Ghostly white northern lights present new auroral mystery
The whitish-gray glows might be cousins of the mauve light streak called STEVE
Scientists are struggling to explain grayish-white glows sometimes woven through the Earth’s northern lights (seen in these all-sky images).
University of Calgary
Mysterious ribbons of grayish white have been spotted woven through the northern lights.
New images reveal that these ghostly glows can appear tucked within or curled up beside red and green auroras. But the pale lights are not auroras themselves, researchers reported December 30 in Nature Communications. Instead, they may arise through a similar process to the one underlying the mauve light streak known as STEVE.
“These observations remind us just how much we still have to learn about the fundamental processes that shape the aurora,” says Claire Gasque, a space physicist at the University of California, Berkeley not involved in the work.
Researchers at the University of Calgary in Canada first noticed something odd in auroral pictures taken by the Transition Region Explorer, or TREx mission. This Canada-based network of low-light cameras and other equipment is dedicated to sensing the near-Earth space environment. While other auroral imagers collect only certain wavelengths of red or green light, TREx cameras capture full-color images.
One day in 2023, space physicist Emma Spanswick was reviewing some of these pictures with a colleague. “We see this really, really weird thing. It was this kind of gray, white patch,” she says. “We were both like, ‘What is that?’”
After glimpsing similar tufts of grayish white in other images, the team decided to make a sweeping search of past TREx data. They found 30 white-laced auroras over Rabbit Lake and Lucky Lake in Saskatchewan from 2019 to 2023.
All-sky images showed these tendrils of white could span tens to hundreds of kilometers. They sometimes appeared alongside red or green auroras. Other times, white light bloomed in places where colorful auroras had just faded away.
Spectral data confirmed that the whitish light is made up of continuum emission. “When you see a continuum emission, you’ve got a little bit of light at all wavelengths,” Spanswick says. This is different from normal auroras, where particles raining into the atmosphere excite atoms to throw off only specific wavelengths of red or green.
The continuum emission of the whitish northern lights looks a lot like that of STEVE, short for Strong Thermal Emission Velocity Enhancement. STEVE is a band of purplish white that runs east to west across the sky, closer to the equator than typical auroras. It’s thought to arise from a river of extremely fast plasma rushing through the atmosphere, heating particles up to glow.
The newly identified whirls of white among the northern lights might also arise from atmospheric heating. “But what’s doing the heating? And why in this patch, and not the region next to it?” Spanswick says. “We have no idea.”
As someone studying STEVE, Gasque says that “observations of similar emissions in the aurora are fascinating.” The patchiness of the white northern lights, compared with the neat arc of STEVE, suggests the two phenomena are not quite the same, she says. But there may be similar chemistry behind both, which could help solve a longtime mystery about STEVE.
Namely, it’s not yet clear just how the torrent of plasma changes atmospheric chemistry to create STEVE’s light. Investigating similar emissions lurking among auroras, Gasque says, “could provide valuable clues.”
