundred. Early development of ccRCC follows well-defined evolutionary trajectories, offering opportunity for early intervention.Structured models of cell migration incorporating molecular binding processesDomschke, PiaTrucu, DumitruGerisch, AlfChaplain, Mark Andrew Josephhttp://hdl.handle.net/10023/131272019-01-06T03:16:03Z2017-12-01T00:00:00ZThe dynamic interplay between collective cell movement and the various molecules involved in the accompanying cell signalling mechanisms plays a crucial role in many biological processes including normal tissue development and pathological scenarios such as wound healing and cancer. Information about the various structures embedded within these processes allows a detailed exploration of the binding of molecular species to cell-surface receptors within the evolving cell population. In this paper we establish a general spatio-temporal-structural framework that enables the description of molecular binding to cell membranes coupled with the cell population dynamics. We first provide a general theoretical description for this approach and then illustrate it with three examples arising from cancer invasion.
DT and MAJC gratefully acknowledge the support of the ERC Advanced Investigator Grant 227619, “M5CGS - From Mutations to Metastases: Multiscale Mathematical Modelling of Cancer Growth and Spread”.
2017-12-01T00:00:00ZDomschke, PiaTrucu, DumitruGerisch, AlfChaplain, Mark Andrew JosephThe dynamic interplay between collective cell movement and the various molecules involved in the accompanying cell signalling mechanisms plays a crucial role in many biological processes including normal tissue development and pathological scenarios such as wound healing and cancer. Information about the various structures embedded within these processes allows a detailed exploration of the binding of molecular species to cell-surface receptors within the evolving cell population. In this paper we establish a general spatio-temporal-structural framework that enables the description of molecular binding to cell membranes coupled with the cell population dynamics. We first provide a general theoretical description for this approach and then illustrate it with three examples arising from cancer invasion.Estimation and simulation of foraging trips in land-based marine predatorsMichelot, TheoLangrock, RolandBestley, SophieJonsen, Ian D.Photopoulou, TheoniPatterson, Toby A.http://hdl.handle.net/10023/131142019-01-13T02:35:10Z2017-07-01T00:00:00ZThe behavior of colony-based marine predators is the focus of much research globally. Large telemetry and tracking data sets have been collected for this group of animals, and are accompanied by many empirical studies that seek to segment tracks in some useful way, as well as theoretical studies of optimal foraging strategies. However, relatively few studies have detailed statistical methods for inferring behaviors in central place foraging trips. In this paper we describe an approach based on hidden Markov models, which splits foraging trips into segments labeled as "outbound", "search", "forage", and "inbound". By structuring the hidden Markov model transition matrix appropriately, the model naturally handles the sequence of behaviors within a foraging trip. Additionally, by structuring the model in this way, we are able to develop realistic simulations from the fitted model. We demonstrate our approach on data from southern elephant seals (Mirounga leonina) tagged on Kerguelen Island in the Southern Ocean. We discuss the differences between our 4-state model and the widely used 2-state model, and the advantages and disadvantages of employing a more complex model.
TM and TP received support from IMBER-CLIOTOP and Macquarie University Safety Net Grant 9201401743. SB was supported under an Australia Research Council Super Science Fellowship. IDJ was supported by a Macquarie Vice-Chancellors Innovation Fellowship. TAP was supported by a CSIRO Julius Career Award and the Villum Foundation.
2017-07-01T00:00:00ZMichelot, TheoLangrock, RolandBestley, SophieJonsen, Ian D.Photopoulou, TheoniPatterson, Toby A.The behavior of colony-based marine predators is the focus of much research globally. Large telemetry and tracking data sets have been collected for this group of animals, and are accompanied by many empirical studies that seek to segment tracks in some useful way, as well as theoretical studies of optimal foraging strategies. However, relatively few studies have detailed statistical methods for inferring behaviors in central place foraging trips. In this paper we describe an approach based on hidden Markov models, which splits foraging trips into segments labeled as "outbound", "search", "forage", and "inbound". By structuring the hidden Markov model transition matrix appropriately, the model naturally handles the sequence of behaviors within a foraging trip. Additionally, by structuring the model in this way, we are able to develop realistic simulations from the fitted model. We demonstrate our approach on data from southern elephant seals (Mirounga leonina) tagged on Kerguelen Island in the Southern Ocean. We discuss the differences between our 4-state model and the widely used 2-state model, and the advantages and disadvantages of employing a more complex model.On well quasi-order of graph classes under homomorphic image orderingsHuczynska, S.Ruškuc, N.http://hdl.handle.net/10023/130912019-01-06T03:11:05Z2017-06-01T00:00:00ZIn this paper we consider the question of well quasi-order for classes defined by a single obstruction within the classes of all graphs, digraphs and tournaments, under the homomorphic image ordering (in both its standard and strong forms). The homomorphic image ordering was introduced by the authors in a previous paper and corresponds to the existence of a surjective homomorphism between two structures. We obtain complete characterisations in all cases except for graphs under the strong ordering, where some open questions remain.
