Electron pitch‐angle diffusion in radiation belts: The effects of whistler wave oblique propagation

Abstract

We calculated the electron pitch‐angle diffusion coefficients in the outer radiation belt forL‐shell ∼4.5 taking into account the effects of oblique whistler wave propagation. The dependence of the distribution of the angleθbetween the whistler wave vector and the background magnetic field on magnetic latitude is modeled after statistical results of Cluster wave angle observations. According to in‐situ observations, the mean value and the variance of theθdistribution rapidly increase with magnetic latitude. We found that inclusion of oblique whistler wave propagation led to a significant increase in pitch‐angle diffusion rates over those calculated under the assumption of parallel whistler wave propagation. The effect was pronounced for electrons with small equatorial pitch‐angles close to the loss cone and could result in as much as an order of magnitude decrease of the electron lifetimes. We show that the intensification of pitch‐angle diffusion can be explained by the contribution of higher order cyclotron resonances. By comparing the results of calculations obtained from two models of electron density distribution along field lines, we show that the effect of the intensification of pitch‐angle diffusion is stronger when electron density does not vary along field lines. The intensification of pitch‐angle diffusion and corresponding decrease of energetic electron lifetime result in significant modification of the rate of electron losses and should have an impact on formation and dynamics of the outer radiation belt.