New insights on how STEVE lights up the night sky

Citizen scientists and satellite data reveal the phenomena behind the odd purple and green glow

Satellite data are helping scientists figure out what’s going on with STEVE, a weird purple streak with green stripes that can glow in the night sky.

Lauri Kangas

We’re one step closer to explaining STEVE. This is an odd glow that can light up the night sky. Unlike the shimmery green ribbons that make up the northern lights, STEVE consists of a reddish-purple band of light. Stretching from east to west, it sometimes has a companion row of vertical green stripes. They’re commonly called a “picket fence.”

STEVE is short for Strong Thermal Emission Velocity Enhancement. Citizen scientists have been snapping pictures of STEVE for years, notes Don Hampton. He’s a space physicist at the University of Alaska Fairbanks. But only now, he says, are scientists starting to figure out how STEVE forms. This knowledge may help researchers predict the effects of space weather on satellite signals, he says. Heated particles in the air produce STEVE’s purple ribbon, new satellite data reveal. And electron showers from space create the green picket fence.

Researchers reported the new findings online April 16 in Geophysical Research Letters.

Yukitoshi Nishimura is a space physicist at Boston University in Massachusetts. He was part of a team that analyzed data collected by satellites that had passed by STEVE events in 2008 and 2016. Those satellites observed particles and waves of energy around the light shows.

Nishimura’s team confirmed that a stream of plasma — electrically charged gas — gives rise to STEVE’s purple smear. That plasma flows west at about 5 kilometers (3 miles) per second. Its flow creates friction. And that heats particles in the air, which causes some to emit a purple light. Different chemicals in the air create different glowing colors, Nishimura notes. His team is not yet sure which cause the purple glow.

“With the picket fence, the story is a little bit different,” says Bea Gallardo-Lacourt. She is a space physicist at the University of Calgary in Canada. When energetic electrons rain down from space, they transfer some of their energy to oxygen molecules in the sky. The excited oxygen glows green.

This is similar to the process behind other auroras, like the northern lights. But electrons don’t usually bombard Earth’s atmosphere where STEVE forms, which is closer to the equator than other auroras. “Something special is happening” at the latitudes where STEVE appears, Gallardo-Lacourt says. Researchers will need to analyze more STEVE events to tease out more details.

Maria Temming is the staff reporter for physical sciences, covering everything from chemistry to computer science and cosmology. She has bachelor's degrees in physics and English, and a master's in science writing.

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