WHISTLER ABSORPTION AND ELECTRON HEATING NEAR THE PLASMAPAUSE

Using the HOTRAY code, we demonstrate that lightning-generated whistle rs which enter the magnetosphere over a broad range of latitudes (Delt a lambda approximate to 5 degrees) just inside the plasmapause are str ongly focused by the steep plasma density gradient into a narrow range of L shells near the equatorial region. The wave normal angle also re mains closely aligned (+/-20 degrees) with the magnetic field directio n along the entire ray path. Under such conditions, Landau resonance i s relatively unimportant, and the wave amplitude is controlled by cycl otron resonant interactions with energetic electrons. All waves with f requencies comparable to or larger than one third of the equatorial el ectron gyrofrequency can be strongly absorbed by resonant electrons, l eading to electron heating perpendicular to the ambient magnetic field at energies above 100 eV. Consequently, in the presence of this stron gly focused source of wave energy, the electron distribution should ev olve toward a marginally stable anisotropic equilibrium distribution w ith T-perpendicular to > T-parallel to. In order to simulate this perp endicular heating, we allow the anisotropy of the electron distributio n to evolve so that damping is minimized at a frequency of 5 kHz, corr esponding to the peak in the power spectrum of spherics above the iono sphere. When the plasmapause is located at L(p) = 4.5, whistlers above 4 kHz experience more than 20 dB attenuation owing mainly to cyclotro n resonance with 0.1 to 1 keV electrons near the equator. It is unlike ly that these waves would be detectable on the ground. This attenuatio n will produce an upper cutoff in the whistler frequency considerably below one half the equatorial electron gyrofrequency for waves that ar e guided along the plasmapause. In contrast, lower-frequency whistlers (f approximate to 1-3 kHz) should be amplified by the anisotropic ele ctron population; such waves are able to propagate to the conjugate io nosphere and thus be detected on the ground. This energy transfer betw een whistlers and cyclotron resonant electrons is relatively unimporta nt when L(p) less than or equal to 3.0, but it should become significa nt for L(p) greater than or equal to 4.5.