New type of aurora revealed in 19-year-old video
Scientists at the University of Iowa said on May 6, 2021, that they’ve given the name diffuse auroral eraser to a new type of aurora not often seen and not well understood. This new and elusive type of aurora – noticed in a 19-year-old video made by a physicist – has never before been described in scientific literature. The new name diffuse auroral eraser describes how this sort of aurora first appears as a brighter stripe within a fainter, more diffuse aurora. When the stripe disappears, it blots out the diffuse light from the background aurora, as though the aurora is erased from that specific patch of sky. Then, after about 20 seconds, the diffuse background aurora reappears.
The scientists examined a video of the auroral phenomenon taken on March 15, 2002, by University of Calgary physicist David Knudsen. He shot the video in Churchill, a town in northern Manitoba, Canada, on the west shore of Hudson Bay. Knudsen and his fellow researchers saw no aurora with their own eyes that night, but the camera did see it. The camera was sensitive enough to pick up the diffuse auroral background and the darkening events. Nineteen years ago, as Knudsen watched the darkening on video, he jotted down the following in his notebook:
… Pulsating ‘black out’ diffuse glow, which then fills in over several seconds.
Knudsen, a co-author of the study, described his memories of the night he filmed along Hudson Bay:
What surprised me, and what made me write it in the notebook, is when a patch brightened and turned off, the background diffuse aurora was erased. There was a hole in the diffuse aurora. And then that hole would fill back in after a half-minute or so. I had never seen something like that before.
Allison Jaynes of the University of Iowa, another study co-author, learned about Knudsen’s video at a scientific meeting in 2010. She alerted Riley Troyer to the auroral phenomenon, and he took it from there. As Jaynes explained:
I knew there was something there. I knew it was different and unique. l had some ideas how it could be analyzed, but I hadn’t done that yet. I handed it to Riley, and he went much further with it by figuring out his own way to analyze the data and produce some significant conclusions.
Troyer is originally from Fairbanks, Alaska, so he’s no stranger to auroras, aka the northern lights. Fairbanks lies under the northern auroral oval, one of two high-latitude regions near the poles where Earth’s magnetic field lines funnel in charged particles from the sun that interact with our atmosphere and excite atoms, which in turn light the sky with the aurora. Troyer said:
I’ve seen hundreds of auroras growing up. They’re part of my heritage, something I can study while keeping ties to where I’m from.
Troyer used a software program he created to analyze the video and locate 22 eraser events in Knudsen’s two-hour video. He discussed his findings:
The most valuable thing we found is showing the time that it takes for the aurora to go from an eraser event (when the diffuse aurora is blotted out) to be filled or colored again, and how long it takes to go from that erased state back to being diffuse aurora. Having a value on that will help with future modeling of magnetic fields.
They calculated the average recovery time between the erasing to the reemergence of diffuse light as 20 seconds, though it varied widely between events.
Jaynes summed up the team’s findings:
The biggest thing about these erasers that we didn’t know before but know now is that they exist. It raises the question: Are these a common phenomenon that has been overlooked, or are they rare? Knowing they exist means there is a process that is creating them, and it may be a process that we haven’t started to look at yet because we never knew they were happening until now.
She also discussed how their findings could have a broader impact on what we know about Earth and climate:
Particles that fall into our atmosphere from space can affect our atmospheric layers and our climate. While particles with diffuse aurora may not be the main cause, they are smaller building blocks that can help us understand the aurora system as a whole, and may broaden our understanding of how auroras happen on other planets in our solar system.
Bottom line: Newly named diffuse auroral erasers rapidly brighten and then darken, taking background auroral light with them. Scientists now have a better understanding of the time scale on which these events happen, which, they say, might lead to a greater understanding of Earth’s atmosphere, our climate system and auroras throughout the solar system.