2017-06-01T00:00:00ZHuczynska, S.Ruškuc, N.In this paper we consider the question of well quasi-order for classes defined by a single obstruction within the classes of all graphs, digraphs and tournaments, under the homomorphic image ordering (in both its standard and strong forms). The homomorphic image ordering was introduced by the authors in a previous paper and corresponds to the existence of a surjective homomorphism between two structures. We obtain complete characterisations in all cases except for graphs under the strong ordering, where some open questions remain.Coronal energy release by MHD avalanches : continuous drivingReid, JackHood, Alan WilliamParnell, Clare ElizabethBrowning, P. K.Cargill, Peterhttp://hdl.handle.net/10023/130892019-01-13T00:33:54Z2018-07-18T00:00:00ZPrevious work has confirmed the concept of a magnetohydrodynamic (MHD) avalanche in pre-stressed threads within a coronal loop. We undertook a series of full, three-dimensional MHD simulations in order to create three threads by twisting the magnetic field through boundary motions until an instability ensues. We find that, following the original instability, one unstable thread can disrupt its neighbours with continued driving. A ‘bursty’ heating profile results, with a series of ongoing energy releases, but no evident steady state. For the first time using full MHD, we show that avalanches are a viable mechanism for the storing and release of magnetic energy in the solar corona, as a result of photospheric motions.
Funding: Carnegie Trust for the Universities of Scotland; Science and Technology Facilities Council (grants ST/N000609/1 and ST/P000428/1).
2018-07-18T00:00:00ZReid, JackHood, Alan WilliamParnell, Clare ElizabethBrowning, P. K.Cargill, PeterPrevious work has confirmed the concept of a magnetohydrodynamic (MHD) avalanche in pre-stressed threads within a coronal loop. We undertook a series of full, three-dimensional MHD simulations in order to create three threads by twisting the magnetic field through boundary motions until an instability ensues. We find that, following the original instability, one unstable thread can disrupt its neighbours with continued driving. A ‘bursty’ heating profile results, with a series of ongoing energy releases, but no evident steady state. For the first time using full MHD, we show that avalanches are a viable mechanism for the storing and release of magnetic energy in the solar corona, as a result of photospheric motions.Enumerating transformation semigroupsEast, JamesEgri-Nagy, AttilaMitchell, James D.http://hdl.handle.net/10023/130642019-01-06T03:17:23Z2017-08-01T00:00:00ZWe describe general methods for enumerating subsemigroups of finite semigroups and techniques to improve the algorithmic efficiency of the calculations. As a particular application we use our algorithms to enumerate all transformation semigroups up to degree 4. Classification of these semigroups up to conjugacy, isomorphism and anti-isomorphism, by size and rank, provides a solid base for further investigations of transformation semigroups.
This work was partially supported by the NeCTAR Research Cloud, an initiative of the Australian Government’s Super Science scheme and the Education Investment Fund; and by the EU Project BIOMICS (Contract Number CNECT-ICT-318202).
2017-08-01T00:00:00ZEast, JamesEgri-Nagy, AttilaMitchell, James D.We describe general methods for enumerating subsemigroups of finite semigroups and techniques to improve the algorithmic efficiency of the calculations. As a particular application we use our algorithms to enumerate all transformation semigroups up to degree 4. Classification of these semigroups up to conjugacy, isomorphism and anti-isomorphism, by size and rank, provides a solid base for further investigations of transformation semigroups.Evaluating the effect of measurement error in pairs of 3D bearings in point transect sampling estimates of densityMarques, Tiago A.Duarte, PedroPeixe, TelmoMoretti, DavidThomas, Lenhttp://hdl.handle.net/10023/130372018-11-11T00:44:52Z2018-03-27T00:00:00Z2018-03-27T00:00:00ZMarques, Tiago A.Duarte, PedroPeixe, TelmoMoretti, DavidThomas, LenTrace-contrast models for capture-recapture without capture historiesFewster, R. M.Stevenson, B. C.Borchers, D. L.http://hdl.handle.net/10023/130102019-01-06T03:33:17Z2016-05-26T00:00:00ZCapture-recapture studies increasingly rely upon natural tags that allow animals to be identified by features such as coat markings, DNA profiles, acoustic profiles, or spatial locations. These innovations greatly increase the number of capture samples achievable and enable capture-recapture estimation for many inaccessible and elusive species. However, natural features are invariably imperfect as indicators of identity. Drawing on the recently developed Palm likelihood approach to parameter estimation in clustered point processes, we propose a new estimation framework based on comparing pairs of detections, which we term the trace-contrast framework. Importantly, no reconstruction of capture histories is needed. We show that we can achieve accurate, precise, and computationally fast inference. We illustrate the methods with a camera-trap study of a partially marked population of ship rats (Rattus rattus) in New Zealand.
