A SAR arc evolving into STEVE during extreme subauroral plasma conditions

A variety of optical structures are observed in the subauroral region, below the typical auroral displays. These phenomena are distinct from aurorae, as their optical signatures appear to be triggered by extreme thermal and kinetic energy in Earth's atmosphere, rather than produced by energetic particles precipitating into the atmosphere. Two such subauroral optical structures are stable auroral red (SAR) arcs and Strong Thermal Emission Velocity Enhancement (STEVE). Although SAR arcs and STEVE both occur during geomagnetically active conditions in the subauroral region, their appearance differs significantly with distinct spectra, scale-size, and duration. We combined images captured by citizen scientist in New Zealand on 17 March 2015 with data from a scientific all-sky imager as well as satellites. We observed the evolution of an unusually bright SAR arc into STEVE. Conjunctions with satellite measurements show a region of fast-moving particles, known as subauroral ion drift (SAID), at the same time the SAR arc occurred. Later, when STEVE was present, these conditions further intensified into a narrower band with higher temperature and faster flows. Another example of a STEVE/ SAR arc connection occurred on 4 November 2021. The all-sky imager measured a SAR arc (630.0 nm only) all night long, but, during a period of ~30 minutes, an arc was also observed in different wavelengths and by citizen scientist images. These observations highlight the benefits of combining citizen scientist observations with scientific data to help discover new connections in Geospace.