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A New Type of Aurora Ripples Across the Sky in Horizontal Green ‘Dunes’

Originally observed by citizen scientists, the unusual light show might help researchers better understand a poorly studied layer of the atmosphere

A new type of aurora, called auroral dunes, produces horizontal, undulating stripes of green that resemble mounds of sand on a beach. (Kari Saari)
smithsonianmag.com

Since time immemorial, people around the world have been beguiled by the wonders of the night sky. But even after millennia of study, the cosmos keeps hitting us with surprises. Reporting this week in AGU Advances, a team of researchers has discovered a brand new type of aurora, thanks to the help of some intrepid citizen scientists.

Described as “dunes,” the spectacular light show manifests as a series of rippling emerald ribbons that extend toward the equator. Unlike typical aurorae, which are oriented vertically like fence posts, the dunes hang horizontally like blinds in a window, giving off the appearance of hills of sand cascading across a beach.

The dunes also occur at an unusual altitude, about 60 or so miles off the Earth’s surface, in a layer of the atmosphere too high to access by balloons and too low to explore with satellites—a sort of scientific no man’s land. This strip of sky, technically known as the mesosphere, is so poorly understood that physicists jokingly dub it the “ignorosphere,” study author Minna Palmroth, a physicist at the University of Helsinki explains in a statement. But sussing out what gives the dunes their dazzling gleam might help researchers figure out what’s going on in the mesosphere as well.

First spotted in October of 2018 by amateur photographers in Finland and Sweden, the dunes quickly caught Palmroth’s attention after images began to pour into her inbox. Struck by the green, finger-like projections that appeared to point toward the equator, Palmroth—who was in the midst of putting together an aurora guidebook—realized they might belong in an entirely new cosmic category.

“They didn’t seem like anything I had seen before,” she told George Dvorsky at Gizmodo.

Auroras occur when solar particles bombard gases in Earth’s atmosphere, spurring the particles to produce colorful light that’s typically most visible near our planet’s north and south poles. The neon green tinge of the dunes suggests they’re being generated by charged-up oxygen atoms banging around the mesosphere. Though Palmroth’s team still isn’t sure what’s sparking the dunes, they think atmospheric waves—undulations of air that act like ripples in a pool of water—could be sloshing oxygen atoms into dense, curving formations that can be energized into producing visible ribbons of light. The auroral dunes, in other words, literally reflect where in the mesosphere oxygen is most dense.

While still a first step, the new study “gives us a great deal of information and … shows us the direction we need to move,” Bea Gallardo-Lacourt, a space physicist at the University of Calgary in Canada who was not involved in the new study, tells AGU News. In 2018, Gallardo-Lacourt led another team of researchers in describing another celestial anomaly called STEVE, originally and incorrectly suspected to be an aurora.

More data on either the mesosphere or the auroral dunes themselves will likely shed some literal light on the other—and perhaps some of the other cosmic phenomena that sparkle through our skies.

“Different auroral forms are like fingerprints,” Palmroth said. “If you see a certain auroral form, you know basically from that form what’s happening further out in space.”

About Katherine J. Wu
Katherine J. Wu

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University. Previously, she served as a Digital Editor at NOVA Next and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.

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