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Theory of dispersive shear Alfvén wave focusing in Earth's magnetosphere
Author(s) -
Rankin R.,
Marchand R.,
Lu J. Y.,
Kabin K.,
Tikhonchuk V. T.
Publication year - 2005
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2004gl021831
Subject(s) - physics , alfvén wave , gyroradius , magnetosphere , field line , amplitude , computational physics , earth's magnetic field , geophysics , wavenumber , shear (geology) , ionosphere , magnetohydrodynamics , magnetic field , optics , geology , quantum mechanics , petrology
It is shown that perpendicular gradients in shear Alfvén wave (SAW) dispersion regulate the localization of wave power on nightside geomagnetic L ‐shells where narrow Field Line Resonances (FLRs) form. We estimate the timescale for this process, ω 0 t c = 1/, and demonstrate that it is analogous to optical wave focusing. Here, β is the gradient in the global wave dispersion parameter across L ‐shells in the equatorial plane, and α is the gradient in the SAW eigenfrequency. It is demonstrated that dispersive SAWs with wave numbers and frequencies in a certain range, are subject to magnetospheric focusing onto L ‐shells where they reach large amplitude and are expected to dissipate. Our theory addresses a class of arc scales that are comparable to the electron inertial length near the ionosphere, or the ion gyroradius near the equatorial plane. We further demonstrate that when the gradient in the SAW frequency reverses across the edges of auroral density cavities, it naturally traps dispersive SAWs and focuses them down to the inertial scale.