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The theoretical foundation of 3-D Alfvén resonances : time-dependent solutions

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Elsden_2017_JGRSP_3_DAlfven_FinalPubVersion.pdf (1.152Mb)
Date
03/2017
Author
Elsden, T.
Wright, A. N.
Keywords
Magnetosphere
ULF waves
Alfvén waves
MHD
Resonance
QA Mathematics
QB Astronomy
QC Physics
NDAS
BDC
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Abstract
We present results from a 3-D numerical simulation which investigates the coupling of fast and Alfvén magnetohydrodynamic (MHD) waves in a nonuniform dipole equilibrium. This represents the time-dependent extension of the normal mode (∝ exp(−iωt)) analysis of Wright and Elsden (2016), and provides a theoretical basis for understanding 3-D Alfvén resonances. Wright and Elsden (2016) show that these are fundamentally different to resonances in 1D and 2D. We demonstrate the temporal behavior of the Alfvén resonance, which is formed within the "Resonant Zone"; a channel of the domain where a family of solutions exists such that the natural Alfvén frequency matches the fast-mode frequency. At early times, phase mixing leads to the production of prominent ridges in the energy density, whose shape is determined by the Alfvén speed profile and the chosen background magnetic field geometry. These off resonant ridges decay in time, leaving only a main 3-D resonant sheet in the steady state. We show that the width of the 3-D resonance in time and in space can be accurately estimated by adapting previous analytical estimates from 1-D theory. We further provide an analytical estimate for the resonance amplitude in 3-D, based upon extending 2-D theory.
Citation
Elsden , T & Wright , A N 2017 , ' The theoretical foundation of 3-D Alfvén resonances : time-dependent solutions ' , Journal of Geophysical Research: Space Physics , vol. 122 , no. 3 , pp. 3247-3261 . https://doi.org/10.1002/2016JA023811
Publication
Journal of Geophysical Research: Space Physics
Status
Peer reviewed
DOI
https://doi.org/10.1002/2016JA023811
ISSN
2169-9402
Type
Journal article
Rights
© 2017 American Geophysical Union. All Rights Reserved. This work is made available online in accordance with the publisher’s policies. This is the final published version of the work, which was originally published at: https://dx.doi.org/10.1002/2016JA023811
Description
Both authors were funded in part by STFC (through Consolidated Grant ST/N000609/1) and The Leverhulme Trust (through Research Grant RPG-2016-071).
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  • University of St Andrews Research
URI
http://hdl.handle.net/10023/11706

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