Several times a year, around the border between the U.S. and Canada, STEVEs take to the sky. STEVEs aren’t plaid-clad dudes with dreams of flight, but rather a celestial phenomenon that illuminates the sky with an enormous crescent of mauve light and an accompanying row of dashes of lime-green light. And STEVE isn’t a nickname for Steven, but rather Strong Thermal Emission Velocity Enhancement. The physics behind STEVEs has stumped scientists almost as soon as they were officially named in 2017, but a recent study published on April 16, 2019, in Geophysical Research Letters sheds new light on the atmospheric reactions that conjure STEVE’s signature hazy purple ribbon in the sky.
Though STEVEs bear an uncanny resemblance to auroras—which gain their glow from charged particles cascading into Earth’s upper atmosphere—they’re not quite the same thing. In 2018, scientists confirmed that STEVE’s mysterious purple light originated from another mechanism entirely. But no one knew precisely what that mechanism was or how it operated, so they coined it a “skyglow” and called it a day.
Now, scientists understand that the elements of a STEVE originate from two distinct atmospheric phenomenon, writes Toshi Nishimura, a space physicist at Boston University and the lead author of the study, in an email. The researchers analyzed data from satellites passing over eight years of STEVE events. They also contrasted measurements of the electric and magnetic fields in Earth’s magnetosphere against photos of STEVE events to see what caused the mysterious glow.
The team confirmed that a STEVE’s row of green lights—which scientists call a picket fence—arises from a mechanism similar to auroras, where energetic electrons streaming in from space collide with oxygen atoms to emit a green light. Except a STEVE’s green fences occur in atmospheres much closer to the equator than normal auroras.
A STEVE’s purple stripe, however, has a much more alluring origin. The light emerges when charged particles in the ionosphere collide, thus creating friction that heats the particles and causes them to emit light the approximate color of an eggplant. “The purple part is like a stream of excited particles zipping through the ionosphere,” Nishimura says. “Instead of knocking off electrons, this actually generated friction, which heats up the particles and causes them to vibrate and jump about a bit.” He likens the reaction to the process that lights incandescent bulbs, where electricity heats a tungsten filament until it glows.
Though the researchers now know what causes a STEVE’s arc of light, they’re not quite sure why it’s purple. Nishimura hopes to resolve this question in a follow-up study, but he believes nitrogen might be involved, as the element has been known to create a mauve-colored auroral emission in similar altitudes.
Beyond further clarifying the mystery of an atmospheric phenomenon that sounds a lot like a dude from your high school, the study has big implications for radio communication, Nishimura says. The new study reveals that STEVE events are associated with troughs, or holes in the plasma density of the ionosphere. These holes can disrupt radio communication between Earth and space, such as GPS navigation. Spotting a STEVE event help researchers visualize where these holes occur and how they evolve and may even help scientists predict areas of radio communication problems in the future, Nishimura says.
STEVEs are quite common and easy to spot, especially if you live in New England, British Columbia, or New Zealand. Nishimura says that photos of STEVEs taken by citizen scientists proved crucial for the team’s analysis. So if you see a STEVE, snap a picture and you can help scientists like Nishimura better understand this stunning ethereal occurrence.