First Simultaneous Observation of STEVE and SAR Arc Combining Data From Citizen Scientists, 630.0 nm All‐Sky Images, and Satellites

Abstract

The subauroral region, located equatorward from typical auroral displays, has been extensively studied by investigating processes like subauroral polarization streams (SAPS), subauroral ion drifts (SAIDs), and stable auroral red (SAR) arcs. Recently a new optical phenomenon known as strong thermal emission velocity enhancement (STEVE) has brought renewed interest in this region (Gallardo-Lacourt, Liang, et al.,2018;MacDonald et al., 2018). A handful of papers have reported characteristics of STEVEs that seem to match those observed during SAIDs (Archer, Nishimura et al., 2019). Spectrograph measurements show that the spectrum of STEVE contains two major contributing components: an overall enhancement of a continuous spectrum between 400 and 730 nm, with a peak at 630.0 nm (Gillies et al., 2019). Nevertheless there are still many issues that need to be understood. The initial observations made the assumption that STEVE was occurring at 130 km altitude, however a study using Citizen Scientist's photographs of a STEVE showed that it spanned altitudes from 130 to 270 km . Altitudes of 150 and 250 km were also found using all-sky imagers (Liang et al., 2019).Enhanced electron temperature and reduced ion density are typical signatures accompanying the observation of SAR arcs and STEVEs, as well as SAPS and SAIDs. These similarities make difficult to distinguish the optical signatures observed. In general, STEVEs are short lived, latitudinally thin, polychromatic (typically purple/mauve and white), and associated with very large westward flows (more than few km/s) (Archer, Gallardo-Lacourt et al., 2019), while SAR arcs are long lived, wide in latitude, monochromatic (630.0 nm),

Rainbow of the Night: First Direct Observation of a SAR Arc Evolving Into STEVE

“…The symbols representing the peaks in N e , T e , and V hor do not fall on top of the arc, which is evidence of a potential change in the height of the emission. A lower altitude emission might be more appropriate and consistent with a SAR arc that has evolved into STEVE, since the latter has been observed to occur at altitudes between ∼170 and 300 km (Archer, St.‐Maurice, et al., 2019; Liang et al., 2019; Martinis et al., 2021) and not near 425 km.…”

“…This mechanism requires a minimum of ∼3 eV, equivalent to 4.4 km/s for NO + (the dominant ion in SAIDs), thus explaining why STEVE is observed during intense SAIDs. Some studies have shown that STEVEs and SAR arcs can occur simultaneously while observed at different geographic locations (Martinis et al., 2021) or that STEVE can merge with a preexisting SAR arc (Liang et al., 2019). However, available observations have never allowed the examination of whether a SAR arc can evolve into STEVE (or vice versa).…”

New Insight Into the Transition From a SAR Arc to STEVE

“…While optically different, SAR arcs and STEVE present many similarities with respect to timescales and geophysical location; therefore, it is important to study if a relationship between these two structures could be established. Previous studies reported observations of STEVE and SAR arcs located temporally and spatially near but not collocated (e.g., Chu et al., 2019; Liang et al., 2019; Martinis et al., 2021); however, they were not able to demonstrate if these two structures were connected. More recently, a study by Martinis et al.…”

Unsolved problems in Strong Thermal Emission Velocity Enhancement (STEVE) and the picket fence

“…Limited investigations have shown that STEVE can occur after the proton aurora (Nishimura et al, 2020b) and that a SAR arc can transition to STEVE (Martinis et al, 2022). Subauroral emissions are not exclusive to each other, but STEVE and a SAR arc can be adjacent to each other simultaneously (Liang et al, 2019;Chu et al, 2019;Martinis et al, 2021). The proton aurora and red arc that transitions to a SAR arc later have also been reported to co-exist during substorms (Nishimura et al, 2022b).…”