In March 2002 physicist Dr David Knudsen noticed something extraordinary while observing auroras in northern Canada. Although he made a note of it, Knudsen didn't take his find any further at the time. Fortunately, however, he was part of a team filming the northern lights. Almost two decades later the video he made has been studied intensively and the previously unrecorded auroral phenomenon made the subject of a scientific paper. Physicists still don't understand what caused what Knudsen saw, or how often it occurs, but they definitely want to find out.
The polar lights have probably fascinated humans since we migrated far enough from the equator to see them. The physics that triggers them in the upper atmosphere and their connection to outbursts from the Sun have been understood, at least roughly, for 60 years. Nevertheless, we're still finding plenty of new aspects and puzzling aspects, such as the narrow reddish-pink band dubbed Steve.
Knundsen, of the University of Calgary, was in Churchill, where the auroras' cause was first established, filming the sky with a camera that could record the faint diffuse light often overshadowed by more attention-grabbing “discrete auroras”. It was just as well he did. Despite clear skies and a location reasonably close to the Earth's North Geomagnetic Pole Knundsen and colleagues couldn't see much auroral activity with their naked eyes.
The more sensitive camera, however, picked up a diffuse aurora, its parts disappeared and returned in an unfamiliar way. “Pulsating ‘black out’ diffuse glow, which then fills in over several seconds,” Knundsen wrote in his notes.
In 2010 Dr Allison Jaynes of the University of Iowa learned about the recording at a scientific meeting. Although Jaynes mentioned it in her thesis on diffuse aurora she didn't research it further.
However, having become an assistant professor she thought this exceptional event required further study and gave a copy of the video to graduate student Riley Troyer. “I knew there was something there. I knew it was different and unique,” Jaynes said in a statement.
Troyer dug deep, and his analysis has now been published in the Journal of Geophysical Research Space Physics. Troyer grew up in Alaska and calls auroras; “part of my heritage,” so was delighted at the opportunity to study one doing something never described before.
Knudsen's video lasts for two hours, during which time Troyer counted 22 “eraser events”. Today Knudsen describes what he saw this way; “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.” So-called black auroras, where holes appear in diffuse auroras, are known, and discrete auroras sometimes pulse, but no one has reported a combination of the two. An event in 2017 marks the only possible repetition, and even that had significant differences.
Troyer and co-authors cannot explain what causes these erasure events. By putting them into the scientific literature they may put aurora watchers on the lookout for a repeat, hopefully allowing us to learn more. Meanwhile, Troyer has conducted a frame-by-frame analysis of the precious video, including detailed measurements of the time each eraser event took to make part of the aurora disappear, and then for the “hole” to return to full brightness.
Knudsen's video was taken as the 23rd solar cycle was passing its peak. The following cycle was much less active, so auroras of all types have been less abundant since, and diffuse auroras particularly so. However, the next few years should see these lights happen more often, increasing the chances someone, whether amateur or professional, will witness an erasure event. If so, Jaynes told IFLScience it is possible a camera phone would be sufficient to record an erasure event. If this was sent to her team it might help explain the atmospheric dynamics responsible.