Relativistic Electron Precipitation Near Midnight: Drivers, Distribution, and Properties

We analyze the drivers, distribution, and properties of the relativistic electron precipitation (REP) detected near midnight by the Polar Orbiting Environmental Satellites (POES) and Meteorological Operational (MetOp) satellites, critical for understanding radiation belt losses and nightside atmospheric energy input. REP is either driven by wave-particle interactions (isolated precipitation within the outer radiation belt), or current sheet scattering (CSS; precipitation with energy dispersion), or a combination of the two. We evaluate the L-MLT distribution for the identified REP events in which only one process evidently drove the precipitation (similar to 10% of the REP near midnight). We show that the two mechanisms coexist and drive precipitation in a broad L-shell range (4-7). However, wave-driven REP was also observed at L < 4, whereas CSS-driven REP was also detected at L > 7. Moreover, we estimate the magnetotail stretching during each REP event using the magnetic field observations from the Geostationary Operational Environmental Satellite (GOES). Both wave-particle interactions and CSS drive REP in association with a stretched magnetotail, although CSS-driven REP potentially shows more pronounced stretching. Wave-driven REP events are localized in L shell and often occur on spatial scales of L. Using either proton precipitation (observed by POES/MetOp during wave-driven REP) as a proxy for electromagnetic ion cyclotron (EMIC) wave activity or wave observations (from GOES and the Van Allen Probes) at the conjugate event location, we find that similar to 73% wave-driven REP events are associated with EMIC waves.