This work was funded by the Royal Society of New Zealand through Marsden Grant 14-UOA-155. Ben Stevenson was supported by EPSRC/NERC Grant EP/1000917/1.
2016-05-26T00:00:00ZFewster, R. M.Stevenson, B. C.Borchers, D. L.Capture-recapture studies increasingly rely upon natural tags that allow animals to be identified by features such as coat markings, DNA profiles, acoustic profiles, or spatial locations. These innovations greatly increase the number of capture samples achievable and enable capture-recapture estimation for many inaccessible and elusive species. However, natural features are invariably imperfect as indicators of identity. Drawing on the recently developed Palm likelihood approach to parameter estimation in clustered point processes, we propose a new estimation framework based on comparing pairs of detections, which we term the trace-contrast framework. Importantly, no reconstruction of capture histories is needed. We show that we can achieve accurate, precise, and computationally fast inference. We illustrate the methods with a camera-trap study of a partially marked population of ship rats (Rattus rattus) in New Zealand.Modelling the effects of bacterial cell state and spatial location on tuberculosis treatment : insights from a hybrid multiscale cellular automaton modelBowness, RuthChaplain, Mark A.J.Powathil, Gibin G.Gillespie, Stephen H.http://hdl.handle.net/10023/129832019-01-06T03:35:17Z2018-06-07T00:00:00ZIf improvements are to be made in tuberculosis (TB) treatment, an increased understanding of disease in the lung is needed. Studies have shown that bacteria in a less metabolically active state, associated with the presence of lipid bodies, are less susceptible to antibiotics, and recent results have highlighted the disparity in concentration of different compounds into lesions. Treatment success therefore depends critically on the responses of the individual bacteria that constitute the infection. We propose a hybrid, individual-based approach that analyses spatio-temporal dynamics at the cellular level, linking the behaviour of individual bacteria and host cells with the macroscopic behaviour of the microenvironment. The individual elements (bacteria, macrophages and T cells) are modelled using cellular automaton (CA) rules, and the evolution of oxygen, drugs and chemokine dynamics are incorporated in order to study the effects of the microenvironment in the pathological lesion. We allow bacteria to switch states depending on oxygen concentration, which affects how they respond to treatment. This is the first multiscale model of its type to consider both oxygen-driven phenotypic switching of the Mycobacterium tuberculosis and antibiotic treatment. Using this model, we investigate the role of bacterial cell state and of initial bacterial location on treatment outcome. We demonstrate that when bacteria are located further away from blood vessels, less favourable outcomes are more likely, i.e. longer time before infection is contained/cleared, treatment failure or later relapse. We also show that in cases where bacteria remain at the end of simulations, the organisms tend to be slower-growing and are often located within granulomas, surrounded by caseous material.
This work was supported by the Medical Research Council [grant number MR/P014704/1] and the PreDiCT-TB consortium (IMI Joint undertaking grant agreement number 115337, resources of which are composed of financial contribution from the European Union’s Seventh Framework Programme (FP7/2007-2013) and EFPIA companies’ in kind contribution.
