Scientists have never seen anything like this on Earth before.
By piecing together a series of images, amateur photographers and a group of astrophysicists have discovered a new kind of aurora — auroral dunes — adding a layer of complexity to the already incredible aurora borealis.
It marks the first observation of an extremely rare phenomenon that takes place in Earth’s mesosphere, and gives scientists an unprecedented window into an unexplored area of our planet’s upper atmosphere.
The discovery came when the photographers and aurora enthusiasts noticed something weird while photographing the Northern Lights in action. One auroral form did not fit into any of the established categories of auroras.
Watch the full time-lapse video to see this strange phenomena in action:
The photographers and researchers described their findings in a study published on Tuesday in the journal AGU Advances.
The new Northern Lights
The Northern Lights are a type of aurora, which are phenomena caused by bouts of space weather. As the Sun emits charged particles in the form of solar wind, they excite oxygen and nitrogen atoms in Earth’s ionized upper atmosphere, causing auroral emissions that result in the varying colors of light that we see in the skies here on Earth.
The Northern Lights aren't the only aurora visible on Earth — if you are in the southern hemisphere close to the South Pole, then you might catch sight of its sister aurora, the aurora australis, or the Southern Lights.
Auroras take different forms: They can appear as patches of light, arcs that curve across the sky, or as rays breaking through the sky — and each one has its own fingerprint.
But the newly discovered aurora — dubbed ‘a dune aurora’ due to its sand dune-like shape — did not fit into any of the established categories.
As the video shows, the formation looks like a green-tinged pattern of waves dancing across a relatively unexplored area which lies between the Earth’s atmosphere and the edge of space — the mesosphere — located some 50 kilometers above the planet's surface.
But in order to figure out what this new type of aurora was, the team first had to identify where it was. And to do that, the researchers looked further out into space — to the stars.
A cosmic puzzle
To identify the aurora, the team first mapped the stars that could be seen behind the aurora. By working out their elevation, the team could use them as a point of reference, enabling them to calculate the altitude and other key characteristics of the new auroral phenomenon.
To their surprise, the auroral dunes occur at a relatively low altitude — 100 kilometers — in the upper parts of the mesosphere. The mesosphere is the part of Earth’s atmosphere located between the stratosphere and the thermosphere. Because this area lies where Earth’s atmosphere and space meet, it is extremely difficult for satellites to observe.
"Due to the difficulties in measuring the atmospheric phenomena occurring between 80 and 120 kilometres in altitude, we sometimes call this area 'the ignorosphere'," Minna Palmroth, professor at the University of Helsinki and lead author of the study, said.
The researchers aren't sure what causes dune auroras, but they have some theories. The unfamiliar pattern could either be the result of wave forms in the particles being emitted from the Sun during solar wind events. But they could also be to do with the density of oxygen atoms in Earth’s atmosphere.
Very rarely, a gravity wave rises up through the Earth's atmosphere. And in even rarer cases, these waves can be funneled between the boundary of the mesosphere and the thermosphere. When that happens, so the theory goes, dune auroras may occur when solar wind excites the oxygen atoms in the channel. That releases light — the green, sandy beach-like waves we can see here on Earth. More research will be needed to confirm whether that is the case, however.
The discovery came somewhat by accident: Palmroth had asked for the help of Northern Lights enthusiasts to gather images of the light display for a book she was writing, A guide for aurora borealis watchers, which was published in late 2018.
"It was like piecing together a puzzle or conducting detective work," Matti Helin, a Northern Lights hobbyist who was involved in the discovery, said in a statement. “Every day we found new images and came up with new ideas.”