Alfvén Wave Generation by a Compact Source Moving on the Magnetopause: Asymptotic Solution

Abstract

AbstractThe spatiotemporal structure of Alfvén waves excited by a moving pressure pulse on the magnetopause is analytically explored. These waves are supposed to be responsible for field‐aligned currents generating traveling convection vortices in the ionosphere. It is found that a moving source generates two wave modes, a primary and a secondary mode, having different azimuthal wave vectors and frequencies. Both modes represent surface waves with amplitudes exponentially decreasing from the magnetopause. At a given azimuthal location, they also decrease with time as the source passes away. For the primary mode, the wave frequency equals the Alfvén resonant frequency on the surface of the source, while for the secondary mode the frequency equals the local resonant frequency. The dependence of the frequency of the secondary mode on the radial coordinate results in phase mixing, which leads to a change of the wave polarization from mixed into toroidal polarization. For both primary and secondary modes, the azimuthal component of the wave vector equals the corresponding wave frequency divided by the speed of the source. Superposition of the primary and secondary modes produces plasma vortices behind the source.