Citizen science helps find ‘auroral dunes’

Atmospheric science meets space science in a particularly difficult area.

The auroral dunes appear as a green-tinged and even pattern of waves resembling a striped veil of clouds or dunes on a sandy beach.
CREDIT: Kari Saari

By Nick Carne

Hobbyists and space researchers working together in Finland say they have discovered a new auroral form they call “dunes”, providing a novel way to investigate conditions in the upper atmosphere.

Writing in the journal AGU Advances, they suggest the phenomenon is caused by waves of oxygen atoms glowing due to a stream of particles released from the Sun.

And its initial discovery came about through a significant stroke of luck.

In 2018, Minna Palmroth, Professor of Computational Space Physics at the University of Helsinki, worked with Northern Lights enthusiasts and amateur photographer to compile a book entitled Revontulibongarin opas (A guide for aurora borealis watchers).

Thousands of pictures were surveyed and categorised, and during the process hobbyists pointed out that a certain auroral form did not fit into any of the pre-existing categories. Palmroth set this aside for later consideration.

By chance, just days after the book was published, the hobbyists saw this unusual form again and immediately informed her. Investigations were launched.

The form appears, the paper says, as a green-tinged and even pattern of waves resembling a striped veil of clouds or dunes on a sandy beach. It was photographed at the same time in Laitila and Ruovesi, both in southwest Finland, with the same detail observed each time.

Palmroth’s colleague Maxime Grandin identified stars behind the emission and determined the azimuths and elevations of the stars with the help of the astronomy software program Stellarium. This made it possible to use the stars as points of reference when calculating the altitude and extent of the auroral phenomenon.

The dunes were seen in a part of the auroral zone where the Earth's electrically-neutral atmosphere meets the edge of space, making it a challenging environment for satellites and other space-borne instruments. Palmroth says it is one of the least studied places on our planet.

"Due to the difficulties in measuring the atmospheric phenomena occurring between 80 and 120 kilometres in altitude, we sometimes call this area the ignorosphere," she says.

The study suggests that what they saw was a mesospheric bore, a rare and little-studied phenomenon that takes place in the mesosphere. A tidal bore is a wave common to many rivers, where the tide travels up the river channel.

Prior to this discovery, mesospheric bores were not observed in the auroral zone, the researchers say, nor have they been investigated via auroral emissions.

"The auroral zone as a whole is usually discounted in studies focused on the bore, as auroral emissions impair the technique used to identify mesospheric bores," says Palmroth.

With the help of measuring devices operated by the Finnish Meteorological Institute, the dunes were found to occur simultaneously and in the same region where the electromagnetic energy originating in space is transferred to the “ignorosphere”.

"This could mean that 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."

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