STEVE and other aurora-like glows perplex scientists with their complex physics

From the pristine dark of his backyard in rural Alberta, Canada, Alan Dyer has taken stunning photos of a rare sky glow called STEVE. To capture this ribbon of mauve, he and other citizen scientists typically let their cameras collect light for seconds at a time. Long exposures smear out STEVE’s finer details in favor of making its color pop. But when a STEVE stretched over his house one August night in 2022, Dyer tried a different approach.

He zoomed in on the sky glow with his camera and took a video of STEVE’s nitty-gritty details at a rate of 24 snapshots per second. Instead of the largely smooth drift of purple seen in past images, Dyer’s footage exposed STEVE as a frenetically flickering torrent of purplish-white fuzz.

“It didn’t look that beautiful,” Dyer says, but on the off chance it might be scientifically useful, he sent the video to Toshi Nishimura, a space physicist at Boston University. 

“I said, ‘Oh my God, no one has ever seen this before,’” says Nishimura, who was eager to analyze such a high-resolution view of STEVE. But upon inspection, STEVE’s fine details didn’t jibe with scientists’ tentative understanding of the atmospheric chemistry behind the airglow. “This fine-scale structure gave us a huge headache, actually,” Nishimura says.

That confusion is par for the course when it comes to the science of STEVE — short for Strong Thermal Emission Velocity Enhancement. Ever since citizen scientists first showed researchers their images of STEVE a few years ago, they’ve raised more questions than they answered.

“Every time we find something new [about STEVE], the number of physics questions that it opens up is triple what we expected,” says Bea Gallardo-Lacourt, a space physicist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

At a meeting of the American Geophysical Union in San Francisco, on December 14, Nishimura’s team presented the new high-res view of STEVE. Other researchers described similarly perplexing observations that another non-aurora sky glow can morph into STEVE. But there was a glimmer of clarity too: a computer model shared by still other sky detectives may explain what causes the “picket fence” of green stripes that sometimes appears below STEVE.

“STEVE and the picket fence are arguably the biggest mystery in space physics right now,” says space physicist Claire Gasque of the University of California, Berkeley. And because satellite signals can be affected by the conditions in Earth’s atmosphere where STEVE appears, explaining this airglow could have uses beyond understanding a pretty light show.  

STEVE’s mysteries are multiplying

When aurora chasers in Canada first introduced STEVE to the scientific community in 2016, researchers knew it was no aurora (SN: 3/15/18). Auroras form when charged particles from the magnetic bubble, or magnetosphere, around Earth rain down into the atmosphere (SN: 2/7/20). Those particles crash into oxygen and nitrogen near Earth’s poles, painting the sky with brushes of red, green and blue. But STEVE was purple. And it appeared closer to the equator than the northern and southern lights do.

“For us here in southern western Canada,” Dyer says, “the aurora is typically to the north.” STEVE, meanwhile, can come right overhead.

STEVE was later linked to a river of charged particles surging through the atmosphere (SN: 4/30/19). That plasma stream, moving at several kilometers per second, is thought to energize the air around 200 kilometers off the ground to the point of glowing purple — but what molecules give STEVE its signature color remain unclear, especially in light of Dyer’s new footage.

Dyer’s video captured details of STEVE down to about 90 meters across — fairly small for an airglow that can span thousands of kilometers. The footage showed a clumpy, speckled stream of purple rushing westward at about 9 kilometers per second, sporting variations in brightness as small as a few kilometers across, some of which popped in and out of view within seconds, Nishimura and colleagues reported in the December JGR Space Physics. 

“The leading theory of the STEVE emission is that there’s nitric oxide that is excited by the fast plasma stream,” Nishimura says. That nitric oxide is thought to give off the purple light. But excited nitric oxide can glow for an hour, Nishimura notes. That’s about how long STEVE lasts overall; the granular bursts of brightness that last mere seconds add a wrinkle to that idea.

Firing a sensor-strapped rocket through STEVE could identify the molecules responsible, Nishimura says. “But the challenge is that we need to know when and where STEVE is going to happen, and that’s extremely difficult.”

STEVE can appear just after the peaks of substorms, which are disturbances in the magnetosphere that can stir up spectacular auroras. “STEVE generally appears after the main aurora show has kind of faded,” Dyer says. But not every substorm comes with a STEVE encore, and research presented by Gallardo-Lacourt and her colleagues at AGU suggests not all STEVEs need a substorm to appear.

One thing that might help researchers refine their STEVE predictions, Nishimura says, is better understanding the light show’s relationship to another non-auroral airglow called a stable auroral red (SAR) arc — which citizen science photos now suggest can morph into STEVE.

How STEVE and SAR arcs interact

In March 2015, citizen scientist Ian Griffin set out to photograph a particularly dazzling auroral display near Dunedin, New Zealand. But just north of the southern lights, he spotted something strange — a wide, red sky glow that morphed into the mauve strand of STEVE. Griffin’s footage offered researchers their first glimpse of a STEVE blooming out of a SAR arc. Space physicist Carlos Martinis of Boston University and colleagues reported it in June 2022 in Geophysical Research Letters.

