Rare spiral lights in the Norwegian sky

On Sunday, December 11, rare auroras in the form of a spiral were seen in the Norwegian city of Tromso.

"The sky was clear and deserted, but suddenly, out of nowhere, green lights appeared, forming a giant vortex," photographer Marek Rybar said.

Unlike other forms of auroras, such as ribbon stripes, arcs or crowns, spiral auroras are observed quite rarely, and scientists have been arguing about the reasons for their appearance for more than half a century. To date, it is known that in the northern hemisphere they twist clockwise, and in the southern hemisphere — in the opposite direction. Their sizes can vary from 10 to more than 1000 km. As a rule, they are painted green.

According to one of the first theories, spirals are formed under the influence of Birkeland currents flowing along geomagnetic lines in the upper atmosphere. The following theory connects their appearance with the Kelvin—Helmholtz instability, when the contacting media move at different speeds. The same process leads to the formation of beautiful wavy clouds.

In a new study published last fall, it is assumed that the swirling of the auroras is due to a certain type of plasma instability that occurs when magnetic fields reconnect and explode.