Favored regions for chorus-driven electron acceleration to relativistic energies in the Earth's outer radiation belt

Abstract

Pitch angle and energy diffusion rates for scattering by whistler-mode chorus waves are proportional to the wave magnetic field intensity and are strongly dependent on the frequency distribution of the waves and to the ratio between the electron plasma frequency (fpe) and the electron gyrofrequency (fce). Relativistic electrons interact most readily with lower-band chorus (0.1 < 0.5) and energy diffusion leading to local acceleration to relativistic energies is most effective in regions of low fpe/fce. We perform statistical studies of CRRES data and show that, outside of the plasmapause, both fpe/fce and lower-band chorus activity are dependent on magnetic activity with regions of low fpe/fce and enhanced lower-band chorus activity occurring over a wide range of geospace during active conditions (AE > 300 nT). Enhanced waves in these regions could play a major role in electron acceleration to relativistic energies during periods of prolonged substorm activity.