STEVE: Mysterious Sky 'Aurora' Is Actually an Entirely New Celestial Phenomenon

Spectacular, glowing ribbons of purple and white light that sometimes appear in the night sky are actually an entirely new celestial phenomenon, not a type of aurora as some previously thought. That's according to scientists.

The streams of light, dubbed STEVE, have been photographed by amateur skywatchers for decades, but it is only in the last couple of years that scientists have become aware of them and begun investigating the phenomenon.

While STEVE looks similar to aurorae, such as the northern and southern lights, a study published in the journal Geophysical Research Letters suggests that it is caused by a different atmospheric process, although what is actually generating it remains a mystery.

For the study, researchers from the University of Calgary and the University of California examined a STEVE event in 2008 to understand whether it was produced in a similar manner to aurorae. These occur when solar winds collide with gases in the Earth's ionosphere—a region of the upper atmosphere filled with charged particles—emitting a variety of dazzling colored lights.

"Our main conclusion is that STEVE is not an aurora," Bea Gallardo-Lacourt, a space physicist at Calgary and lead author of the study, said in a statement. "So right now, we know very little about it. And that's the cool thing, because this has been known by photographers for decades. But for the scientists, it's completely unknown."

In 2016, a group of photographers known as the Alberta Aurora Chasers—who had noticed white and purple lights in the Canadian sky, thinking they were simply part of the aurorae—brought STEVE to the attention of researchers.

While it resembles a typical aurora and has so far only been spotted in the presence of one, there are some key differences. STEVE has only been observed in certain seasons, unlike aurorae, and it also appears closer to the equator. It takes the form of a single, narrow arc of purple and white light extending for hundreds or thousands of miles, aligned east-to-west. In contrast, the northern and southern lights are much broader and usually consist of green, blue and red hues.

The photographers named the phenomenon in reference to a scene in the 2006 animated film Over the Hedge in which the animal characters encounter a hedge for the first time and decide to name it Steve. Subsequently, scientists proposed converting the name into the "backronym" STEVE, which now stands for Strong Thermal Emission Velocity Enhancement.

Initially, the aurora chasers thought the ribbons of light were being generated by excited protons, but these can only be photographed with special equipment because the wavelengths they emit cannot be picked up by normal cameras.

Then, in March this year, the first scientific study was published on STEVE based on satellite data and images from ground-based observatories, casting new light on the phenomenon. The results showed that a stream of fast-moving ions—charged atoms or molecules—and super-hot electrons passed through the ionosphere right where a STEVE event was taking place.

While the research team—which also involved Gallardo-Lacourt—suspected that these particles were connected to the event somehow, limits to the data meant that it could not be determined whether the observed phenomenon was being generated by particles raining down into the atmosphere, like normal aurorae, or a different—and possibly undocumented—process in the ionosphere.

For the latest study, Gallardo-Lacourt and her colleagues analyzed a STEVE event that took place over eastern Canada in March 28, 2008, using images from ground-based cameras that record aurorae over North America. The scientists also combined this information with data collected by a National Oceanic and Atmospheric Administration satellite that happened to pass directly over the cameras during the STEVE event.

Alberta Aurora Chasers capture STEVE, the new-to-science upper atmospheric phenomenon, on the evening of April 10 in Prince George, British Columbia, Canada. Ryan Sault

The results showed no evidence that charged particles were raining down from the upper atmosphere during the STEVE event, meaning the lights are likely produced by an entirely different mechanism to normal aurorae. Instead, the scientists describe STEVE as a kind of "skyglow."

Studying STEVE will help scientists to better understand the upper atmosphere and the processes that generate light in the sky.