The drivers of radial diffusion-dominated radiation belt acceleration events

Abstract:

High-energy electron populations within the Van Allen radiation belts are highly dynamic, and seen to increase and decrease on timescales as short as hours. One of the lingering questions about the observed flux increases concerns which type of plasma wave interaction is the dominant cause for these enhancements. The two sparring candidates are waves that occur at two vastly different frequency bands: VLF and ULF. Studies of enhancement events are often complicated by the fact that both wave modes typically occur simultaneously as a result of heightened geomagnetic activity. In this study, we analyze seven years of Van Allen Probes satellite electron observations in the multi-MeV energy range to find cases with only ULF waves acting on the population, isolated from any VLF-driven energy diffusion. We examine the phase space density profiles of these events and the solar driving conditions that lead to this kind of isolated wave activity. Our results show that ULF-driven radial diffusion can often be the dominant mechanism behind ultrarelativistic electron enhancements; although high populations of lower energies, likely produced by VLF interactions, are a necessary precondition.