Whistler and Electron Firehose Instability Control of Electron Distributions in and Around Dipolarizing Flux Bundles

Abstract

Electromagnetic fluctuations associated with various plasma instabilities have been previously identified near dipolarization fronts in Earth's magnetotail. However, the potential effect of these fluctuations on particle distributions and energy conversion is poorly understood. The most important instabilities responsible for electron acceleration and scattering are the whistler instability and the electron firehose instability. Utilizing 10 years of Time History of Events and Macroscale Interactions during Substorms satellite observations, we explore the occurrence probability and intensity of these instabilities near dipolarization fronts, as function of electron anisotropy and plasma beta. Electron temperature anisotropies are well constrained by the marginal stability conditions of the whistler and oblique electron firehose instabilities. In fact, the observed enhancement of magnetic field fluctuations near the instability thresholds provides good evidence for the operation of these instabilities on electrons near fronts. Since the build-up of electron anisotropy is limited by wave-particle interactions, we conclude that such interactions are important enough to affect electron dynamics and energetics.