Whistler mode wave growth and propagation in the prenoon magnetosphere

Abstract

Pitch angle scattering of electrons can limit the stably trapped particle flux in the magnetosphere and precipitate energetic electrons into the ionosphere. Whistler mode waves generated by a temperature anisotropy can mediate this pitch angle scattering over a wide range of radial distances and latitudes, but in order to correctly predict the phase space diffusion, it is important to characterize the whistler mode wave distributions that result from the instability. We use previously published observations of number density, pitch angle anisotropy, and phase space density to model the plasma in the quiet prenoon magnetosphere (defined as periods when AE < 100 nT). We investigate the global propagation and growth of whistler mode waves by studying millions of growing raypaths and demonstrate that the wave distribution at any one location is a superposition of many waves at different points along their trajectories and with different histories. We show that for observed electron plasma properties, very few raypaths undergo magnetospheric reflection; most rays grow and decay within 30 degrees of the magnetic equator. The frequency range of the wave distribution at large L can be adequately described by the solutions of the local dispersion relation, but the range of wave normal angle is different. The wave distribution is asymmetric with respect to the wave normal angle. The numerical results suggest that it is important to determine the variation of magnetospheric parameters as a function of latitude, as well as local time and L‐shell.