Journal Article

Radiation Belt Electron Acceleration Inside the Plasmasphere

Hua, Man, Bortnik, Jacob

2025

Overview

We report a new population of outer belt electron acceleration events ranging from hundreds of keV to ∼1.5 MeV that occurred inside the plasmasphere, which we named “Inside Events” (IEs). Based on 6 year observations from Van Allen Probes, we compare the statistical distributions of IEs with electron acceleration events outside the plasmasphere (OEs). We find that most IEs were observed at L < 4.0 at energies below ∼1.5 MeV, with weaker acceleration ratio (<10) and larger event numbers (peaking value reaching >200), compared to stronger but less frequently occurred (peaking event numbers only reaching ∼80) OEs that were mostly observed at L > 4.0. The evolution of electron phase space density of a typical IE shows signature of inward radial diffusion or transport. Our study provides a feasible mechanism for IE, which is the results of the inward radial transport of the electron acceleration in the outer region of outer belt. Plain Language Summary Since the discovery of the Earth's Van Allen radiation belts in 1958, extensive studies have advanced our understanding of outer belt electron acceleration mechanisms. However, most previous studies focused on the electron acceleration process occurring in the low electron density region, outside the plasmasphere. Although limited previous studies reported the electron flux enhancements penetrated down to very low L‐shells (L = 2.5), involving flux enhancements inside the plasmasphere, these studies did not specify the plasmapause location. In this letter, we report a new population of outer belt electron acceleration events ranging from hundreds of keV to ∼1.5 MeV observed inside the plasmasphere. The “inside electron acceleration events” (IEs) are weaker but occur much more frequently compared to the stronger acceleration events observed outside plasmasphere, and cannot be neglected when investigating radiation belt electron dynamics. The evolution of electron phase space density (PSD) of a typical IE event demonstrates signature of inward radial transport, showing gradual flux enhancements over several days and monotonically increasing radial profile of electron PSD. Our study provides convincing evidence that this observed IE in the low L‐shell region (L = 2.5) was dominantly caused by inward radial transport of electron acceleration in the outer region of the outer belt. Key Points We report a new population of outer belt electron acceleration events from 300 keV to ∼1.5 MeV observed inside the plasmasphere (IE) At >300 keV, the IEs are weaker but occur more frequently compared to the stronger acceleration events observed outside the plasmasphere The evolution of electron phase space density of a typical IE shows monotonically increasing radial profile, suggesting the crucial role of radial transport

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Electron acceleration / Electron density / Electron flux / Electrons / Evolution / Fluctuations / inward radial transport / Plasmapause / Plasmasphere / Radiation / radiation belt electron fluxes / Radiation belts / Space density / statistical distribution / Statistical distributions