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Abstract 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.
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Magnetospheric Conditions for STEVE and SAID: Particle Injection, Substorm Surge, and Field‐Aligned Currents
Abstract To understand magnetosphere‐ionosphere conditions that result in thermal emission velocity enhancement (STEVE) and subauroral ion drifts (SAID) during the substorm recovery phase, we present substorm aurora, particle injection, and current systems during two STEVE events. Those events are compared to substorm events with similar strength but without STEVE. We found that the substorm surge and intense upward currents for the events with STEVE reach the dusk, while those for the non‐STEVE substorms are localized around midnight. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite observations show that location of particle injection and fast plasma sheet flows for the STEVE events also shifts duskward. Electron injection is stronger and ion injection is weaker for the STEVE events compared to the non‐STEVE events. SAID are measured by Super Dual Auroral Radar Network during the STEVE events, but the non‐STEVE events only showed latitudinally wide subauroral polarization streams without SAID. To interpret the observations, Rice Convection Model (RCM) simulations with injection at premidnight and midnight have been conducted. The simulations successfully explain the stronger electron injection, weaker ion injection, and formation of SAID for injection at premidnight, because injected electrons reach the premidnight inner magnetosphere and form a narrower separation between the ion and electron inner boundaries. We suggest that substorms and particle injections extending far duskward away from midnight offer a condition for creating STEVE and SAID due to stronger electron injection to premidnight. The THEMIS all‐sky imager network identified the east‐west length of the STEVE arc to be ~1900 km (~2.5 h magnetic local time) and the duration to be 1–1.5 h.
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- Award ID(s):
- 1907698
- PAR ID:
- 10374573
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 125
- Issue:
- 8
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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