Global Distribution of Relativistic Electron Precipitation and the Dependences on Substorm Injection and Solar Wind Ram Pressure: Long-Term POES Observations

Relativistic electron precipitation (REP) into the Earth's atmosphere is one of the fundamental mechanisms causing radiation belt electron flux dropout. It is important to know the preferred regions of REP events and the interplanetary and magnetospheric conditions under which the events take place. In this study of the POES relativistic electron data during 1998-2017, we have examined all REP events from L = 3-10 using an automatic algorithm. The REP events preferentially occur in MLT = 17-03 at L = 4-6.5, where the precipitation fluxes also peak. The monthly average REP event number is correlated with both monthly average AE (auroral electrojet index) and P-d (solar wind ram pressure) indices, so we separately investigate the spatial dependences of the events on one index by restricting the other one to a low level. The occurrence rate of REP events increases with both the AE* and the indices (where AE* and are time-modified from the AE and P-d indices). During large AE* but low , the events occur mainly in the dusk sector (MLT = 17-22) near the plasmapause. For large but low AE* intervals, most of the events occur outside the plasmapause in the midnight sector (MLT = 23-03). Possible REP loss mechanisms are discussed based on our findings. Our study provides new insights into the distribution and dependences of REP in the Earth's magnetosphere.