The Characteristics of Three-Belt Structure of Sub-MeV Electrons in the Radiation Belts

After the launch of Van Allen Probes, the three-belt structures of ultra-relativistic electrons are discovered. In this study, we investigate the three-belt structures of sub-MeV electrons, which may form under different mechanism compared with those of ultra-relativistic electrons and are worth in-depth analysis. Based on the differential flux data from Magnetic Electron Ion Spectrometer onboard Radiation Belt Storm Probes-B satellite, we find 54 events, in which two comparable peaks of sub-MeV electron fluxes and a slot appear where there should be the outer radiation belt. Through the statistical analysis, the three-belt structures of sub-MeV electrons are found to be closely related to symmetric H-component (SYM-H) and Auroral Electrojet (AE) indices. The 2-day SYM-H minimum and AE maximum before the event have a linear trend with the remnant belt and the "second slot" locations. The L-values of the remnant belt and the "second slot" of different energy electrons decrease as energy increases in general and show interesting characteristics during their temporal evolution. Moreover, the lifetime of the remnant belt of different energy electrons increases as energy increases. We find similarities and differences between sub-MeV and ultra-relativistic electrons three-belt events, which provides a new perspective in three-belt structure study. Plain Language Summary An important result of the Van Allen Probes mission was that the ultra-relativistic electron (>2MeV) radiation belts, containing an inner belt, a slot region and an outer belt, often show a three-belt structure. That is, two belts and a slot region appear where there should be the outer radiation belt. The dominate mechanism to form the structure is still under debate. Recently, such structure for sub-MeV electrons was also reported in a case study. Since the physical properties of these two types of electrons are different, the characteristics and mechanism to form the structure may be different. In this work, we show that common occurrence and general characteristics of sub-MeV electron three-belt events. The location, formation, and temporal evolution of the structure are influenced by electron energy and magnetospheric conditions reflected by geomagnetic indices. We have also determined the relatively short lifetime of the intermediate belt (remnant belt) and explained why threebelt events for sub-MeV electrons are less visible.