Evolution of energetic electron pitch angle distributions during storm time electron acceleration to megaelectronvolt energies

Abstract

We investigate the pitch angle distributions of 0.15–1.58 MeV electrons observed during the 9–15 October 1990 storm measured by the Combined Release and Radiation Effects Satellite (CRRES) spacecraft. This storm period is characterized by an enhancement in the electron flux at L ≈ 4 by more than an order of magnitude over the prestorm level. The overall change in flux at L ≈ 6.6 is small in comparison. Previous work shows that radial diffusion underestimates the flux enhancement by up to a factor of 5 for L ≤ 4.5 [Brautigam and Albert, 2000], indicating the need for an additional acceleration process. The pitch angle distributions presented here are examined for evidence of the acceleration mechanism. The distributions at L ≈ 2 are rounded and are dominated by Coulomb collisions. They show little variation during the storm. The distributions at L ≈ 3 are pancake‐shaped before the storm, characteristic of pitch angle scattering by plasmaspheric hiss. During the main phase, they become broad and flat, and they evolve back into pancake distributions during the recovery phase. At L ≈ 4–6, the pitch angle distributions are characterized as butterfly distributions at storm onset, and they become broad flat top distributions during the recovery phase. The flat top distributions persist throughout the ∼3‐day recovery phase and are observed in the region of highest flux enhancement. The flat top distributions are energy dependent and are broader at lower energies (30°–150°) than at higher energies (50°–130°). The higher energies exhibit a much faster fall off toward the loss cone than at lower energies. Inward radial diffusion should result in anisotropic distributions peaked near 90° and does not explain the observed energy dependence. Furthermore, the direction of diffusion is outward at higher energies. Model calculations of the pitch angles resonant with whistler mode waves show that flat top distributions are consistent with pitch angle and energy scattering in regions where fpe/fce ∼ 1. Although radial diffusion may be very important for particle energization, the observed pitch angle distributions provide strong evidence that wave particle interactions play an important role in the energization process.