On the origin and geomagnetic conditions of STEVE's formation

The recent discovery of the subauroral phenomenon called STEVE (Strong Thermal Emission Velocity Enhancement) has been an exciting development in auroral research. Although STEVE has been documented by amateur night sky watchers for decades, it is a new upper atmospheric phenomenon to the scientific community. A fundamental question on the origin of STEVE is whether or not STEVE is a manifestation of particle precipitation. We analyzed an event on March 28, 2008 that was observed by THEMIS ground-based All-Sky Imagers (ASI) and a POES satellite. The POES-17 satellite crossed STEVE at the center of the ASI field-of-view allowing us to collect particle and optical data simultaneously. These concurrent measurements show that STEVE might not be associated with particle precipitation (electrons or ions) and suggests that STEVE's generation mechanism is different than aurora.

In addition, we will present our analysis of the geomagnetic conditions during 28 STEVE occurrences. These events were identified using THEMIS ASI and the REGO database. We found that STEVE occurs about one hour after substorm onset at the end of a prolonged expansion phase. On average, the magnitude of the AL index is larger and the expansion phase has a longer duration for STEVE compared to SAID or Substorm events. STEVE also persists for about one hour and has a latitudinal width of 20km.