Low-Frequency Whistler Mode

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Abstract
Resonant growth (or damping) of electromagnetic waves propagating at an angle to the magnetic field in a model plasma is assumed to arise from a two component velocity distribution consisting of a dense, cold background plasma propagation medium, and a diffuse energetic nonthermal ``tail,'' responsible for resonant particle effects. The low‐frequency whistler mode, cB/M+c « ω « eB/M−c, is treated in detail. When the energetic resonant electrons have a sufficiently hard‐energy spectrum, and a pitch angle anisotropy corresponding to more electron energy perpendicular than parallel to the magnetic field, the whistler can be unstable over a significant cone of wave propagation angles to the magnetic field. Detailed computations of this growth rate for various energy and pitch angle distributions are presented as a function of wave normal angle to the field. The unstable cone is small when the high‐energy tail is not well populated. For this reason, whistlers are usually thought to be heavily damped in low β (= 8πp/B2) plasmas. Whistler instability may limit the intensity of trapped high‐energy electrons in the earth's Van Allen belts.