Finnish amateur astronomers have discovered a new kind of aurora, similar in appearance to stratus clouds or sand dunes. Their existence is presumably associated with high-altitude atmospheric waves, the press service of the University of Helsinki said on Wednesday, citing an article in the AGU Advances magazine.
"The differences in brightness between the 'waves' of these celestial dunes may be associated with the existence of similar fluctuations in the streams of particles falling to the Earth from space, or in the distribution of oxygen atoms in the planet's atmosphere. We assume that the latter theory is closer to the truth." said Professor of the University of Helsinki Minna Palmroth, whose words are quoted by the university's press service.
Auroras are a product of the interaction between the Earth's magnetosphere and the solar wind - a stream of plasma and charged particles that the Sun emits into space. These particles move along the lines of force of the planet's magnetic field, the "legs" of which are at the poles of the Earth.
Due to this arrangement of field lines, charged particles, which usually do not enter the atmosphere, freely penetrate into it at the poles, colliding with nitrogen and oxygen molecules, which causes a blue-green glow, which we call the northern or polar lights.
In recent years, amateur photographers helping NASA study these phenomena have begun talking about a new type of flare in the Arctic sky. A year ago, these outbreaks, similar to a purple rainbow, were investigated in detail by scientists. They found that they are generated by a unique combination of processes associated with the activity of the Sun and the movement of ion streams in the Earth's atmosphere.
Polar dunes
Palmroth and her colleagues discovered another type of polar flares of light after several amateur astronomers approached them for help. They saw several flashes of light in the night sky of Finland at once, which did not fit into the well-known categories of auroras and were not similar to STEVE - purple rainbows.
Scientists and amateur astronomers joined forces and began to observe. They called these flashes dunes. Images from different points helped the researchers understand that such a glow occurs in the upper layers of the mesosphere at a relatively low altitude - about 100 kilometers. This part of the atmosphere, as Palmroth notes, has hardly been studied before - it is extremely difficult to observe the region separating the electrically neutral part of the atmosphere from the ionosphere. For this reason, physicists cannot yet say for sure what exactly gives rise to the polar dunes.
On the other hand, the distance between the crests of the "wave" of this type of aurora (about 45 kilometers) suggests that the flares are associated with the existence of special mesospheric waves. So scientists call very large fluctuations in the air that occur in the atmosphere due to the presence of irregularities on the surface of the Earth. Sometimes they "rise" to the level of the boundary between the ionosphere and the rest of the Earth's air envelope.
These waves, Palmroth and her colleagues suggest, create irregularities in the distribution of oxygen atoms in the mesosphere. In turn, their collisions with "cosmic" electrons falling into the atmosphere from the ionosphere and space, cause oxygen to create these flashes of light. Differences in the concentration of this gas, scientists believe, are the reason why polar dunes are similar in structure to cirrus clouds or wave crests of their counterparts from terrestrial deserts.
If this theory is confirmed, then such auroras can be used to study atmospheric waves that arise in the mesosphere and other hard-to-reach areas of the Earth's air shell, as well as the phenomena that generate and maintain their existence. Further observations of the polar dunes, scientists hope, will answer this question.
