First it was STEVE. Now it’s “dunes.” That’s the name given to a newly recognized form of aurora that resembles rippled sand dunes at a beach. I see spreading cirrus clouds. The discovery found its way into the world through Taivaanvahti , a Finnish Facebook group devoted to sharing photos and information about astronomical and atmospheric phenomena. It has more than 11,000 members including Minna Palmroth, a physics professor at the University of Helsinki. Palmroth studies space weather, the interaction between particle winds from the sun and Earth’s magnetic field.
This video, shot by Kari Saari, University of Helsinki, shows dune-like auroras (upper left) appearing over Savojärvi, southern Finland, in October 2018.
Palmroth recently published Revontuli — Bongarin Opas, an auroral field guide. “Revontuli” is the Finnish word for northern lights. After publication, a few of the Facebook group contributors still had questions about a type of aurora not mentioned in the guide. Most auroral displays exhibit a mix of arc-like and vertical forms, but several Finnish amateur photographers picked up on a new form in their photos they dubbed “the dunes,” which spread out horizontally like fingers.
“Some of the citizens saw the dunes in the sky, and they told me I had excluded one auroral form from the book,” Palmroth said. Like the related but non-auroral phenomenon STEVE, the dunes were discovered by amateur aurora hunters.
On seeing the images, Palmroth was curious about the dunes’ altitude. Knowing their height could help to explain what causes the phenomenon, so she suggested that multiple observers photograph the dunes from widely separated locations in Finland at the exact same time. By measuring the shift in the aurora’s position against the background stars from the different locations, scientists could triangulate its altitude.
Two of the group’s photographers spaced 75 miles (120 km) apart captured the dunes at the at the exact same second. That allowed the space weather researchers to pin the altitude of the dunes to a thin layer at around 60 miles (100 km) high. Palmroth and her colleagues suspect the dunes are visible manifestations of a kind of atmospheric gravity wave called a mesospheric bore.
Gravity waves are undulations of air caused by disturbances in the lower atmosphere such as airflow over a mountain range or a frontal system. The bores or waves propagate through the atmosphere,like ripples spreading in a pool and are ducted into a narrow layer in the mesosphere, the layer of air above the stratosphere.
Dunes occur where there’s a lot of electrical activity created by the arrival of electrons and protons from the sun. Collisions with air molecules release the red and green light of the aurora, but the impacts also heat the air. Palmroth and her colleagues suspect that heating from those collisions may be linked to the creation of the mesospheric bores.
“We now suggest that this additional energy from Joule heating (an electric current that produces heat when passing through a conductor) could give you the circumstances where the bores can originate… and this is a totally new topic,” she said. “We are rather excited.”
Is that the understatement of the year?
Now that we know about the dunes or feathers or bores (whatever you’d like to call them!) we can anticipate their appearance in upcoming displays of the northern lights. No auroras are forecast for middle latitudes at the moment, but that could change in March, a traditionally good time for activity. Keep watch with your eye and your camera!