The Broadband Excitation of 3‐D Alfvén Resonances in a MHD Waveguide. Issue 1 (27th January 2018)

• Record Type: Journal Article
• Authors: Elsden, T.; Wright, A. N.
• Is Part Of: Journal of geophysical research. Volume 123:Issue 1(2018)
• Page Start: 530
• Page End: 547
• Publication Date: 2018-01-27
• Subjects: field line resonance -- MHD waves -- Alfven resonance -- mode coupling -- Earth's magnetosphere; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2017JA025018
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: 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. Key Points: Given a broadband stimulus, it is the natural fast waveguide modes which drive Alfvén resonances Resonances of significant amplitude can form at extremely inclined angles (intermediate polarizations) Resonance location and structure is fairly insensitive to the form of the driver