The 600 keV electron injections in the Earth’s outer radiation belt: A statistical study

Relativistic electron injections are one of the mechanisms of enhancements of relativistic (≥ 0.5 MeV) electrons in the Earth’s outer radiation belt. In this study, we present the statistical observation of 600 keV electron injections in the outer radiation belt using the Van Allen Probes data. Based on the different injection characteristics, 600 keV electron injections in the outer radiation belt are divided into “pulsed electron injections” and “non-pulsed electron injections”. The 600 keV electron injections are observed at over 4.5 < L < 6.4 under the geomagnetic conditions of 450 nT < AE < 1450 nT. L ~ 4.5 is an inward limit for 600 keV electron injections. Before the electron injections, the flux negative L shell gradient for ≤ 0.6 MeV electrons or the low electron fluxes in the injected region are observed. For 600 keV electron injections at different L shells, the source populations from the Earth’s plasma sheet are different. For 600 keV electron injections at higher L shells, the source populations are higher energy electrons (~ 200 keV at X ~ –9 RE), while the source populations for 600 keV electron injections at lower L shells are lower energy electrons (~ 80 keV at X ~ –9 RE). These results are important for our further understanding of electron injections and rapid enhancements of 600 keV electrons in the Earth’s outer radiation belt.