STEVE, the enigmatic atmospheric phenomenon, explained (sort of)

JOEL WILLIAMS joel.williams@photonics.com

“I propose we call it Steve,” said photographer Chris Ratzlaff in a 2016 post in a Facebook group for aurora chasers.

Ratzlaff was referring to an atmospheric optical phenomenon that appears as reddish and green light in the sky. The name “Steve” is a nod to the 2006 animated film “Over the Hedge,” in which its characters chose “Steve” as a benign name for something unknown. During a December 2016 meeting of the American Geophysical Union (AGU), the name was given a backronym — “Strong Thermal Emission Velocity Enhancement” (STEVE) — which has since been adopted by NASA as the phenomenon’s official name.

STEVE, photographed on Aug. 17, 2015, at Little Bow Resort in Lomond, Alberta, Canada. Courtesy of Elfie Hall/Wikipedia/CC BY-SA 4.0.

To the untrained eye (and even the trained one), STEVE resembles an aurora. It appears as a long, glowing reddish ribbon with a green picket-fence pattern.

Ratzlaff’s 2016 Facebook post linked to a Weather.com article that highlighted the luminous enigma. The prevailing theory at the time was that STEVE was a proton aurora; however, as University of Calgary physics professor Eric Donovan noted in a 2017 BBC article, proton auroras are not visible.

In 2017, the European Space Agency (ESA) took measurements of STEVE using data from its SWARM mission. The ESA determined that STEVE was located 300 km above the surface of Earth and that the temperature of the air within the phenomenon was 3000 °C hotter than the air outside of it. Inside, the 25-km-wide ribbon of gas flowed at 6 km/s. The study dubbed STEVE a type of “skyglow,” but did not go much further than that.

A more recent study uncovered additional information, though the phenomenon is still not fully understood. Research published in April 2019 in AGU’s Geophysical Research Letters analyzed satellite data and ground images of STEVE events and concluded that the reddish arc and the green picket fence are phenomena caused by different mechanisms.

The reddish streaks are caused by heating of charged particles higher up in the atmosphere, similar to how a light bulb works. When STEVE is on display, energetic electrons pour into Earth’s ionosphere, the layer of the atmosphere where atoms lose electrons due to solar and cosmic radiation. The friction heats particles, which creates a reddish glow.

Satellite information explained how the picket-fence effect occurs. The data showed that waves moving from Earth’s magnetosphere to the ionosphere both energized electrons and moved them out of the magnetosphere, creating a green picket-fence-patterned glow.

Researchers said the new information will help them learn how to predict the paths of particles flowing through the ionosphere, which, like STEVE, is something they’d like to understand better.