2018-06-07T00:00:00ZBowness, RuthChaplain, Mark A.J.Powathil, Gibin G.Gillespie, Stephen H.If improvements are to be made in tuberculosis (TB) treatment, an increased understanding of disease in the lung is needed. Studies have shown that bacteria in a less metabolically active state, associated with the presence of lipid bodies, are less susceptible to antibiotics, and recent results have highlighted the disparity in concentration of different compounds into lesions. Treatment success therefore depends critically on the responses of the individual bacteria that constitute the infection. We propose a hybrid, individual-based approach that analyses spatio-temporal dynamics at the cellular level, linking the behaviour of individual bacteria and host cells with the macroscopic behaviour of the microenvironment. The individual elements (bacteria, macrophages and T cells) are modelled using cellular automaton (CA) rules, and the evolution of oxygen, drugs and chemokine dynamics are incorporated in order to study the effects of the microenvironment in the pathological lesion. We allow bacteria to switch states depending on oxygen concentration, which affects how they respond to treatment. This is the first multiscale model of its type to consider both oxygen-driven phenotypic switching of the Mycobacterium tuberculosis and antibiotic treatment. Using this model, we investigate the role of bacterial cell state and of initial bacterial location on treatment outcome. We demonstrate that when bacteria are located further away from blood vessels, less favourable outcomes are more likely, i.e. longer time before infection is contained/cleared, treatment failure or later relapse. We also show that in cases where bacteria remain at the end of simulations, the organisms tend to be slower-growing and are often located within granulomas, surrounded by caseous material.Theory and observations of the magnetic field in the solar coronaCarcedo, Laurahttp://hdl.handle.net/10023/129482018-06-14T23:17:02Z2005-01-01T00:00:00ZAlthough the solar corona is one of the most studied areas in solar physics, its activity, such as flares, prominence eruptions and CMEs, is far from understood. Since the solar corona is a low-ß plasma, its structure and dynamics are driven by the magnetic field. The aim of this PhD thesis to study the magnetic field in the solar corona. Unfortunately, high quality direct measurements of the coronal magnetic field are not available and theoretical extrapolation using the observed photospheric magnetic field is required. The thesis is mainly divided in two parts. The first part deals with the comparison between theoretical models of magnetic fields and observed structures in the corona. For any theoretical model, a quantitative method to fit magnetic field lines to observed coronal loops is introduced. This method provides a quantity C that measures how closely a theoretical model can reproduce the observed coronal structures. Using linear force-free field extrapolation, the above field line fitting method is used to study the evolution of an active region. The method is also illustrated when the theoretical magnetic field depends on more than one parameter. The second part of the thesis focuses on the linear force-free field assumption using two different geometric configurations. Firstly a vertical rigid magnetic flux tube is considered. The analytical expression of the magnetic field is obtained as an expansion in terms of Bessel functions. The main properties of this system are discussed and compared with two cylindrically symmetric twist profiles. For the second system, the photosphere is assumed to be an infinite plane. Using translational geometry, the analytical expression of the linear force-free magnetic field that matches a prescribed line of sight magnetic field component is obtained. This solution is compared with the non-linear solution obtained by Roumeliotis (1993).
2005-01-01T00:00:00ZCarcedo, LauraAlthough the solar corona is one of the most studied areas in solar physics, its activity, such as flares, prominence eruptions and CMEs, is far from understood. Since the solar corona is a low-ß plasma, its structure and dynamics are driven by the magnetic field. The aim of this PhD thesis to study the magnetic field in the solar corona. Unfortunately, high quality direct measurements of the coronal magnetic field are not available and theoretical extrapolation using the observed photospheric magnetic field is required. The thesis is mainly divided in two parts. The first part deals with the comparison between theoretical models of magnetic fields and observed structures in the corona. For any theoretical model, a quantitative method to fit magnetic field lines to observed coronal loops is introduced. This method provides a quantity C that measures how closely a theoretical model can reproduce the observed coronal structures. Using linear force-free field extrapolation, the above field line fitting method is used to study the evolution of an active region. The method is also illustrated when the theoretical magnetic field depends on more than one parameter. The second part of the thesis focuses on the linear force-free field assumption using two different geometric configurations. Firstly a vertical rigid magnetic flux tube is considered. The analytical expression of the magnetic field is obtained as an expansion in terms of Bessel functions. The main properties of this system are discussed and compared with two cylindrically symmetric twist profiles. For the second system, the photosphere is assumed to be an infinite plane. Using translational geometry, the analytical expression of the linear force-free magnetic field that matches a prescribed line of sight magnetic field component is obtained. This solution is compared with the non-linear solution obtained by Roumeliotis (1993).Loop oscillations in the coronaJames, Lornahttp://hdl.handle.net/10023/129472018-06-14T23:17:13Z2004-01-01T00:00:00ZMagnetic loops in the Sun's corona have been discovered to oscillate in a variety of modes. The oscillations are observed to exhibit strong damping. A number of theories have been put forward to explain the damping, including resonant absorption and phase mixing. Here we consider the modelling of loop oscillations, paying particular attention to two effects: gravity, and the addition of a chromospheric layer below the corona. We develop an acoustic model of coronal loop oscillations and consider two ways of describing the effects of the gravitational stratification and the chromospheric layers, considering either two media separated by a discontinuous interface or a single medium with a sound speed that varies along the loop. A dispersion relation for the two-layer isothermal atmosphere case is obtained and investigated numerically using a bisection code. On comparison with roots obtained for a single isothermal atmosphere, it was found that the effect of chromospheric footpoints on the period of a mode is slight. However, the effect of gravity was found to be more notable, rising up to a twenty percent change in period when considering the longer observed loops. This result is of especial interest since gravity is often ignored by authors discussing loop oscillations. The case of a linear sound speed has been investigated analytically, obtaining a dispersion relation in terms of Bessel functions. Our results show that the Bessel equation is a possible solution for describing the wave modes.
