Acceleration of electrons to ultra-relativistic energies in the Earth's radiation belts.

In this study we focus on the acceleration of the most energetic part of the radiation belt population so-called ultra-relativistic electrons. We perform simulations with coupled cold plasma and radiation belt codes. Our simulations show that the acceleration to such high energies occurs only when cold plasma density is extremely depleted. Our coupled simulations demonstrate that when realistic density variability is included, we can accurately reconstruct the dynamics of the radiation belts. We also perform statistical analysis of all storms during the Van Allen Probes era that show acceleration to 7.7 MeV and have observations on the dawn side. These observations show that the presence of 2 MeV seed population and the presence of prolonged deplitions in density are required for acceleration to 7.7MeV.This study also reveals the intricate interplay between cold plasma and the enhancements of ultra-relativistic electrons that are millions of times more energetic than plasma particles.Similar acceleration may occur in planetary radiation belts, for lab plasmas, at exoplanets, and in other magnetized astrophysical objects.