Citizen scientists, along with satellite and ground-based sensors, have revealed a new arc boundary at subauroral latitudes.

We present three STEVE (strong thermal emission velocity enhancement) events in conjunction with Time History of Events and Macroscale Interactions (THEMIS) in the magnetosphere and Defense Meteorological Satellite Program (DMSP) and Swarm in the ionosphere, for determining equatorial and interhemispheric signatures of the STEVE purple/mauve arc and picket fence. Both types of STEVE emissions are associated with subauroral ion drifts (SAID), electron heating, and plasma waves. The magnetosphere observations show structured electrons and flows and waves (likely kinetic Alfven, magnetosonic, or lower‐hybrid waves) just outside the plasmasphere. Interestingly, the event with the picket fence had a >~1 keV electron structure detached from the electron plasma sheet, upward field‐aligned currents (FACs), and ultraviolet emissions in the conjugate hemisphere, while the event with only the mauve arc did not have precipitation or ultraviolet emission. We suggest that the electron precipitation drives the picket fence, and heating drives the mauve as thermal emission.

We present the first observations of ionospheric phenomena using the newly deployed Transition Region Explorer (TREx) Spectrograph. On the night of 10 April 2018, STEVE (Strong Thermal Emission Velocity Enhancement) and the Picket Fence optical structures were observed by the spectrograph in Lucky Lake, Saskatchewan. STEVE contains an enhancement of the OI red‐line (630‐nm) emission and a continuum which spans the visible wavelengths. Based upon its spectrum, we assert that the characteristic mauve color of STEVE is a result of this continuum. The spectrum of the Picket Fence contains a strong OI green‐line (557.7‐nm) emission similar to that produced in typical auroral structures. From their spectra, we assert that the Picket Fence is caused by particle precipitation and thus that the Picket Fence is a form of aurora, while STEVE's spectrum confirms that it is not aurora.

There has been an exciting recent development in auroral research associated with the discovery of a new subauroral phenomenon called STEVE (Strong Thermal Emission Velocity Enhancement). Although STEVE has been documented by amateur night sky watchers for decades, it is as yet an unidentified upper atmosphere phenomenon. Observed first by amateur auroral photographers, STEVE appears as a narrow luminous structure across the night sky over thousands of kilometers in the east-west direction. In this paper, we present the first statistical analysis of the properties of 28 STEVE events identified using Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager and the Redline Emission Geospace Observatory (REGO) database. We find that STEVE occurs about 1 hr after substorm onset at the end of a prolonged expansion phase. On average, the AL index magnitude is larger and the expansion phase has a longer duration for STEVE events compared to subauroral ion drifts or substorms. The average duration for STEVE is about 1 hr, and its latitudinal width is~20 km, which corresponds to~¼ of the width of narrow auroral structures like streamers. STEVE typically has an equatorward displacement from its initial location of about 50 km and a longitudinal extent of 2,145 km. Plain Language Summary Strong Thermal Emission Velocity Enhancement (STEVE) is anatmospheric phenomenon that manifests across the night sky as an extremely thin yet long ribbon of vibrant purple and white hues. Although STEVE has been well documented by amateur auroral photographers for several decades, the scientific community only recently stumbled upon this phenomenon. In this paper, we report on the first statistical analysis of STEVE's optical characteristics using ground-based all-sky imagery and examined satellite data to determine the geomagnetic conditions favorable for the formation of STEVE. Our results verify that STEVE is narrow in the north-south direction, but it extends over a wider east-west region. We have also determined that STEVE displaces southward over its lifetime in most observations. More interestingly, all 28 STEVE events identified in this study were observed at the end of a prolonged substorm expansion phase. More recently, Gallardo-Lacourt et al. (2018) analyzed data from the Polar-Orbiting Environmental Satellite (POES)-17 satellite for one STEVE event identified by Time History of Events and Macroscale Interactions GALLARDO-LACOURT ET AL. 9893