A Sub‐relativistic Electron Three‐Belt Event in the Earth's Radiation Belts: Observation and Explanation

The Van Allen Probes mission contributed to the discovery of the relativistic (∼500 keV–2 MeV) and ultra‐relativistic (∼>2 MeV) electron three‐belt structure in Earth's radiation belts. This structure results from the partial depletion of the preexisting outer belt and the replenishment of a new outer belt. Ultra‐low frequency and very‐low frequency waves are believed to play important roles in these processes, and substorm injections are usually not responsible for the formation of the external outer belt. In this study, based on observations from the Arase and NOAA‐18 (National Oceanic and Atmospheric Administration‐18) satellites, we report an electron three‐belt event in the energy range of ∼100–200 keV (i.e., sub‐relativistic electrons). According to the evolution of the phase space density and the dawn‐dusk asymmetric flux enhancement during this event, we conclude that the depletion of the upper part of the original outer belt was due to outward radial diffusion accompanied by magnetopause shadowing effect and convection, and the formation of the external outer belt was due to increased electron flux from convection as well as substorm injections. This discovery and its preliminary explanation may help understand the electron three‐belt structure in radiation belts more comprehensively. Key Points We reported a sub‐relativistic electron three‐belt event that occurred in October 2020 Electrons from injections and convection after the partial loss of the outer belt contributed to the formation of the external outer belt The flux enhancement event during the geomagnetic storm on 24 October destroyed the sub‐relativistic electron three‐belt structure