STEVE—the name given to strange purple ribbons of light that seem to accompany auroras—is weirder than we thought, a new study suggests.
In 2016, members of the Alberta Aurora Chasers Facebook group noticed violet-tinged stripes of light across beautiful shots of swirling auroras posted online and upon further investigation found them in images spanning the last few decades. With typical internet logic, they decided to name the phenomenon "Steve" after a joke in the cartoon movie Over the Hedge (luckily they didn’t choose Aurora McAuroraface). Scientists later turned the name into a “backronym,” dubbing the ribbons STEVE which stands for Strong Thermal Emission Velocity Enhancements.
Carly Stagg at the CBC reports the glowing bands are not auroras at all but something completely unknown, as researchers determined in only the second study ever done on the phenomena.
So, if STEVE isn’t an aurora, what is the mysterious purple light? It’s hard to say. Sarah Lewin at Space.com reports that skywatchers at first believed the lights were proton arcs. But Eric Donovan, a physicist at University of Calgary and co-author of the new study in Geophysical Research Letters, notes that protons create a very diffuse light that is barely picked up by cameras while STEVE is very bright. The researchers really don’t know yet where the purple one gets its energy, writing, “Interestingly, its skyglow could be generated by a new and fundamentally different mechanism in the ionosphere.”
Auroras—both the aurora borealis in the Northern Hemisphere and aurora australis in the Southern Hemisphere—light up when solar wind and solar flares from the sun blow energetic, charged plasma particles through space to Earth. Near the poles, plasma particles evade Earth’s magnetic field, which is usually strong enough to deflect most of those particles. When the particles interact with oxygen, they produce the classic green glow. The lights turn red if the particles mix with high-altitude oxygen or blue or purple if they excite nitrogen.
According to a press release for the latest study, researchers wanted to see if the same conditions that produce auroras also produce STEVEs. They compared images of a STEVE from March 2008 captured using ground-based cameras with images gathered by NOAA's Polar Orbiting Environmental Satellite 17, which passed over the same area and collected data about the ionosphere.
The data shows that the STEVE appeared at a time when charged particles were not deluging the ionosphere, indicating that the same process that powers aurorae does not bring STEVEs to life. Instead, it’s a distinct phenomenon from an aurora that, for now, is simply classified as a “skyglow.”
"Our main conclusion is that STEVE is not an aurora. So right now, we know very little about it. And that's the cool thing,” physicist Bea Gallardo-Lacourt also of the University of Calgary and lead author of the study tells the CBC’s Stagg.
STEVE doesn’t behave much like an aurora either. Instead of shimmering in the sky in a diffuse horizontal glow, STEVEs are narrow vertical bands that pierce the sky—sometimes stretching 600 miles—like the trail from a giant purple rocket. They are also hot, reaching 5,500 degrees Fahrenheit.
“The aurorae you see in the sky, at least from our data, is moving at a certain speed, and then you have this guy moving crazy fast at lower latitudes, passing from east to west, super narrow, almost like a comet,” Gallardo-Lacourt tells the CBC’s Stagg. “I like to describe it to my friends as the aurora moves like Wile E. Coyote, while Steve moves like the Road Runner.”
STEVEs have likely been streaking across the sky for millennia, but it took the development of several new technologies for humanity to notice. Lewin reports that STEVEs are a relatively common phenomenon, but seeing one means being in the right place at the right time. In the last 20 years, Canada has gone from having one whole-sky imager, a camera that take snapshots of the sky every night, to more than 100, which means there’s more data about what's going on above us.
In the last decade crowdsourcing tools like the app Aurorasaurus, in which citizen scientists help map the aurorae, as well as many aurora watching groups on Facebook and other platforms, have also improved our view of the sky. Donovan tells Lewin that just 15 years ago it would have taken scientists 10 years and $200 to $300 million to find and study a phenomenon like STEVE.