Boundaries and enhancements: ULF wave-driven dynamics of energetic particles in the Van Allen belts

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 radiation belt dynamics overall is which types of plasma waves are responsible for the various changes we observe. Here, we present two studies that illuminate how ULF waves shape the boundaries and enhancements of relativistic electrons. One result, using seven years of Van Allen Probes satellite data, shows that ULF waves can create multi-MeV flux enhancements following geomagnetically active periods. Thus, 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. A second analysis, looking at decades of POES data, shows that relativistic breaches of the lower boundary of the outer belt happen in concert with elevated ULF wave power yet are not associated with particular type of solar driving. How relativistic electrons can cross this barrier and enter into the slot region and inner zone is crucial for understanding the radiation environment in this regime closest to Earth.