The ephemeral glow of the northern and southern lights has entranced humans throughout the ages, sparking countless creative works, myths, and legends as well as scientific study of the mechanism behind the phenomenon. But the celestial fireworks still have the capacity to surprise experts, as scientists and sky-watchers learned when they met “Steve,” a whole new kind of light show first spotted by citizen scientists in 2016 in Alberta, Canada.
Featuring an elongated purple stream and sometimes a green, picket fence-like structure, this odd illumination can be seen lingering at latitudes far lower than typical aurorae. Bemused space physicists couldn’t ascertain whether the entity was a genuine aurora, albeit a weirdly shaped one, or something else entirely.
Now, research suggests that perhaps neither answer is correct: Steve could instead be an electrical hybrid.
“We found that the picket fence is an aurora, but the purple arc is not an aurora,” explains study lead author Toshi Nishimura, a space physics researcher at Boston University.
This advance in knowledge matters because typical aurorae and Steve’s purple river appear in the ionosphere. This charged layer of Earth’s atmosphere influences the paths of radio waves, which means perturbations there may affect how our communication networks operate, says study coauthor Bea Gallardo-Lacourt, a space physicist at the University of Calgary.
“It’s fundamental for us to understand what is going on up there,” she says.
Aurorae appear when energetic particles in space slide down Earth’s magnetic field lines, which stretch out of the planet’s magnetic poles like a colossal fishing net. When these particles slam into the upper atmosphere, they give electrons there an energy boost, leading to a release of varying colors of light. At the same time, when the magnetic field lines of Earth and the sun meet, they break and reassemble, taking charged particles along for the ride. This ensures that particles fly into the atmosphere on both sides of the planet, creating daytime and nighttime aurorae.
With its very different appearance and its low-latitude locale, it was obvious early on that Steve wasn’t your run-of-the-mill aurora.
The citizen scientists who spotted the strange new light show over Canada gave it this amusingly ordinary name as a reference to the 2006 animated comedy Over The Hedge, in which the woodland animal characters name a shrubbery Steve because they didn’t know what it was. Elizabeth MacDonald, a space physicist at NASA’s Goddard Research Center and founder of the first citizen science network for the auroras, enjoyed the name, so her team turned it into the backronym STEVE, for Strong Thermal Emission Velocity Enhancement.
Using data from a magnetic field-analyzing probe that flew across a Steve event, MacDonald and her colleagues initially reported in a 2018 study that it was a new “species” of aurora. It was forming in a slightly different way from traditional aurorae, but it still featured all the charged particle mayhem you’d expect.
But Steve clearly existed long before people spotted it, and with the increased interest of citizen scientists combing through past photographs, it has since been captured in even more detail. A separate 2018 study led by Gallardo-Lacourt used satellites to look at a single Steve event from 2008 lacking the green component. Those researchers found that there were nowhere near enough charged particles involved for that event to classify as a true aurora.
The chimera of light
The newest study, published earlier this month in Geophysical Research Letters, looked at three Steve events, again with help from aurora photographers. Various probes, including members of NASA’s aurora-investigating THEMIS satellite swarm, analyzed the electrical characteristics of each event.
Steve is definitely created in the ionosphere, Nishimura’s team reports, but the purple slither doesn’t appear to be an aurora, which is defined as light emissions caused by energetic electrons. Instead, what’s happening here is that low-energy streams of free-moving, electrically excited particles are bouncing off neutral particles, creating friction and generating heat that we see as purple haze. It’s a bit like a lightbulb filament heating up and giving off a glow–and not really like an aurora.
“We’d rather call it a sky glow,” Nishimura says.
Unlike its purple cousin, though, the green feature is very aurora-esque. Disturbances in the magnetic bubble that envelopes the planet can cause “waves” to crash into the ionosphere, sending electrons screaming down to the upper atmosphere and causing them to collide with atmospheric particles, exciting them. That is where the green picket fence feature comes from, the team says.
“This emission mechanism is the same as for the typical green aurora,” Nishimura says. The results show that each part of Steve should be treated as a separate phenomenon in follow-up studies.
A little help from my friends
The new researchis “clearly a significant leap forward” in understanding the origin of Steve, says Larry Lyons, an expert in magnetospheric and ionospheric physics at the University of California, Los Angeles, who not involved with the work. Thanks to an “imaginative use” of both low-altitude observations and the THEMIS satellites, we now better understand Steve’s connection with the planet’s upper atmosphere and its magnetic bubble, a pairing that drives all kinds of beautiful and potentially dangerous space weather effects.
But the case isn’t closed just yet. The new paper only assessed three events–a small sample size–and more data are required before both the picket fence structures and purple streams can be fully explained. It’s not clear, for example, why those streams are purple in the first place, or why the purple and green parts can appear both together and independently.
Still, with this work, “we’re moving, little by little, closer to understanding Steve,” says Gallardo-Lacourt, who also emphasizes that none of this research would have been possible without the work of citizen scientists.
“Although scientists are doing the research for Steve, this really is a discovery by the photographers,” she says, with everyone trying to figure out this mystery together. “For me, this is the most romantic way of doing science.”