Last fall, Dave Markel, a photographer based in Kamloops, British Columbia was out photographing the northern lights when he saw something really strange. Crossing over his head and running almost perfectly from east to west, a narrow arc of light glowed for almost an hour. Markel took photos of the phenomenon and posted it on social media.
“It looked like a massive contrail moving rapidly across the sky,” said Markel in an e-mail. The arc was pierced with what he described as “green pickets” or short, parallel streaks of aurora. Perhaps you’ve also seen this peculiar form of aurora. It’s not too common, but I’ve viewed it on 5 or 6 occasions and always wondered exactly what it was and how it formed.
That’s where aurora researcher Eric Donovan of the University of Calgary steps in. Prof. Donovan met with members of the popular Facebook group Alberta Aurora Chasers. Looking at their photos, he came across Markel’s purple streak and additional photos like it taken by other photographers.
You’ll often here the band described as a “proton arc,” but it’s not, according to Jason Ahrns, an aurorologist and physicist at the University of Alaska-Fairbanks. In a recent post on the Aurorasaurus Facebook page, Ahrns describes a true proton arc:
“It’s green, featureless, and very large. Even a good proton aurora is around 75 times dimmer than the bright, active green aurora.” The accompanying photo shows a faint, greenish veil hanging in the southern sky. At the very least, proton arcs are hardly noticeable and don’t resemble the curious curl captured by Markel. Here’s a photo of one.
The familiar reds and greens of northern light come from oxygen and nitrogen atoms that get clobbered at high speed by tiny electrons in the sun’s particle wind. When they relax back to their pre-collision state, they emit red and green light. Proton arcs in contrast are spawned by protons (basically hydrogen atoms without their electrons) that capture an electron. If they’re excited by another collision, they spit a bit of light out but far less than a typical aurora.
Let’s return to the strange arc. Donovan knew it had to be something else, but what? To find out, he and his colleagues looked through data from the European Space Agency’s Swarm magnetic field mission as well as his network of all-sky cameras. Swarm is comprised of three identical satellites that orbit the Earth and measure the magnetic fields arising from Earth’s core, mantle, crust and oceans, as well as from the ionosphere and magnetosphere high overhead.
He not only discovered that the narrow-band-type aurora was much more common than he’d expected but was able to match a ground sighting of the phenomenon to an overpass of one of the three Swarm satellites.
“As the satellite flew straight though Steve (the nickname given to Markel-type auroras), data from the electric field instrument showed very clear changes,” said Donovan.
“The temperature 186 miles (300 km) above Earth’s surface jumped by 3000°C and the data revealed a 15.5-mile-wide (25 km) ribbon of gas flowing westwards at about 6 km/second compared to a speed of about 10 meters/second either side of the ribbon.”
Wow — the feature really was something unique! After hearing the description, a work colleague of mine compared it to a fluorescent light without the glass, an apt analogy.
In decades of aurora watching I’ve only seen this form of aurora a handful of times. On most of those occasions, there was either no other aurora visible or minor activity in the northern sky. The narrow arc, which typically lasted for an hour or so, pulsed and flowed with light and occasionally displayed the little pickets described by Markel.
Goes to show you that even today dedicated skywatchers and nighttime photography buffs can help answer scientific questions or even crack open ripe new areas for research. Groups like the Great Lakes Aurora Hunters and Aurorasaurus serve as clearinghouses for observers to report auroral displays, and I encourage you to use them to share your own aurora observations and photos. The path to scientific discovery often begins with a deceptively simple question:
“What is that?”