Synthetic observational signatures of coronal heating mechanisms from 3D numerical simulations

Abstract

In this thesis, we investigate the synthetic observables from 3D MHD simulations which explore coronal heating mechanisms. These models include the twisting of magnetic flux tubes, the propagation of transverse oscillations through complex braided magnetic fields and a coronal arcade driven by footpoint motions of different characteristic time scales.

Through the use of forward modelling, the numerical model results are transformed into synthetic emission data. Examining such data will teach us more about, and help us identify, the observable features caused by the dynamics and heating of the coronal plasma. It will also help us build a catalogue of characteristics of energy release in the solar corona. This brings numerical models and observations closer together by allowing us to compare models and observations in a meaningful way.

Most of the observables examined within this thesis are as one would expect with knowledge of the plasma parameters (i.e. the density, temperature and velocity field) but some of this information is not readily available from observations. We expected and observed line broadening in regions of fast outflows as a result of magnetic reconnection, high frequency signals when shorter time scale motions are present at the footpoints of a coronal arcade, and the impact the line-of-sight has on estimated kinetic energies. There are also signatures which are not necessarily obvious until the forward modelling is complete but nonetheless are intuitive after the fact. For example, waves helping identify regions of complex magnetic fields; coronal arcade structures are visible in Doppler velocity signatures as well as intensity images, and signatures of Alfvén and fast waves are present within coronal arcades. There is one feature we encounter which could be misinterpreted. During the model which examines transverse oscillations through a braided magnetic field, Doppler signatures are generated which look like those due to torsional motions. In fact, with prior knowledge of the simulation, these are the result of phase mixing and counter-propagating waves through a complex magnetic field.

Finally we round up by examining if there is a relation (more specifically ratio) between wave amplitudes and non-thermal line widths. In order to estimate wave energies, previous studies have used such a relation where the non-thermal line widths are a factor of the square root of 2 smaller than the root mean squared wave amplitudes; however different factors have been used in other studies. We focus on determining whether one true value does exist by examining the simulations already presented in this thesis.