Finite-beta effects on quasi-linear diffusion coefficients

Abstract

[1] One of the most important theoretical tools for modeling the sources and losses of Earth's radiation belt electrons is that based on quasi-linear theory. However, one of the key assumptions made in such efforts is that the waves are described by cold plasma theory. In this paper, quasi-linear diffusion coefficients for hot/warm (or finite-beta) plasmas are formulated and computed on the basis of a recently developed empirical theory in which exact numerical hot/warm plasma dispersion relations are modeled by analytical functions. It is found that finite-beta effects associated with whistler mode waves lead to a reduction of the diffusion coefficients for relativistic electrons near 90° pitch angles, which tends to reduce the efficiency of electron acceleration. Finite-beta effects associated with electron scattering by electromagnetic ion cyclotron (EMIC) waves broaden the range of wave-particle interaction in pitch angle space, and hence increase the efficiency of pitch angle diffusion. The rate of pitch angle diffusion of protons by EMIC waves is also generally enhanced by finite-beta effects.