For millennia, humans in the high latitudes have been enthralled by
auroras—the northern and southern lights. Yet even after all that time, it
appears the ethereal, dancing ribbons of light above Earth still hold some
secrets.
In a new study, physicists led by the University of Iowa report a new
feature to Earth’s atmospheric light show. Examining video taken nearly two
decades ago, the researchers describe multiple instances where a section of
the diffuse aurora—the faint, background-like glow accompanying the more
vivid light commonly associated with auroras—goes dark, as if scrubbed by a
giant blotter. Then, after a short period of time, the blacked-out section
suddenly reappears.
The researchers say the behavior, which they call “diffuse auroral erasers,”
has never been mentioned in the scientific literature. The findings appear
in the Journal of Geophysical Research Space Physics.
Auroras occur when charged particles flowing from the sun—called the solar
wind—interact with Earth’s protective magnetic bubble. Some of those
particles escape and fall toward our planet, and the energy released during
their collisions with gases in Earth’s atmosphere generate the light
associated with auroras.
“The biggest 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 Iowa and study co-author. “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,” Jaynes
continues, “and it may be a process that we haven’t started to look at yet
because we never knew they were happening until now.”
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 along Hudson Bay in
Canada, to film auroras. Knudsen’s group was a little disheartened; the
forecast called for clear, dark skies—normally perfect conditions for
viewing auroras—but no dazzling illumination was happening. Still, the team
was using a camera specially designed to capture low-level light, much like
night-vision goggles.
Though the scientists saw only mostly darkness as they gazed upward with
their own eyes, the camera was picking up all sorts of auroral activity,
including an unusual sequence where areas of the diffuse aurora disappeared,
then came back.
Knudsen, looking at the video as it was being recorded, scribbled in his
notebook, “pulsating ‘black out’ diffuse glow, which then fills in over
several seconds.”
“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.
It went away,” says Knudsen, a Fort Dodge, Iowa, native who has studied
aurora for more than 35 years and is a co-author on the study. “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.”
The note lay dormant, and the video unstudied, until Iowa’s Jaynes handed it
to graduate student Riley Troyer to investigate. Jaynes learned about
Knudsen’s recording at a scientific meeting in 2010 and referenced the
eraser note in her doctoral thesis on diffuse aurora a few years later. Now
on the faculty at 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. “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, from Fairbanks, Alaska, took up the assignment with gusto.
“I’ve seen hundreds of auroras growing up,” says Troyer, who is in his third
year of doctoral studies at Iowa. “They’re part of my heritage, something I
can study while keeping ties to where I’m from.”
Troyer created a software program to key in on frames in the video when the
faint erasers were visible. In all, he cataloged 22 eraser events in the
two-hour recording.
“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,” says Troyer, who is the paper’s
corresponding author, “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.”
Jaynes says learning about diffuse auroral erasers 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 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 how auroras happen on other planets in our solar system.”
Reference:
“The Diffuse Auroral Eraser” by R. N. Troyer, A. N. Jaynes, S. L. Jones, D.
J. Knudsen and T. S. Trondsen, 18 February 2021, JGR Space Physics.
DOI: 10.1029/2020JA028805