Abstract

Ultrarelativistic electron remnant belts are frequently observed at low L shells between the inner belt and a re-forming outer belt following geomagnetic disturbances that led to a dropout of electron fluxes at higher radial distances from the Earth. Using wave, particle, and plasma measurements from the Van Allen Probes and Pc3-Pc5 ultra low frequency (ULF) wave data from ground magnetometers from September 2012 to November 2017, we find significant correlations between the upper edge of the remnant belts and the minimum plasmapause and last closed drift shell locations. The maximum 2-hr-averaged radial diffusion rate based on ULF wave power recorded during the dropouts is correlated with the upper edge of the remnant belts and last closed drift shell position. Frequently, ULF wave power is sufficiently strong down to the upper edge of the remnant belts to allow a fast outward radial diffusion of electrons up to the last closed drift shell and to account for the observed confinement of remnant belts to low L shells. The electron phase space density often exhibits the needed negative or oscillating outward gradients in the region of flux loss. Accordingly, fast outward radial diffusion by ULF waves turns out to be a crucial contributor to the depletion of the outer belt that leads to the formation of remnant belts of ultrarelativistic electrons, although we show that multi-MeV electron precipitation through combined scattering by contemporaneous electromagnetic ion cyclotron (EMIC) and lower-band chorus waves probably contributes in a limited number of cases.