Recent Results on the Understanding of Earth’s Double Outer Radiation Belt Events and Remnant Belt confinement

Abstract:

One of the first results of the Van Allen Probes mission was the discovery of a completely new configuration of the Earth’s electron radiation belts in which the outer belt, following a partial depletion of fluxes at ultrarelativistic energies (>3 MeV), left a remnant belt at low altitudes. This was subsequently followed by a newly reformed outer belt at high altitude with a gap between them, resulting in a three-belt configuration never seen or predicted before. This configuration, assumed to be relatively unique, has been shown to be a frequent occurrence, generally associated with moderate geomagnetic storms, and its decay process has been found to be consistent with scattering by hiss waves. The formation processes leading to remnant belts are still subject to debate within the community. Here we show that ultrarelativistic remnant belt occurrence and confinement to low L-shells is significantly correlated with the minimum plasmapause and last closed drift shell locations, and that they occur during dropouts associated with geomagnetic storms. Additionally, 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. ULF wave power is often 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. This accounts for the observed occurrence and confinement of remnant belts in most cases, although EMIC wave-driven loss likely contributes in at least some events.