Strong Thermal Emission Velocity Enhancement (STEVE) is a “subauroral optical structure” identified by a thin band of white to mauve dancing lights bending across the sky. Its causes remain largely unknown, but now for the first time we have images of a more familiar stable aurora red (SAR) arc turning into a STEVE, and some insight into atmospheric processes at the time
Auroras have probably fascinated humans since we journeyed close enough to the poles to see them. The first likely surviving description is almost 3,000 years old and more recent accounts exist from many cultures. Nevertheless, the more widespread availability of digital cameras has revealed previously unrecorded auroral phenomena.
STEVE is the latest example, with the first scientific publication on it appearing in 2018. There have been several developments since, including a suggestion it is nothing but skyglow, but a paper in Geophysical Research Letters not only appears to confirm STEVE, but to offer some insight into its origins and nature.
According to the paper, neither STEVEs nor SARs are true auroras. Instead they might be considered aurora-adjacent, appearing at lower latitudes than the true auroral zone. “Their optical signatures appear to be triggered by extreme thermal and kinetic energy in Earth's atmosphere, rather than produced by energetic particles raining down into our atmosphere,” the paper notes.
In March 2015 a citizen scientist in Dunedin, New Zealand photographed an unusually bright SAR in a series of ten second exposures, which turned into a STEVE during the image taking. This being before STEVE had been named, the photographer didn't know what they had seen. When the footage was passed over to Professor Carlos Martinis of Boston University and colleagues, however, they compared it to images from the all-sky imager at Mount John Observatory, around 200 km (120 miles) to the north and satellite observations taken at the same time.
The Dunedin film reveals a bright SAR red arc replaced by a purplish-white STEVE. There was more cloud at Mount John that night, making images much more patchy. However, combining what was available from the two locations allowed the paper's authors to pinpoint the source of the initial emissions to an altitude of 425 kilometers, although this later fell. This is a typical height for a SAR arc, but the brightness was anything but – 10-12 times the usual SAR.
Like true auroras, the events coincided with a geomagnetic storm. https://www.iflscience.com/geomagnetic-storms-coming-our-way-as-dead-sunspot-throws-plasma-at-earth-63289
Satellites passing overhead measured fast-moving particles, which the authors call “subauroral ion drift” coinciding with the SAR and intensifying at increased temperatures during the STEVE.
The paper suggests the ions running into nitrogen molecules provides a “plausible generation mechanism” for Steve, but is mostly content to bring the data together from the three sources for others to interpret. The authors conclude; “These observations highlight the benefits of combining citizen scientist observations with scientific data to help discover new connections in Geospace.” This may be another way of saying; “If anyone has any STEVE photographs please send them to scientists.”
STEVE's are usually quite faint, helping to explain why they were not scientifically described until recently, but as the images show, there are exceptions.
H/T Live Science