Standing Alfven waves excited in the dayside magnetosphere by extra-magnetospheric sources

Abstract:

It is shown that monochromatic fast magnetosonic (FMS) waves penetrating from the solar wind into the magnetosphere excite standing Alfven waves on multiple resonant shells simultaneously. The fundamental mode of standing Alfvén waves is excited at the resonance shell nearest to the Earth. Higher harmonics of the standing waves are excited on outer magnetic shells. FMS wave field reaches the ionosphere with a very small amplitude, in contrast to the resonant Alfvén waves they excite. Oscillations related to the resonant Alfven waves are observed on the Earth's surface. If the FMS wave source is broadband in the solar wind, but has a rather narrow spectrum, the mean amplitude distribution of the resonant Alfven waves has two local maxima in the magnetosphere (and on the Earth): in the middle and high latitudes, as observed in the daytime Pc3 geomagnetic pulsations.

Another feature of the resonant Alfven waves is related to their transverse dispersion which determines their structure and propagation across magnetic shells. Transition from one type of kinetic Alfven wave dispersion to another takes place in the plasmopause region. A "hot" dispersion associated with a finite Larmor radius of ions occurs in the outer magnetosphere, and a "cold" dispersion determined by the electron skin-length takes place in the plasmasphere. The dispersion length becomes complex in the plasmapause transition layer, where standing Alfven waves are completely absorbed by the plasma thermoelectrons. This process leads to epithermal electron precipitations in the ionosphere resulting in stable auroral red (SAR)  arcs.