Some spectacularly lucky timing helped a number of amateur photographers in Southwest Finland capture a new and very rare form of the Northern Lights that doesn't fit into any pre-existing categories. They're calling them "dunes."
The beautiful auroral emissions that have delighted and entranced humans since the dawn of time are the result of solar winds – charged particles released from the sun that excite oxygen and nitrogen atoms in the ionosphere, high above the ground. Typically, they appear in one of three forms: as arcs, bands, or pillars. Sometimes they can appear as coronas, but generally this is just the result of standing under a pillar structure.
Now, there's another. The dune-shaped emissions shown here have been recognized as a new form of aurora after being photographed by amateur enthusiasts.
Minna Palmroth, Professor of Computational Space Physics at the University of Helsinki, has been working on developing the world's most accurate simulations of the space weather and near-earth conditions required to kick off an auroral emission. In 2018, she also published a book for aurora watchers, assembled from thousands of photos taken by hobbyists she was communicating with on an aurora-watching Facebook page.
In the process of categorizing this trove of images, Palmroth and the hobbyists began noticing a few images that didn't fit the pre-existing categories. She put them aside to look at later. Then, just days after the publication of her book in 2018, messages started coming in saying these unusual forms were happening again.
"One of the most memorable moments of our research collaboration was when the phenomenon appeared at that specific time and we were able to examine it in real time," says Northern Lights and astronomy hobbyist Matti Helin. The group describes the new form as "a green-tinged and even pattern of waves resembling a striped veil of clouds or dunes on a sandy beach."
From there, research work began to try to isolate the conditions leading to these sky dunes, and over time the culprit materialized: increased oxygen atom density caused by gravity waves bending to run through a mesospheric bore – a channel that forms between the mesopause and the inversion layer below it. When this high concentration of oxygen is hit by solar wind, the shower of electrons excites the oxygen atoms, which release the auroral light.
The finding could shed new light on how the upper atmosphere interacts with electromagnetic energy coming in from space.
"The energy transmitted from space to the ionosphere may be linked with the creation of the inversion layer in the mesosphere," says Palmroth. "In terms of physics, this would be an astounding discovery, as it would represent a new and previously unobserved mechanism of interaction between the ionosphere and the atmosphere."
Enjoy the dune aurora in the short video below.
Source: University of Helsinki
Seems like some clever physicist somewhere ought to be able to turn this phenomenon into a gravity wave detector - one much smaller than the LIGOs.