Modelling the acceleration of radiation belt electrons to ultra-relativistic energies during a geomagnetic storm using VERB-4D

The most energetic electrons exceeding energies of 7 MeV are observed in the Earth’s outer radiation belt. In the past, numerical models of the radiation belts fell short to reproduce the acceleration to these ultra-relativistic energies, while the main acceleration process remains a debated topic.In this work, we use the VERB-4D (Versatile Electron Radiation Belt) model to examine a geomagnetic storm that occurred on April 20th, 2017 to investigate the acceleration of electrons to ultra-relativistic energies. Using the observations from NASA’s Van Allen Probes spacecraft and quasi-linear plasma theory, we show that such acceleration is achievable only under extremely low plasma density conditions. The full 3-D simulation with a statistical model of plasma density fails to reproduce the acceleration to such high energies, whereas the simulation with plasma density variations taken from observations successfully reproduces the observed energization to ultra-relativistic energies at all radial location.This study demonstrates the intricate interplay between the cold plasma and the acceleration of electrons to ultra-relativistic energies, and showcases our improved understanding of the high energy particle population.