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The Broadband Excitation of 3-D Alfven Resonances in a MHD Waveguide

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journal contribution
posted on 08.04.2020, 11:20 by T Elsden, AN Wright
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.

Funding

Funding Information Leverhulme Trust. Grant Number: RPG-2016-071 STFC. Grant Number: ST/N000609/1

History

Citation

Elsden, T., & Wright, A. N. ( 2018). The broadband excitation of 3‐D Alfvén resonances in a MHD waveguide. Journal of Geophysical Research: Space Physics, 123, 530– 547. https://doi.org/10.1002/2017JA025018

Version

VoR (Version of Record)

Published in

JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS

Volume

123

Issue

1

Pagination

530 - 547 (18)

Publisher

AMER GEOPHYSICAL UNION

issn

2169-9380

eissn

2169-9402

Acceptance date

02/01/2018

Copyright date

2018

Available date

16/01/2018

Publisher version

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JA025018

Language

English