Decay of Ultrarelativistic Remnant Belt Electrons Through Scattering by Plasmaspheric Hiss

Abstract

AbstractUltrarelativistic 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.