In October 2018, several aurora enthusiasts who regularly photograph the shimmering lights noticed something unusual.
Skywatchers in Finland are no stranger to auroras.
These dramatic nighttime light shows typically grace the Finnish skies roughly 200 nights out of the year. But in October 2018, several aurora enthusiasts who regularly photograph the shimmering lights noticed something unusual.
Where the lights of regular auroras are typically arranged vertically to create a rippling, curtain-like effect, the skywatchers instead saw luminous green arcs structured horizontally and extending hundreds of miles toward the horizon.
Now, new research has found that these unusual displays are a new type of aurora, nicknamed "the dunes" because their appearance in the night sky resembles wind-sculpted ridges. Scientists say studying how these auroras form could help them better understand the process by which charged particles from space interact with Earth's upper atmosphere.
"There are all kinds of things that happen in an aurora that are signs of what is happening in outer space," said Minna Palmroth, a space physicist at the University of Helsinki and lead author of a study published Tuesday in thejournal AGU Advances that details the dunes. "The auroras are like fingerprints."
Auroras occur when electrons ejected from the sun bombard Earth's magnetic field and interact with gases in the planet's atmosphere to create the stunning green, purple, red and blue lights of the northern and southern lights.
Palmroth and her colleagues studied the dunes and noticed that in addition to their horizontal alignment, the lights appeared to undulate like a wave in the night sky.
The researchers think the dunes may get their appearance from so-called atmospheric waves in a layer of the Earth's upper atmosphere, 50 to 75 miles above the surface. It's a region of the atmosphere that is not well studied, but Palmroth said the dunes could offer some unique insights.
"When electrons from space collide with atmospheric oxygen in denser parts of the wave, they emit more light," she said. "In the wave's truss, you don't see as much light."
Palmroth thinks this slice of the atmosphere, which encompasses the boundary between the planet's atmosphere and the cosmos beyond, is hypersensitive to changes in solar activity. Energy transferred in this part of the atmosphere when charged particles collide with atmospheric oxygen could explain why the dunes behave differently from other auroras.
More research is needed to confirm how the dunes form, and Palmroth and her colleagues hope their follow-up studies could include observations from a spacecraft.
Palmroth, who published a guidebook in October 2018 for aurora enthusiasts, said she was first alerted to the existence of the dunes by amateur skywatchers across Finland. Though she has not witnessed the dunes herself, she worked closely with seasoned aurora photographers to study the light displays.
Since Palmroth and her colleagues published their findings, they have been contacted by skywatchers in the U.S. and Canada who have reported other instances of the dunes, which could prove useful as the scientists look to expand their research.
For now, though, Palmroth is hopeful that her research will allow her to witness the stunning lights in person.
"Perhaps I'll get to go on an aurora tour and see them soon," she said.