Side-by-side images show fish-eye-style photographs of a starry evening sky illuminated by different subauroral events. The left image shows a SAR arc, a reddish arc that spans the sky. Beneath the arc the sky transitions from bright green at the horizon to purple and is partially obscured by clouds. The right image shows a STEVE phenomenon. Part of the red SAR arc is still visible but is much fainter and more of a pink color than its original red. The green and purple sky is more muted in color, and more stars and the band of the Milky Way are now visible. In both images, the silhouette of some shrubs is visible.
A SAR arc (left) appeared over New Zealand in March 2015, and half an hour later it evolved into a STEVE (right). Credit: Ian Griffin
Source: Geophysical Research Letters

Stable auroral red (SAR) arcs and strong thermal emission velocity enhancement (STEVE) phenomena are optical structures that have been detected in the subauroral upper atmosphere. Whereas aurorae are created by energetic particles raining down from above, SAR arcs and STEVE phenomena are generated by extreme thermal and kinetic energy in Earth’s atmosphere.

The two optical structures differ from each other in terms of spectra, size, and duration. The structure and occurrence of SAR arcs are better understood than those of STEVE, and although they are known to happen in the same region, whether or not SAR arcs can change into instances of STEVE had not been documented.

On 17 March 2015, that changed. In a new study about that event, Martinis et al. report on a SAR arc that evolved into a STEVE phenomenon during a geomagnetic storm. At 09:27 UT, A SAR arc was observed from New Zealand, running east to west, shining brighter than previously reported arcs. Images of the optical structure were captured by amateur photographers as well as an all-sky imager at the Mount John Observatory. According to the observatory’s information, the SAR arc reached a brightness of about 6 kilorayleighs, 10 times brighter than a typical SAR arc and similar to visible red aurorae. The researchers made sense of the images and geolocation of the structure by assigning each pixel a coordinate; they deduced that the emission height reached around 425 kilometers (264 miles).

Martinis and colleagues also analyzed data from the European Space Agency’s Swarm B satellite and found that the SAR arc’s characteristics were associated with strong ion drifts and high electron temperature. Over the next 30 minutes, these conditions became stronger as the structure eventually morphed into a STEVE—a thin, white-mauve arc.

According to the authors, this is the first reported instance of a SAR arc transitioning into a STEVE. Scientists look forward to future missions with instruments that can gather data from such extreme events as well as deciphering the mechanisms that form these subauroral phenomena to begin with. (Geophysical Research Letters,, 2022)

—Alexandra K. Scammell, Associate Editor

Citation: Scammell, A. K. (2022), From SAR arc to STEVE: An atmospheric evolution, Eos, 103, Published on 06 September 2022.
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