Fast Non-Equilibrium Pitch Angle Diffusion in a Plasmaspheric Plume Associated with BARREL Precipitation

This summary paper reports progress on an ongoing investigation of radiation belt electron pitch-angle scattering by plume hiss waves. We further analyze an electron precipitation event observed by BARREL, first reported in [1]. We find that the bremsstrahlung x-ray spectrum observed by BARREL is not consistent with an exponential precipitating electron energy spectrum, and is harder than the trapped electron spectrum measured by Van Allen Probes. We extend our quasi-linear diffusion model to consider a more realistic initial electron energy spectrum. The evolution of the electron phase space density towards a harder energy spectrum is qualitatively consistent with the BARREL observations and is attributed to the rapid loss of low energy particles at early times during their non-equilibrium interaction with the plume hiss waves.