Scientists have studied SAR arcs for decades. Like STEVE, these airglows stretch east-to-west across the sky closer to the equator than the northern and southern lights. But unlike STEVE’s roughly hour-long set, SAR arcs can stain the sky for hours to days at a time — visible with cameras, though usually too dim to see with the naked eye.

SAR arcs form when disturbances in Earth’s magnetosphere cause charged particles thousands of kilometers out in space to collide, creating heat that seeps down into the ionosphere — the layer of the atmosphere home to STEVE. That heat energizes electrons to excite oxygen atoms to shed red light that’s normally about one-tenth as bright as auroras. But the SAR arc that Griffin saw was radiant enough to rival red southern lights.

“It was just stunning,” says Megan Gillies, who studies auroras at the University of Calgary in Canada. Griffin’s footage inspired her to search for other cases of STEVE emerging from SAR arcs. Her team found one spotted by the Transition Region Explorer, or TREx, Spectrograph at Lucky Lake, Saskatchewan in April 2022. The group reported it in Geophysical Research Letters in March. STEVE’s bright purple streak emerged from a SAR arc’s red glow, hung around for about half an hour, then gave way to more red.

“It’s like watching a fire smoldering, and then you throw more wood on it and then it blazes up … Whoosh, there it goes! And then it kind of dies back down,” says Gillies, whose group described the SAR arc–STEVE connection at the AGU meeting. “There’s something that happens that triggers a STEVE,” she says, but because not all SAR arcs mutate into STEVEs, it’s not clear what causes this transition.

It might have something to do with the plasma torrent that powers STEVE. SAR arcs have similarly been linked to westward plasma flows in the atmosphere — though not as fast as the plasma flows that power STEVEs, Martinis notes. As the SAR arc seen in 2015 evolved into STEVE, satellite data did show a wide stream of plasma in the atmosphere narrow and quicken into the kind of intense filament typical of STEVE. But what triggered this switch remains an open question, Martinis says. Further complicating matters: citizen scientists have also spotted STEVEs and SAR arcs existing alongside but seemingly independent of each other.

With researchers left scratching their heads over these observations, “this is where modeling comes in,” Gillies says. Theorists can use computers to test whether the physics they think is happening produces light patterns resembling STEVE, she explains. Computer models are already helping piece together another STEVE-related puzzle: the source of the picket fence.

The picket fence may be built right in Earth’s backyard

At first, researchers thought STEVE’s sometimes sidekick of green stripes was a plain old aurora. After all, the picket fence’s bright green glow is a similar hue as some normal northern lights. But the specific wavelengths of light emanating from the picket fence hint that it might not be an aurora, after all (SN: 11/12/20).

Showers of charged particles from way out in the magnetosphere light up normal auroras. “When they collide with the atmosphere, they’re going to create a pretty wide spectrum of colors,” Gasque says. That includes green from oxygen and red and blue from nitrogen. “That blue is kind of the smoking gun that we didn’t see with the picket fence,” Gasque says. Its absence hints that the picket fence’s green spires don’t arise from the same process as auroras.

An alternative explanation for the picket fence might be electric fields embedded within Earth’s atmosphere that run parallel to the planet’s magnetic field, Gasque says. Those fields could energize local electrons to excite oxygen into glowing green and coax nitrogen to give off a bit of red but not blue. Gasque and colleagues ran a computer model of Earth’s atmosphere with electrons energized by electric fields. The team compared the light produced inside their simulated atmosphere with light from a picket fence seen by the TREx Spectrograph at Lucky Lake in April 2018.

The model did indeed reproduce the ratio of red to green light seen in the real-life picket fence without a tinge of blue — bolstering the idea that atmospheric electric fields could construct the picket fence, the researchers reported November 16 in Geophysical Research Letters and at the AGU meeting. But scientists need to confirm that such electric fields actually exist at the altitudes where picket fences appear. 

“The plan now is to try and fly a rocket through one of these structures,” says Gallardo-Lacourt. Gasque and her colleagues have just proposed such a mission to NASA. The rocket wouldn’t fly through the picket fence — which, like STEVE, is too hard to predict. Instead, it would target phenomena with similar coloring that are far more common: enhanced auroras.

“With enhanced aurorae, you have kind of these sharp, bright layers within the aurora,” Gasque says. The sharpness of those variations in auroral light and their picket fence–like color scheme hints that they might be powered by electric fields as well. If a future rocket mission detects electric fields threaded through enhanced auroras, that would help confirm that similar fields build the picket fence.

NASA’s Geospace Dynamics Constellation mission will also launch a fleet of spacecraft as early as 2027 to probe Earth’s magnetosphere and ionosphere — which might yield more data that help explain aspects of STEVE, Gallardo-Lacourt notes. In the meantime, STEVE’s dedicated paparazzi of citizen scientists will continue snapping photos of the phenomenon from the ground.

“We’re out specifically looking for STEVE and knowing that there’s scientific interest in it,” Dyer says. “Prior to the era of STEVE … you might have thought, well, there’s nothing amateurs can contribute now to aurora research, it’s all done with rockets and satellites and the like. But nope! There’s a lot we can contribute” — even if those contributions are often new puzzles for scientists to solve.