2004-01-01T00:00:00ZJames, LornaMagnetic loops in the Sun's corona have been discovered to oscillate in a variety of modes. The oscillations are observed to exhibit strong damping. A number of theories have been put forward to explain the damping, including resonant absorption and phase mixing. Here we consider the modelling of loop oscillations, paying particular attention to two effects: gravity, and the addition of a chromospheric layer below the corona. We develop an acoustic model of coronal loop oscillations and consider two ways of describing the effects of the gravitational stratification and the chromospheric layers, considering either two media separated by a discontinuous interface or a single medium with a sound speed that varies along the loop. A dispersion relation for the two-layer isothermal atmosphere case is obtained and investigated numerically using a bisection code. On comparison with roots obtained for a single isothermal atmosphere, it was found that the effect of chromospheric footpoints on the period of a mode is slight. However, the effect of gravity was found to be more notable, rising up to a twenty percent change in period when considering the longer observed loops. This result is of especial interest since gravity is often ignored by authors discussing loop oscillations. The case of a linear sound speed has been investigated analytically, obtaining a dispersion relation in terms of Bessel functions. Our results show that the Bessel equation is a possible solution for describing the wave modes.Magnetic annihilation, null collapse and coronal heatingMellor, Christopherhttp://hdl.handle.net/10023/129462018-06-14T23:17:06Z2004-01-01T00:00:00ZThe problem of how the Sun's corona is heated is of central importance to solar physics research. In this thesis we model three main areas. The first, annihilation, is a feature of non-ideal MHD and focusses on how magnetic field of opposite polarity meets at a null point and annihilates, after having been advected with plasma toward a stagnation point in the plasma flow. Generally, the null point of the field and the stagnation point of the flow are coincident at the origin, but in chapter 2 a simple extension is considered where an asymmetry in the boundary conditions of the field moves the null point away from the origin. Chapter 3 presents a model of reconnective annihilation in three dimensions. It represents flux being advected through the fan plane of a 3D null, and diffusing through a thin diffusion region before being annihilated at the spine line, and uses the method of matched asymptotic expansions to find the solution for small values of the resistivity. The second area of the thesis covers null collapse. This is when the magnetic field in close proximity to a null point is disturbed, causing the field to fold up on itself and collapse. This is a feature of ideal MHD, and causes a strong current to build up, allowing non-ideal effects to become important. When using linearised equations for the collapse problem, we are in fact looking at a linear instability. If this instability initiates a collapse, this is only a valid model until non-linear effects become important. By talking about collapse in chapters 4 and 5 (as it is talked about in the literature), we mean that the linear instability initiates collapse, which in principle, non-linear effects could later stop. Chapter 4 introduces a two-dimensional model for collapse, using the ideal, compressible, linearised MHD equations. It is a general solution in which all spatially linear nulls and their supporting plasma flows and pressure gradients can be checked for susceptibility to collapse under open boundary conditions. Chapter 5 uses the model introduced in chapter 4 to investigate the collapse of three-dimensional, potential nulls (again, spatially linear) for all possible supporting plasma flows and pressure gradients. Using this model, all nulls under consideration are found to collapse and produce large currents, except for a group of 2D O-type nulls supported by highly super-Alfvenic plasma flows. The third area of this thesis involves numerically simulating a model of heating by coronal tectonics (Priest et al, 2002). A simple magnetic field is created and the boundary is driven, also in a simple manner. Current sheets which scale with grid resolution are seen to build up on the quasi-separatrix layers, and there is some evidence of magnetic reconnection.
