The Role of the Dynamic Plasmapause in Outer Radiation Belt Electron Flux Enhancement

Abstract

The plasmasphere is a highly dynamic toroidal region of cold, dense plasma around Earth.Plasma waves exist both inside and outside this region and can contribute to the loss and acceleration of high energy outer radiation belt electrons. Early observational studies found an apparent correlation on long time scales between the observed inner edge of the outer radiation belt and the modeled innermost plasmapause location. More recent work using high-resolution Van Allen Probes data has found a more complex relationship. For this study, we determine the standoff distance of the location of maximum electron flux of the outer belt MeV electrons from the plasmapause following rapid enhancement events. We find that the location of the outer radiation belt based on maximum electron flux is consistently outside the plasmapause, with a peak radial standoff distance of ΔL~1. We discuss the implications this result has for acceleration mechanisms. Plain Language SummaryWe present a statistical study of the location of particles, particularly electrons, in currents around the Earth whose intensity is increased, or enhanced, rapidly (within 1 day). We use over 5 years of particle intensity and plasma density data from the Van Allen Probes satellites. We find that these quick enhancement events occur when the particles are outside the edge of the plasmasphere, an ever-changing, often asymmetric donut-shaped region of plasma around the Earth. Specifically, we find that the locations of particles are correlated with the location of plasma waves outside the plasmasphere, which supports models of electron acceleration from interactions with these waves over short periods of time. Understanding these wave-particle interactions will improve our understanding of how the plasmasphere under differing conditions can either shield Earth from or worsen the impacts of geomagnetic activity and space weather.

The Effects of Geomagnetic Activities on Acceleration Regions of Radiation Belt Electrons

“…For the storm of 1 June 2013, the variations of the convection electric field could be responsible for the evolution of the plasmapause and enhancements of lower energy electrons (μ = 209 and 302 MeV/G) at different L shells. Previous studies have found a good correlation between enhancements of radiation belt electrons and the plasmapause location (e.g., Bruff et al, 2020;Goldstein et al, 2016;Khoo et al, 2019;X. Li et al, 2006).…”

“…For the storm of 1 June 2013, the variations of the convection electric field could be responsible for the evolution of the plasmapause and enhancements of lower energy electrons ( μ = 209 and 302 MeV/G) at different L shells. Previous studies have found a good correlation between enhancements of radiation belt electrons and the plasmapause location (e.g., Bruff et al., 2020; Goldstein et al., 2016; Khoo et al., 2019; X. Li et al., 2006). During the strong storm recovery phase, the plasmapause was highly dynamic, which may be an important effect on the formation of the two acceleration regions of relativistic electrons in the outer radiation belt.…”

Differentiating Between the Leading Processes for Electron Radiation Belt Acceleration

“…When electron flux enhancements occur during geomagnetic storms, the location of the peak in MeV electron flux enhancements during recovery phase is strongly correlated with the magnitude of the storm, as quantified by the Dst index (Tverskaya et al, 2003) or, equivalently, with the plasmapause location (O'Brien et al, 2003;Moya et al, 2017;Bruff et al, 2020). The equatorial pitch angle distribution of MeV electron flux enhancements at the center of the outer belt is most anisotropic (i.e., 90 °peaked) within a day of the start of the recovery phase, and the degree of anisotropy increases with energy (e.g., Ozeke et al, 2022).…”