Early last year, a little-regarded atmospheric phenomenon gained the attention of researchers when citizen scientists drew attention to it on social media: Giving it the highly-scientific name of "Steve", this luminous purple ribbon that occasionally streaks across the sky is a fast-moving stream of hot gases that occurs high in the atmosphere. Initially assumed to be part of the aurora borealis, a recent study of the phenomenon has found that it has nothing to do with the display of charged particles that light up the polar atmosphere — so, just what is Steve?
Backronymed to stand for "Strong Thermal Emission Velocity Enhancement", STEVE is a stream of hot gas (3,000ºC / 5,432ºF) that is streaks through an upper layer of the atmosphere called the thermosphere, at an altitude of 300 kilometers (186 miles). It is 25 kilometers (15.5 miles) wide, thousands of kilometers (or miles) long, and flows at 6 kilometers (3.7 miles) per second, despite the surrounding air only moving at about 10 meters (32.8 feet) per second.
Using data that had been collected by a network of ground-based cameras and data collected from one of the National Oceanic and Atmospheric Administration’s Polar-orbiting Operational Environmental Satellites, researchers with Canada’s University of Calgary looked for clues from a known appearance of STEVE from March 28, 2008 — nearly a decade before scientists recognized the phenomenon — and found that there was a lack of charged particles present that would normally light up a traditional aurora. This meant that STEVE is formed under a completely different process than the Northern Lights… so, just what the heck is STEVE?
"Our main conclusion is that STEVE is not an aurora," explains Bea Gallardo-Lacourt, a space physicist with the University of Calgary, and lead study author. "So right now, we know very little about it. And that’s the cool thing, because this has been known by photographers for decades. But for the scientists, it’s completely unknown."
For the time being, the researchers involved are classifying STEVE as a "skyglow", as it doesn’t fall under the definition of a traditional aurora. Their next step is to try to determine if the stream of fast ions and hot electrons in the ionosphere are generating the phenomenon’s light, or if that light is being produced from higher up in the atmosphere.