On the Confinement of Ultrarelativistic Electron Remnant Belts to Low L Shells

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. Plain Language Summary The Earth's Van Allen radiation belts are a very dynamic region in the near-Earth magnetosphere where extremely energetic electrons can be trapped, transported, energized, and removed over periods of minutes to weeks. There are generally two belts filled with significant electron flux and a gap in between them. However, after certain geomagnetic disturbances, a three-belt configuration of electron flux appears following a partial removal of the outer belt, followed by a partial re-population, leaving a gap in between the remnant belt and new outer belt. In this work, we study possible mechanisms that determine the extent of the partial removal of the outer belt for a significant number of events that occurred between September 2012 and November 2017. We found that the main mechanism removing the outer belt is likely fast transport of particles to regions far from Earth, followed by loss of such electrons to the interplanetary medium, although sometimes precipitation of particles into the Earth's atmosphere can also contribute.