The tail flank magnetopause is known to be subject to magnetosheath pressure variations, and may possibly develop a Kelvin-Helmholz instability. We have examined the effect of low-frequency long-wavelength perturbations induced by magnetosheath waves on the flank magnetopause and on the plasma sheet and the plasma sheet boundary layer. We find that a certain fraction of the energy input contained in the magnetosheath waves can cross the magnetopause and can be transported to and locally dissipated in the plasma sheet boundary layer. 

The accompanying figure shows the propagation of an incident magnetosheath wave (the magnetosheath is on the right of the figure) across the magnetopause (near x = 20 R_E away from the center of the tail, through the lobe, and across the plasmasheet boundary up to the central plasma sheet (at x = 0 R_E).Part of this wave is reflected at the magnetopause. Part of the wave is resonantly absorbed at the magnetopause, as shown by the singularity in the wave amplitude (left panel) and the jump in the energy flux profile (middle panel).

Although the wave is evanescent in the tail lobe, it still has a non-zero amplitude by the time it reaches the plasmasheet boundary layer, where resonant absorption can occur again (peak in wave amplitude, jump in energy flux). Resonance occurs in this example when the field-aligned wave vector matches the local Alfven wave vector (frequency divided by the Alfven velocity), as shown in the right panel.

At these sites of resonance, wave energy is converted from the imposed compressional waves to Alfven waves (as in this example) or to slow mode waves. The jumps in the energy flux indicate how much of the energy carried by the wave is dissipated in the resonant layer.

Magnet ohydrodynamic wave mode conversion in the Earth's magnetotail
J. De Keyser 
J. Geophys. Res., 1999. Accepted.

We study magnetohydrodynamic wave mode conversion as a mechanism contributing to the observed low frequency electromagnetic fluctuation level in the plasma sheet.  Low frequency waves originating in the magnetosheath can transport energy across the tail flank magnetopause and through the tail lobes toward the
plasma sheet. In the plasma sheet boundary layer and in the central plasma sheet local conditions permit mode conversion to occur in resonant sheets. Resonant coupling to both Alfv\'en and slow mode waves is possible.  Energy is fed to the plasma in these resonant sheets, which may help to explain the acceleration and heating of particles observed in the plasma sheet boundary layer and the central plasma sheet.
 
 

Author: J. De Keyser   Curator: J. De Keyser  Johan.DeKeyser@oma.be