Decay of Ultrarelativistic Remnant Belt Electrons Through Scattering by Plasmaspheric Hiss

Ultrarelativistic electron remnant belts appear frequently following geomagnetic disturbances and are located in-between the inner radiation belt and a reforming outer belt. As remnant belts are relatively stable, here we explore the importance of hiss and electromagnetic ion cyclotron waves in controlling the observed decay rates of remnant belt ultrarelativistic electrons in a statistical way. Using measurements from the Van Allen Probes inside the plasmasphere for 25 remnant belt events that occurred between 2012 and 2017 and that are located in the region 2.9 < L < 4.0, we demonstrate that, in general, the observed decay rates and their scaling with electron energy are in good agreement with theoretical lifetimes of ultrarelativistic electrons calculated for hiss-induced loss based on recent statistics of hiss waves and plasmaspheric density. A slight variation of electron lifetimes with energy is noticed for several peculiar events. We show that this behavior is apparently unrelated to inward radial diffusion. Moreover, we show that simultaneously observed electromagnetic ion cyclotron waves in the hydrogen band, despite their very low time-averaged intensity of about 9 pT, can sufficiently increase electron scattering at low pitch angles as compared with hiss alone to account for the observations. Plain Language Summary Following geomagnetic disturbances, partial depletions of the outer radiation belt can leave behind a stable remnant belt located between the inner belt and a reforming outer belt, resulting in three-belt events. Remnant belts decay in a way such that their lifetimes increase as electron energy increases. Here we statistically analyze the observed decay rates of remnant belt ultrarelativistic electrons to identify the main physical mechanisms responsible for such losses. We use Van Allen Probes measurements during 25 events that occurred between 2012 and 2017. We demonstrate that both the observed loss rates and their dependence on electron energy are in good agreement with theoretical estimates of losses due to electron scattering by plasmaspheric hiss waves. An unusually weak variation of electron loss rates with energy, noticed during some peculiar events, is attributed to simultaneously observed electromagnetic ion cyclotron waves of small time-averaged intensity, showing that such waves can still produce significant effects at low L-shells in spite of their very low occurrence rate and average power.