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A collaboration between photo enthusiasts and Finnish astrophysicists has allowed us to identify a new form of northern lights that draws patterns similar to sand dunes. The finding, described in the first issue of the journal AGU Advances , has been located in a region of the atmosphere little studied so far, the mesopause, which lies between the mesosphere and the ionosphere , on the border with outer space.
The discovery of the northern dunes began to take shape when Minna Palmroth, professor of Computational Space Physics at the University of Helsinki, was invited to participate in a Facebook group of auroras fans, to answer questions about the origin of the different variants of this phenomenon. The experience led him to write a book and, as part of the project, he asked the members of that same community to capture images with unusual forms and send them to them . Among them appeared an unknown pattern in the scientific literature.
This new type of aurora was identified in a region where the atmosphere is electrically neutral, on the border with space . It is an extremely difficult environment for satellites and other space instruments, so the authors remember that it is the least studied places on our planet. "Due to the difficulties in studying the atmospheric phenomena that occur between 80 and 120 kilometers of altitude, we sometimes call this area the ignorosphere," Palmroth explains.
The northern dunes were photographed at the same time in Laitila and Ruovesi, two locations in southwestern Finland. The researchers identified the stars behind the emissions and used them as a reference point. From their angles and elevations, they were able to calculate the altitude and extent of the phenomenon, placing it exactly in the ignorosphere.
"It has been a bit like putting together a puzzle, almost like doing detective work, every day we found new images and we came up with new ideas, " says Matti Helin, one of the astronomy fans who participated in the discovery. "One of the most special moments of our collaboration was when we could study it in real time, while the dawn was taking place."
Atmospheric tides
Polar auroras - called borealis in the northern hemisphere, southern in the south - occur when electrically charged particles from the sun (the so-called solar winds) are channeled to the poles by the magnetic field that envelops the Earth. There they interact with the gases in the atmosphere, including oxygen and nitrogen, increasing their energy until the charge is released in the form of light particles.
At the levels of the atmosphere where they appear there are usually many different types of gravity waves, which travel in different directions at different wavelengths. All this generates chaotic forms and it is very unusual for repeated patterns as uniform as those exhibited by the dunes. For that reason, at first the regularity of the forms puzzled its discoverers .
But his research has shown that the dunes are the result of mesospheric tides, a rare and poorly studied phenomenon. The behavior of these tides is similar to that which occurs at the mouth of some rivers, when the waves move against the direction of the flow. Only in this case it is waves of luminous particles. "We believe that these differences in brightness within the waves that produce the shapes of the dunes are the result of the higher density of oxygen atoms," says Palmroth.
"The region where the auroras appear is often avoided in many studies, since the same auroral particles interfere with the techniques we use to study the atmosphere, " adds the researcher. But, with the help of measuring devices operated by the Finnish Meteorological Institute, the team has been able to track the dunes and see that they occur in the same region where electromagnetic energy originated in space is transferred to the ignorosphere. "This could mean a new mechanism of interaction between the ionosphere and the exosphere that had not been observed so far," he says.
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