For millennia, humans in high latitudes have been fascinated by the Northern Lights – the Northern Lights and the Southern Lights. Yet even after all this time, it seems the ethereal, dancing ribbons of light above Earth still hold secrets.
In a new study, physicists led by the University of Iowa report a new feature to Earth’s atmospheric light show. Looking at a video taken nearly two decades ago, the researchers describe several instances where a section of the diffused aurora – the faint background glow accompanying the brighter light commonly associated with aurorae – becomes dark, as if it was cleaned by a giant blotter. . Then, after a short period of time, the blackened section suddenly reappears.
The researchers say the behavior, which they call “diffuse auroral gums,” has never been mentioned in the scientific literature. The results appear in the Journal of Geophysical Research Space Physics.
The Northern Lights occur when charged particles from the sun – called the solar wind – interact with the Earth’s protective magnetic bubble. Some of these particles escape and fall towards our planet, and the energy released when they collide with gases in the Earth’s atmosphere generates the light associated with the auroras.
“The greatest thing about these erasers that we didn’t know before, but know now, is that they exist,” says Allison Jaynes, assistant professor in the Department of Physics and Astronomy at the ‘Iowa and study co-author. “This begs the question: is this a common occurrence that has been overlooked or is it rare?
“Knowing that they exist means that there is a process that creates them,” Jaynes continues, “and it may be a process that we haven’t yet begun to examine because we never knew that they have been performing so far. “
It was on March 15, 2002 that David Knudsen, a physicist at the University of Calgary, set up a video camera in Churchill, a town on Hudson’s Bay in Canada, to film the Northern Lights. Knudsen’s group were a little disheartened; the forecast was for clear, dark skies – normally perfect conditions for viewing the Northern Lights – but no dazzling illumination was occurring. Still, the team used a camera specifically designed to capture low-level light, much like night vision goggles.
Although scientists only saw most of the darkness as they gazed upward with their own eyes, the camera picked up all manner of auroral activity, including an unusual sequence where areas of the diffuse aurora were disappearing, and then were coming back.
Knudsen, watching the video as it was recorded, scribbled in his notebook, “a pulsating ‘black’ diffuse glow, which then filled in several seconds.”
“What surprised me, and what made me write it down in the notebook, was when a patch cleared up and went out, the diffuse background aurora was erased. It’s gone,” says Knudsen, a Fort Dodge, Iowa native who has studied Aurora for over 35 years and is a co-author of the study. “There was a hole in the diffuse aurora. And then that hole would fill up after about half a minute. I had never seen anything like this before.”
The note remained dormant and the video was not investigated, until Jaynes from Iowa turned it over to graduate student Riley Troyer for investigation. Jaynes learned of Knudsen’s recording at a science meeting in 2010 and referred to the gum note in his doctoral dissertation on diffuse auroras a few years later. Now at college in Iowa, she wanted to learn more about the phenomenon.
“I knew there was something there. I knew it was different and unique, ”says Jaynes, assistant professor in the Department of Physics and Astronomy. “I had some ideas on how this could be analyzed, but I hadn’t done it yet. I handed it to Riley, and he took it a lot further by finding his own way to analyze the data and come up with meaningful conclusions.
Troyer, of Fairbanks, Alaska, took up the mission with enthusiasm.
“I’ve seen hundreds of auroras grow,” says Troyer, who is in his third year of a doctorate at Iowa. “They are part of my heritage, something that I can study while still having a connection to my home country.”
Troyer created software to capture images from video when weak erasers were visible. In all, he cataloged 22 gum events during the two-hour recording.
“The most valuable thing we have found is to show the time it takes for the aurora to go from a gum event (when the diffuse aurora is erased) to be filled or colored again,” explains Troyer, who is the corresponding author of the article, “and how long it takes to go from this erased state to the diffuse aurora state. Having a value on this will help in future modeling of magnetic fields. “
Jaynes says learning diffuse auroral gums is akin to studying DNA to understand the entire human body.
“Particles that fall into our atmosphere from space can affect our atmospheric layers and our climate,” Jaynes says. “While particles with diffuse auroras may not be the primary cause, they are smaller building blocks that can help us understand the Northern Lights system as a whole, and may broaden our understanding of how whose auroras occur on other planets in our solar system. “