Dynamics of Auroral Precipitation Boundaries Associated With STEVE and SAID

Using Defense Meteorological Satellite Program (DMSP) and National Oceanic and Atmospheric Administration (NOAA) satellite observations and ground-based observations by the THEMIS all-sky imagers (ASIs) and SuperDARN radars, we determine how the equatorward boundary locations of ring current ions and plasma sheet electrons at pre-midnight relate to occurrence of strong thermal emission velocity enhancement (STEVE) and intense subauroral ion drifts (SAID) during substorms. We found that the STEVE events are associated with a sharper gradient of electron precipitating flux, lower precipitating ion flux, and a narrower (<1°) latitudinal gap between the equatorward boundaries of trapped ring current ions and precipitating plasma sheet electrons and narrower region-2 field-aligned currents (FACs) than for the non-STEVE events. The narrow gap of the particle boundaries contains intense SAID, higher upflow velocity, lower trough density, and slightly higher electron temperature than those for the non-STEVE events. The non-STEVE substorms have much wider gaps between the trapped ions and precipitating electrons, and subauroral polarization streams (SAPS) do not show intense SAID. These results indicate that subauroral flows and downward FACs for the STEVE events can only flow within the latitudinally narrow subauroral low-conductance region between the ion and electron boundaries, resulting in intense SAID and heating. During the non-STEVE events, the SAPS flows can flow in the latitudinally wide region without forming intense SAID.