Nonresonant scattering of relativistic electrons by electromagnetic ion cyclotron waves in Earth's radiation belts

TL;DR

This paper introduces a generalized diffusion model that explains how short electromagnetic ion cyclotron wave packets can cause nonresonant scattering of relativistic electrons in Earth's radiation belts, accounting for observed precipitation at energies below resonance.

Contribution

It extends the quasi-linear diffusion model to include nonresonant effects, explaining simultaneous electron precipitation across a broader energy range.

Findings

Model shows exponential decay of scattering rates below resonant energies.

Explains observed nonresonant electron precipitation in the hundreds of keV range.

Accounts for effects of short wave packets on electron scattering.

Abstract

Electromagnetic ion cyclotron waves are expected to pitch-angle scatter and cause atmospheric precipitation of relativistic ($> 1$ MeV) electrons under typical conditions in Earth's radiation belts. However, it has been a longstanding mystery how relativistic electrons in the hundreds of keV range (but $<1$ MeV), which are not resonant with these waves, precipitate simultaneously with those $>1$ MeV. We demonstrate that, when the wave packets are short, nonresonant interactions enable such scattering of $100$s of keV electrons by introducing a spread in wavenumber space. We generalize the quasi-linear diffusion model to include nonresonant effects. The resultant model exhibits an exponential decay of the scattering rates extending below the minimum resonant energy depending on the shortness of the wave packets. This generalized model naturally explains observed nonresonant electron precipitation in the hundreds of keV concurrent with $>1$ MeV precipitation.