On the Source of Energetic Electron Precipitation during Auroral Substorms

Precipitating auroral electrons are believed to originate mainly from parallel electric fields set up at the auroral acceleration region (AAR) extending up to 20,000 km altitude. However, electrons of energy greater than 100 keV are probably generated by acceleration processes beyond the AAR. Observational evidence for the source location of these energetic electrons are hard to come by. In our current work, we present simultaneous magnetically conjugate measurements of energetic electron spectra estimated at the ionosphere using the Poker Flat Incoherent Scatter Radar (PFISR) and measured at the inner plasma sheet by the THEMIS spacecraft. The flux of precipitating electrons of energy greater than 100 keV demonstrate a striking spatio-temporal correlation with that of the inner plasma sheet electrons. This suggests that the source of the energetic electrons lie at or beyond the inner plasma sheet, and that the acceleration processes within the auroral acceleration zone don't contribute substantially to their energization. Using simultaneous THEMIS measurements of wave power, we speculate that the electromagnetic ion cyclotron (EMIC) and Chorus waves are likely candidates for electron acceleration within the inner plasma sheet apart from the usual candidates of betatron and fermi acceleration. However, between the ionosphere and the plasma sheet, electrons of energy less than 100 keV show significant differences in their energy spectra after the substorm onset suggesting an active AAR