High-latitude Ionospheric Electrodynamics during STEVE Events

The mysterious subauroral phenomenon STEVE (Strong Thermal Emission Velocity Enhancement) was recently discovered in 2016 by citizen observers, yet its physical mechanism is not completely understood. STEVE events are characterized by the presence of a thin, dynamic aurora-like structure with purple emission that propagates westward, typically located in the pre-midnight mid-latitude sector [MacDonald et al., 2018; Gallardo-Lacourt et al., 2018].

The focus of this paper is to investigate the magnetosphere-ionosphere coupling processes during STEVE events by characterizing global and local high-latitude electrodynamics features using an AMGeO (Assimilative Mapping of Geospace Observation) procedure [e.g., Richmond and Kamide, 1989; Matsuo, 2020]. AMGeO is designed to derive maps of high-latitude electrodynamic variables by optimally combining geospace observations with climatological models of ionospheric convection and aurora. The following investigates 33 STEVE events between the years of 2008 through 2018 identified using THEMIS and REGO all-sky imagers [Gallardo-Lacourt, 2018]. For this purpose, SuperDARN plasma drifts and ground-based magnetic field measurements are ingested into AMGeO to estimate the electrostatic potential distribution with a five-minute temporal resolution for the entirety of STEVE events. Precipitating auroral particle flux measured by the Defense Meteorological Satellite Program (DMSP) F16, F17, and F18 satellites are also used. Preliminary analysis of the convection patterns shows the development of enhanced westward plasma drifts (>1 km/s) in the pre-midnight mid-latitude sector, which is characteristic of subauroral ion drift, accompanied by the westward extension of a region of sharp convection reversal, likely associated with a substorm. Principal Component Analysis is applied to electrostatic potential maps for all 33 STEVE events to characterize modes of variability associated with this westward extension.