Relativistic electron precipitation and substorm auroras

English Abstract:

We report evidence for relativistic electron precipitation in substorm auroras. Multi-beam measurements by the electronically steerable Poker Flat Incoherent Scatter Radar (PFISR) reveal transient patches of ionization extending below 70 km altitude. These regions are restricted to the poleward edge of the auroral expansion phase. Reconstructed vertical density profiles, analyzed using a particle penetration model, suggest that the primary electron spectrum contains a significant population in the 300-600keV range. Proton precipitation is ruled out due to the limited horizontal extent of these regions and the extreme energies required for protons to penetrate to these altitudes. This electron population is not likely produced by near-Earth acceleration mechanisms. The likely source lies in the magnetotail, where substorm dipolarization can readily energize electrons to the relativistic range. Our interpretation is supported by magnetically conjugate measurements from the THEMIS satellite for one of these events, which reveal a remarkably impulsive reconfiguration of the magnetotail preceding one of these events. These results exemplify the unique scientific capability of common volume measurements by electronically steerable ISR and wide-field cameras.