Lately, skygazers have noticed something odd appearing in Canada’s night sky: a thin, purple ribbon of light waving in the darkness. The cause of the phenomenon, which scientists have dubbed STEVE, continues to remain a mystery.

Observers originally thought STEVE was part of the aurora borealis—colorful bands of light in the nighttime sky that usually occur in extreme northern latitudes. Auroras occur when charged particles from the sun and Earth’s upper atmosphere interact with gas molecules in the lower atmosphere. These collisions cause the molecules to glow in an array of colors.

But recently, a passing satellite got an in-depth look at STEVE and found no traces of the charged particles that cause auroras. “Although STEVE resembles an aurora, it does not have the same physical characteristics,” says Bea Gallardo-Lacourt. She’s a space physicist at the University of Calgary in Canada. “But the two phenomena may be related. We recently found that STEVE tends to occur about an hour after very intense auroras.”