Relativistic theory of wave‐particle resonant diffusion with application to electron acceleration in the magnetosphere

TLDR

The study constructs fully relativistic resonant diffusion curves for electron cyclotron resonance with R‑mode and EMIC waves and proposes a model combining EMIC pitch‑angle scattering and whistler‑mode energy diffusion to explain relativistic electron variations during geomagnetic storms. Analytical solutions for a single‑ion plasma and a numerical scheme for multi‑ion plasmas are used to generate diffusion curves for magnetospheric parameters inside and outside the plasmapause. Whistler‑mode waves can accelerate electrons from ~100 keV to >1 MeV outside the plasmapause during storm recovery, while EMIC waves produce minimal energy change, are ineffective for stochastic acceleration, but can scatter electrons above 1 MeV near the duskside plasmapause.

Abstract

Resonant diffusion curves for electron cyclotron resonance with field‐aligned electromagnetic R mode and L mode electromagnetic ion cyclotron (EMIC) waves are constructed using a fully relativistic treatment. Analytical solutions are derived for the case of a single‐ion plasma, and a numerical scheme is developed for the more realistic case of a multi‐ion plasma. Diffusion curves are presented, for plasma parameters representative of the Earth's magnetosphere at locations both inside and outside the plasmapause. The results obtained indicate minimal electron energy change along the diffusion curves for resonant interaction with L mode waves. Intense storm time EMIC waves are therefore ineffective for electron stochastic acceleration, although these waves could induce rapid pitch angle scattering for ≳ 1 MeV electrons near the duskside plasmapause. In contrast, significant energy change can occur along the diffusion curves for interaction between resonant electrons and whistler ( R mode) waves. The energy change is most pronounced in regions of low plasma density. This suggests that whistler mode waves could provide a viable mechanism for electron acceleration from energies near 100 keV to above 1 MeV in the region outside the plasmapause during the recovery phase of geomagnetic storms. A model is proposed to account for the observed variations in the flux and pitch angle distribution of relativistic electrons during geomagnetic storms by combining pitch angle scattering by intense EMIC waves and energy diffusion during cyclotron resonant interaction with whistler mode chorus outside the plasmasphere.