Early-Time Non-Equilibrium Pitch Angle Diffusion of Electrons by Whistler-Mode Hiss in a Plasmaspheric Plume Associated with BARREL Precipitation

R M Millan,J.-F Ripoll,O Santolík,W S Kurth

doi:10.3389/fspas.2021.776992

Abstract

In August 2015, the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) observed precipitation of energetic (<200 keV) electrons magnetically conjugate to a region of dense cold plasma as measured by the twin Van Allen Probes spacecraft. The two spacecraft passed through the high density region during multiple orbits, showing that the structure was spatial and relatively stable over many hours. The region, identified as a plasmaspheric plume, was filled with intense hiss-like plasma waves. We use a quasi-linear diffusion model to investigate plume whistler-mode hiss waves as the cause of precipitation observed by BARREL. The model input parameters are based on the observed wave, plasma and energetic particle properties obtained from Van Allen Probes. Diffusion coefficients are found to be largest in the same energy range as the precipitation observed by BARREL, indicating that the plume hiss waves were responsible for the precipitation. The event-driven pitch angle diffusion simulation is also used to investigate the evolution of the electron phase space density (PSD) for different energies and assumed initial pitch angle distributions. The results show a complex temporal evolution of the phase space density, with periods of both growth and loss. The earliest dynamics, within the ∼5 first minutes, can be controlled by a growth of the PSD near the loss cone (by a factor up to ∼2, depending on the conditions, pitch angle, and energy), favored by the absence of a gradient at the loss cone and by the gradients of the initial pitch angle distribution. Global loss by 1-3 orders of magnitude (depending on the energy) occurs within the first ∼100 min of wave-particle interaction. The prevalence of plasmaspheric plumes and detached plasma regions suggests whistler-mode hiss waves could be an important driver of electron loss even at high L-value (L ∼6), outside of the main plasmasphere.

Highlights

On August 10, 2015, BARREL balloon payload 3 A observed bremsstrahlung x-rays (∼5–150 keV) attributed to precipitation of energetic electrons with energies < 200 keV
About 40 min later, at 1525 UT, RBSP-A encounters a similar localized increase in density. Both spacecraft encountered the high density region near apogee (L 5.9) and in the same magnetic local time range (∼16–17 MLT), indicating that the density increase was a stable spatial structure which we identify as a plasmaspheric plume or detached plasma region
A quasi-linear pitch angle diffusion model was developed using wave and plasma parameters observed by RBSP as they passed through a stable region of dense cold plasma (n ∼80 cm−3) near L

Summary

Introduction

On August 10, 2015, BARREL balloon payload 3 A observed bremsstrahlung x-rays (∼5–150 keV) attributed to precipitation of energetic electrons with energies < 200 keV. Strong whistler-mode hiss waves were observed by both spacecraft in the high density region. Plasmaspheric hiss is a major driver of radiation belt loss inside the plasmasphere, contributing to the creation of the slot region between the inner and outer radiation belts (Lyons and Thorne, 1973). These low-frequency (∼50 Hz-2 kHz) electromagnetic whistler-mode waves are broadband and incoherent. Their power is proportional to the density, with more power in dense regions (Malaspina et al, 2016; Malaspina et al, 2018). More information about hiss waves and the dynamics of the radiation belts can be found in the review of Ripoll et al (2020a)

Methods

Results

Conclusion