Whistler absorption and electron heating near the plasmapause

Using the HOTRAY code, we demonstrate that lightning-generated whistlers which enter the magnetosphere over a broad range of latitudes (Δλ ≈ 5°) just inside the plasmapause are strongly focused by the steep plasma density gradient into a narrow range of L shells near the equatorial region. The wave normal angle also remains closely aligned (±20°) with the magnetic field direction along the entire ray path. Under such conditions, Landau resonance is relatively unimportant, and the wave amplitude is controlled by cyclotron resonant interactions with energetic electrons. All waves with frequencies comparable to or larger than one third of the equatorial electron gyrofrequency can be strongly absorbed by resonant electrons, leading to electron heating perpendicular to the ambient magnetic field at energies above 100 eV. Consequently, in the presence of this strongly focused source of wave energy, the electron distribution should evolve toward a marginally stable anisotropic equilibrium distribution with T⊥>T‖. In order to simulate this perpendicular heating, we allow the anisotropy of the electron distribution to evolve so that damping is minimized at a frequency of 5 kHz, corresponding to the peak in the power spectrum of spherics above the ionosphere. When the plasmapause is located at Lp = 4.5, whistlers above 4 kHz experience more than 20 dB attenuation owing mainly to cyclotron resonance with 0.1 to 1 keV electrons near the equator. It is unlikely that these waves would be detectable on the ground. This attenuation will produce an upper cutoff in the whistler frequency considerably below one half the equatorial electron gyrofrequency for waves that are guided along the plasmapause. In contrast, lower-frequency whistlers (ƒ ≈ 1–3 kHz) should be amplified by the anisotropic electron population; such waves are able to propagate to the conjugate ionosphere and thus be detected on the ground. This energy transfer between whistlers and cyclotron resonant electrons is relatively unimportant when Lp ≤ 3.0, but it should become significant for Lp ≥ 4.5.

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