2004-01-01T00:00:00ZMellor, ChristopherThe problem of how the Sun's corona is heated is of central importance to solar physics research. In this thesis we model three main areas. The first, annihilation, is a feature of non-ideal MHD and focusses on how magnetic field of opposite polarity meets at a null point and annihilates, after having been advected with plasma toward a stagnation point in the plasma flow. Generally, the null point of the field and the stagnation point of the flow are coincident at the origin, but in chapter 2 a simple extension is considered where an asymmetry in the boundary conditions of the field moves the null point away from the origin. Chapter 3 presents a model of reconnective annihilation in three dimensions. It represents flux being advected through the fan plane of a 3D null, and diffusing through a thin diffusion region before being annihilated at the spine line, and uses the method of matched asymptotic expansions to find the solution for small values of the resistivity. The second area of the thesis covers null collapse. This is when the magnetic field in close proximity to a null point is disturbed, causing the field to fold up on itself and collapse. This is a feature of ideal MHD, and causes a strong current to build up, allowing non-ideal effects to become important. When using linearised equations for the collapse problem, we are in fact looking at a linear instability. If this instability initiates a collapse, this is only a valid model until non-linear effects become important. By talking about collapse in chapters 4 and 5 (as it is talked about in the literature), we mean that the linear instability initiates collapse, which in principle, non-linear effects could later stop. Chapter 4 introduces a two-dimensional model for collapse, using the ideal, compressible, linearised MHD equations. It is a general solution in which all spatially linear nulls and their supporting plasma flows and pressure gradients can be checked for susceptibility to collapse under open boundary conditions. Chapter 5 uses the model introduced in chapter 4 to investigate the collapse of three-dimensional, potential nulls (again, spatially linear) for all possible supporting plasma flows and pressure gradients. Using this model, all nulls under consideration are found to collapse and produce large currents, except for a group of 2D O-type nulls supported by highly super-Alfvenic plasma flows. The third area of this thesis involves numerically simulating a model of heating by coronal tectonics (Priest et al, 2002). A simple magnetic field is created and the boundary is driven, also in a simple manner. Current sheets which scale with grid resolution are seen to build up on the quasi-separatrix layers, and there is some evidence of magnetic reconnection.Sir Arthur Eddington and the foundations of modern physicsDurham, Ian T.http://hdl.handle.net/10023/129332018-06-14T23:16:37Z2005-01-01T00:00:00ZIn this dissertation I analyze Sir Arthur Eddington's statistical theory as developed in the first six chapters of his posthumously published Fundamental Theory. In particular I look at the mathematical structure, philosophical implications, and relevancy to modern physics. This analysis is the only one of Fundamental Theory that compares it to modern quantum field theory and is the most comprehensive look at his statistical theory in four decades. Several major insights have been made in this analysis including the fact that he was able to derive Pauli's Exclusion Principle in part from Heisenberg's Uncertainty Principle. In addition the most profound general conclusion of this research is that Fundamental Theory is, in fact, an early quantum field theory, something that has never before been suggested. Contrary to the majority of historical reports and some comments by his contemporaries, this analysis shows that Eddington's later work is neither mystical nor was it that far from mainstream when it was published. My research reveals numerous profoundly deep ideas that were ahead of their time when Fundamental Theory was developed, but that have significant applicability at present. As such this analysis presents several important questions to be considered by modern philosophers of science, physicists, mathematicians, and historians. In addition it sheds new light on Eddington as a scientist and mathematician, in part indicating that his marginalization has been largely unwarranted.
2005-01-01T00:00:00ZDurham, Ian T.In this dissertation I analyze Sir Arthur Eddington's statistical theory as developed in the first six chapters of his posthumously published Fundamental Theory. In particular I look at the mathematical structure, philosophical implications, and relevancy to modern physics. This analysis is the only one of Fundamental Theory that compares it to modern quantum field theory and is the most comprehensive look at his statistical theory in four decades. Several major insights have been made in this analysis including the fact that he was able to derive Pauli's Exclusion Principle in part from Heisenberg's Uncertainty Principle. In addition the most profound general conclusion of this research is that Fundamental Theory is, in fact, an early quantum field theory, something that has never before been suggested. Contrary to the majority of historical reports and some comments by his contemporaries, this analysis shows that Eddington's later work is neither mystical nor was it that far from mainstream when it was published. My research reveals numerous profoundly deep ideas that were ahead of their time when Fundamental Theory was developed, but that have significant applicability at present. As such this analysis presents several important questions to be considered by modern philosophers of science, physicists, mathematicians, and historians. In addition it sheds new light on Eddington as a scientist and mathematician, in part indicating that his marginalization has been largely unwarranted.Photospheric observations of surface and body modes in solar magnetic pores