The northern lights have been seen in many forms, including "silent arches", spirals, crowns, and they have long captivated amateurs and scientists alike.
In fact, astronomer Galileo Galilei coined the term ‘aurora’ in Latin. Auroras occur when charged particles, such as electrons, are ejected by the sun and channelled to the Earth's poles by our planet’s magnetic field.
There they interact with the gases in the atmosphere, increasing their energy and subsequently releasing it in the form of light.
This new form of aurora is caused by waves of oxygen that flow through a stream of solar particles. The light show is dyed uniformly green and extends for more than 48 kilometres and up to almost 100 kilometres, in the mesosphere.
Space physics professor at the University of Helsinki (Finland), Minna Palmroth, leads a research group that develops the most accurate simulation of the world of space and near-Earth space weather that cause auroral emissions.
Palmroth asked a team of observers to send her pictures for a book and one of them highlighted the images of an aurora that did not seem to fit any known type and simply called them 'auroral dunes'.
Each auroral form resembles an atmospheric fingerprint, formed by specific conditions in the atmosphere. As the name implies, the green-coloured lights dance in the northern sky like dunes spread across sandy beaches in an undulating pattern. Shortly after the discovery, a similar dawn appeared and was later photographed in two different places in southwestern Finland.
Experts traced its origin to a waveguide formed within the mesosphere and its limit, known as the Karman line. The study also reveals that this new form of aurora provides researchers with a renewed way to investigate the qualities of the upper atmosphere.
PhD researcher, Maxime Grandin, who is part of Palmroth’s team, identified the stars and determined their azimuths and elevations with the help of the Stellarium astronomy software program.
Stellarium is an open-source free software that has made it possible to use the stars as benchmarks when calculating the altitude and extent of the aurora. The team detected a total of seven similar 'auroral dunes ' events and a camera recorded the same uniform wave pattern in all of them.
“The waves have different frequencies, different wavelengths, different amplitudes and thus, to observe something very even, like the dunes, means that there has to be some active mechanism which is making them so even,” explains Palmorth.
What could be causing this new aurora shape?
Palmroth noted that the mechanism could be an unusual event called a mesospheric bore, through which a gravity wave is filtered and bent, allowing it to travel horizontally between two layers in the atmosphere.
According to Palmroth, this can happen when the temperature of one layer of the atmosphere is slightly higher than the two layers above and below. The dune appearance is due to the wave that produces peaks and valleys in oxygen density.
With the help of measuring devices controlled by the Finnish Meteorological Institute, her team discovered that the dunes occur simultaneously and in the same region where the electromagnetic energy that originates in space is transferred to the mesosphere.
“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,” adds Palmroth.
“In terms of physics, this would be a surprising discovery, as it would represent a new and previously unobserved mechanism of interaction between the ionosphere and the atmosphere.”
Obstacles and challenges
The part of the auroral zone where Earth’s electrically neutral atmosphere meets the edge of space is an extremely challenging environment for satellites and other space instruments.
That is why it is one of the least studied places on our planet. The interaction between the neutral atmosphere and the electromagnetic ionosphere have different observation techniques. The biggest obstacle is that it is too high to be reached by radars and balloons and too low to be observed by a spacecraft.
Reference: Citizen scientists discover a new auroral form: Dunes provide insight into the upper atmosphere” by M. Palmroth, M. Grandin, M. Helin, P. Koski, A. Oksanen, M. A. Glad, R. Valonen, K. Saari, E. Bruus, J. Norberg, A. Viljanen, K.4 Kauristie, and P. T Verronen. AGU Advances 1, 1-12, xx.1.2020. DOI: 10.1002/aga2.20017