Seeing the northern lights ranks highly on many people's bucket lists, but did you know that Finnish amateur photographers working together with space researchers have discovered a new type of aurora? The lights have been termed “dunes,” because they appear as an even, green-tinged pattern of waves resembling a striped veil of clouds or dunes on a sandy beach.
This differentiates them from typical aurorae, which are oriented vertically, and they occur at an unusual altitude, about 100km from the Earth’s surface. They came to the attention of Minna Palmroth, a computational science physics professor at the University of Helsinki, who was interacting with northern lights enthusiasts in a Facebook group while publishing a book entitled 'Revontulibongarin opas' – a guide for aurora borealis watchers. Thousands of photographs of the lights taken by amateur photographers were examined for the book, and during the classification, Minna and the photographers realised that a certain auroral form did not fit into any of the pre-existing categories of aurorae.
Investigations were launched into the phenomenon and a study was conducted that has been published in the journal AGU Advances. It was found that the auroral dunes occur in the upper parts of the mesosphere, which is a layer of the atmosphere that is too low to explore with satellites and is not well studied. Palmroth’s team is unsure of what is causing the dunes to occur but feels that they may reflect where oxygen is most dense in the mesosphere. "The differences in brightness within the dune waves could be due to either waves in the precipitating particles coming from space, or in the underlying atmospheric oxygen atoms," she says. "We ended up proposing that the dunes are a result of increased oxygen atom density."
You can read the full study in AGU Advances here.