Particle physics laboratory

Mysterious STEVE light emissions emanate from Earth’s magnetosphere


Photograph of the STEVE events of July 17, 2018. Credit: Neil Zeller

For years, dawn watchers across Canada have noticed mysterious streaks of pale purple and green light that appear to dance across the night sky. But it wasn’t until 2016 that they shared their colorful images with scientists, who quickly identified the light show as a new kind of phenomenon in the upper atmosphere jokingly called STEVE. Additional contributions from Citizen Scientists are now helping researchers identify different types of STEVE emissions and determine how and where they are generated in Earth’s atmosphere.

“Scientists have known for several years that STEVE is not a regular type of aurora because it appears at lower latitudes and has different colors,” says Xiangning Chu, a researcher at the University’s Atmospheric and Space Physics Laboratory. from Colorado to Boulder. “But exactly how these light emissions form in Earth’s atmosphere remains a mystery.”

STEVE, who received the backronym Strong improvement in thermal emission speed, usually appears as a narrow purple band that crosses the sky. This ribbon is sometimes accompanied by rippling green lights that look like a palisade in the photos. Because the purple and green lights don’t always appear together, Chu wanted to determine if the two lights shared the same conductor, but he wasn’t sure how to do it, until a happy coincidence happened on July 17, 2018. .

That night, two citizen scientists 400 kilometers apart took images of the same STEVE event. Because the photos were taken simultaneously from two different angles, Chu and his team were able to perform a simple triangulation, which provided them with the exact location of each point in the images.

The researchers then analyzed this information in an attempt to resolve the location of STEVE’s driver. “We weren’t sure whether we should look in a part of the ionosphere, 100-400 kilometers above the Earth, or in the magnetosphere about 30,000 kilometers above the surface,” says Chu.

The researchers already knew that the green “stakes” follow the magnetic lines of the Earth. What Chu’s team analyzes show is that the purple and green emissions are located along the same magnetic field lines, albeit at different altitudes. Because they follow the same field lines over a wide elevation range, Chu says there is no known local mechanism in the ionosphere that can explain them.

Analysis of photos of Citizen Scientists from a STEVE 2018 event shows that purple and green light emissions occur at different altitudes but along the same magnetic field lines. This indicates that they are being driven by the same region of the magnetosphere nearly 30,000 kilometers above the Earth’s surface. Credit: Neil Zeller via Chu et al. 2020.

“Our analysis indicates that the purple and green STEVE emissions are driven by the same narrow region, so it must be the magnetosphere,” he says. “This supports our previous study, which provided evidence that the STEVE pilot region is located at a sharp boundary of the magnetosphere marked by strong waves and particle acceleration. “

One of the fundamental topics of space physics is the coupling of the ionosphere and the magnetosphere, although this process is generally only considered at high latitudes, where auroras occur. But Chu says their results indicate that strong coupling could also occur at lower latitudes, carrying large amounts of energy and momentum between the magnetosphere and the ionosphere.

According to Chu, this research would not have been possible without the help of citizen scientists. “I want to thank Neil Zeller and Colin Chatfield, who were so kind in providing their photographs for this analysis,” he says. “I encourage other citizen scientists to submit their images to the Alberta Aurora Hunters FaceBook group.

There are still many unresolved questions about STEVE, including how it is generated, Chu says, and citizen scientists will likely continue to play an important role in this research. “We are limited by the number of STEVE events and appropriate observations,” explains Chu. “This is a whole new mechanism that has never been reported before. If we can fix this, we’ll have more information to add to our manuals.

Chu go gift the team’s analysis on Wednesday, December 15 at the American Geophysical Union’s fall 2021 meeting. Its co-authors are Lukas Wolter, David Malaspina, Laila Andersson, Martin Connors, Colin Chatfield and Neil Zeller.