Here’s what causes the aurora-like glow known as STEVE

We’re one step closer to understanding the mysterious atmospheric light show called STEVE.

Short for Strong Thermal Emission Velocity Enhancement, STEVE is an unusual type of sky glow that appears closer to the equator than auroras (SN: 4/14/18, p. 5). Unlike the shimmery green ribbons that make up the northern lights, STEVE consists of a mauve band of light that stretches east to west, sometimes accompanied by a row of vertical green stripes called the picket fence.

Analysis of celestial photos and satellite data reveals that heated atmospheric particles produce STEVE’s mauve ribbon, while electron showers from space create the picket fence, researchers report online April 16 in Geophysical Research Letters.

Citizen scientists have been snapping pictures of STEVE events for years, but only now are scientists beginning to understand the atmospheric conditions that create these sky glows, says Don Hampton, a space physicist at the University of Alaska Fairbanks, who wasn’t involved in the study. Learning more about such atmospheric processes may help researchers anticipate the effects of space weather on satellite signals, he says.

Space physicist Yukitoshi Nishimura of Boston University and colleagues analyzed data from satellites that passed near two STEVE events in 2008 and 2016. These satellites observed particles and electromagnetic waves in the atmosphere and near-Earth space.

Nishimura’s team found that STEVE’s purple smear arises from a westward-flowing stream of plasma. Charged particles in the plasma, flowing at about 5 kilometers per second, heat other atmospheric particles through friction and cause these particles to emit purple light. “Different [chemical] species in the atmosphere create different colors of glow,” Nishimura says, but the researchers don’t yet know which molecules produce this purple light.

“With the picket fence, the story is a little bit different,” says study coauthor Bea Gallardo-Lacourt, a space physicist at the University of Calgary in Canada. Energetic electrons rain down from space and excite oxygen molecules in the atmosphere, making them glow green. This is similar to the process behind typical auroras, but electrons don’t typically bombard the atmosphere so close to the equator (SN: 8/9/14, p. 32).

“Something special is happening” at the latitudes where STEVE appears that allows electrons to tumble into the atmosphere and create the picket fence, Gallardo-Lacourt says, but researchers will need to analyze more STEVE events to tease out exactly what’s going on.