Source: Geophysical Research Letters
Steady red aurora arcs (SAR) and strong thermal emission velocity enhancement phenomena (STEVE) are optical structures that have been detected in the subauroral upper atmosphere. While the aurora borealis is created by energetic particles coming from above, SAR arcs and STEVE phenomena are created by the extreme thermal and kinetic energy in the Earth’s atmosphere.
The two optical structures differ from each other in spectrum, size and duration. The structure and occurrence of SAR arcs are better studied than in STEVEand although they are known to occur in the same region, it has not been documented whether SAR arcs can evolve into STEVE instances.
On March 17, 2015, that changed. In a new study on this event, Martinis and others. report on a SAR arc that developed into a STEVE phenomenon during a geomagnetic storm. At 09:27 UT, a SAR arc was observed from New Zealand that ran from east to west and was brighter than previously reported arcs. The images of the optical structure were taken by amateur photographers as well as the all-sky camera at Mount John Observatory. According to the observatory, the brightness of the SAR arc reached about 6 kilorays, which is 10 times brighter than a normal SAR arc and similar to the visible red auroras. The researchers made sense of the images and geolocation of the structure by assigning coordinates to each pixel; they concluded that the ejecta reached an altitude of about 425 kilometers (264 miles).
Martinis and his colleagues also analyzed data from the European Space Agency Swarm B satellite and found that the SAR arc characteristics were associated with strong ion drifts and high electron temperatures. Over the next 30 minutes, these conditions intensified as the structure eventually evolved into STEVE, a thin white-purple arc.
According to the authors, this is the first reported case of SAR arc transition in STEVE. Scientists look forward to future missions with instruments that will be able to collect data on such extreme events, as well as decipher the mechanisms that form these subauroral phenomena. (Geophysical Research Letters, https://doi.org/10.1029/2022GL0985112022)
— Alexander K. Scammell, deputy editor
