A girl pointing at the night sky
Amateur astronomers spotted a new auroral feature, nicknamed “the dunes,” on 7 October 2015. Credit: Matti Helin

Earth’s atmosphere is relatively thin—imagine an apple’s skin compared to the fruit itself—but there’s still much to learn about it. Now, using a recently discovered aurora as an atmospheric spotlight, a team of amateur astronomers and researchers has discovered evidence for high-altitude atmospheric waves. This discovery sheds light on the structure of our planet’s upper atmosphere, a region that’s notoriously tough to monitor with either spacecraft or balloons.

Mysterious Green Stripes

Aurorae, formed when energetic particles bombard Earth’s atmosphere, are a captivating sight—their shimmering, waving forms have been recorded for millennia.

“They are always so mysterious and beautiful,” said Matti Helin, an amateur astronomer and photographer in Lieto, Finland, who spotted his first aurora while a teenager.

Helin and other members of the Ursa Astronomical Association, Finland’s amateur astronomy association, regularly observe the sky in search of aurorae. On 7 October 2015, they were treated to a spectacle.

Green stripes covered a wide swath of the sky that night, the amateur astronomers found when they trained their cameras skyward. Green is a common auroral color—it’s associated with oxygen—but aurorae typically resemble arcs, spirals, or curtains. Seeing stripes was a surprise, said Helin. “Nobody knew what they were.” The features dissipated after a few hours, and the sky watchers filed their pictures away.

Three years later to the day, Helin and other Ursa members were gathered online to celebrate the release of a new book about aurorae. The book, cowritten by Minna Palmroth, a space physicist at the University of Helsinki in Finland, features club members’ photography.

Dashing Outside

Out of habit, Helin took a few pictures of the sky during the celebration. He was astonished to find that he had captured the green stripes again, which he hadn’t seen since 2015. “I immediately informed Minna and the others,” he said. Aurora afficionados across Finland and Sweden rushed outside to photograph the elusive feature.

Emma Bruus, a member of Ursa, took pictures from central Finland. It was fun to participate in such a coordinated effort, she said. And it was particularly exciting because the observations ended up launching a research collaboration, said Bruus. “We didn’t know it at the time that we were doing science research.”

“You could imagine you’re actually looking at sandy dunes.”

Bruus and Helin, along with other Ursa members, collaborated with Palmroth and a few of her colleagues to analyze the observations collected on 7 October 2018. By then, the green stripes had acquired a nickname: “the dunes.” (“You could imagine you’re actually looking at sandy dunes,” said Palmroth.)

The dunes are a true aurora, the collaboration believes, unlike the atmospheric phenomenon known as STEVE (strong thermal emission velocity enhancement), which made headlines a few years ago. STEVE is thought to be caused by moving plasma rather than by particles slamming into Earth’s atmosphere.

The team started by analyzing a pair of images of the dunes, both taken at 19:41 local time from two different locations in Finland. Using planetarium software, team members pinpointed the stars behind six of the dunes’ stripes. They then applied trigonometry to calculate that all of the stripes were at an altitude of roughly 100 kilometers, consistent with the altitude of other known auroral features.

A Long-Ignored Region

That part of Earth’s atmosphere, the upper mesosphere, is particularly hard to study. Sending spacecraft there is tough because of the significant amounts of frictional heating that close to Earth. At the same time, it’s too high for most balloons to reach. “It’s a region that is extremely hard to measure,” said Palmroth. For that reason, scientists have jokingly taken to calling this swath of the atmosphere the “ignorosphere.” Perhaps the dunes could shed some literal light on the upper mesosphere, the team hoped.

The researchers started by asking a basic question: What dictates the dunes’ structure? Its stripes are hundreds of kilometers long and typically separated by about 50 kilometers. Something must be varying to explain the aurora’s alternating regions of relative brightness and darkness, the team concluded.

They proposed two hypotheses: Either the flux of particles bombarding the atmosphere (the source) is varying, or the number of oxygen atoms in the atmosphere (the target) is varying.

To test the first theory, the researchers used measurements from orbiting GPS satellites to estimate the vertical total electron content above the dunes. (Electrons constitute a large fraction of the particles that bombard Earth’s atmosphere and produce visible-light aurorae.) They found that the dunes tended to be coincident with regions with higher-than-normal electron contents. However, the limited spatial resolution of the data prevented the team from investigating changes in electron density on the scale of the dunes’ stripes.

They next used ground-based magnetometers to trace the movement of electrons—in other words, electric currents—within Earth’s atmosphere at an altitude below that of the dunes. The team found a pronounced eastward moving current at the location of the dunes.

These measurements reveal enhancements over a wide range of altitudes, the team concluded. But the dunes themselves occur over only a relatively thin swath of atmosphere near an altitude of 100 kilometers. Therefore, it’s unlikely that a difference in the flux of electrons bombarding the atmosphere is responsible for the dunes’ structure, the team suggested.

A Wave in the Sky

That leaves the second hypothesis, the collaboration concluded: The dunes look the way they do because of variations in the number of oxygen atoms in the atmosphere. And a rare atmospheric wave known as a “mesospheric bore” could plausibly be responsible for those variations, the team suggested.

Temperature inversions and wind shear in the atmosphere can trigger mesospheric bores, which manifest as changes in air density that can propagate over long distances. (They’re similar to the tidal bores observed in some rivers.)

“The aurora illuminates this wave which is already in the atmosphere.”

The varying densities of oxygen atoms in mesospheric bores are responsible for the dunes’ characteristic stripes, the team proposed. “The aurora illuminates this wave which is already in the atmosphere,” said Palmroth. It’s the first time that a mesospheric bore has been traced with an aurora, she said. “This is a new phenomenon.”

These results were published last year in AGU Advances on page 1 of issue 1. “We’re really proud to be the first-ever article to be published in AGU Advances,” said Palmroth.

This discovery turns a literal spotlight on mesospheric bores, and it’s surprising to find them so close to Earth’s poles, said Bea Gallardo-Lacourt, a space physicist at NASA Goddard Space Flight Center in Greenbelt, Md., not involved in the research. “It’s a completely novel result.”

This work also highlights the important contributions of science enthusiasts and community groups, Gallardo-Lacourt said. Camera-toting amateurs have an important leg up on stationary research facilities, she said. “They have the advantage of going where the phenomenon is happening.”

—Katherine Kornei (@KatherineKornei), Science Writer

Citation:

Kornei, K. (2021), Auroral “dunes” light up Earth’s atmosphere, Eos, 102, https://doi.org/10.1029/2021EO155491. Published on 05 March 2021.

Text © 2021. The authors. CC BY-NC-ND 3.0
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