Humans have been watching the aurora borealis for thousands of years, with scientific studies of the phenomenon underway for centuries. Despite all that watching and studying, however, there are still some auroral forms that remain a mystery–namely, the “proton arc.” This one appeared over the Grande Cache area of Alberta, Canada, on July 29th:
“As I was driving to the Kakwa river, I saw a purple ‘proton arc’ crossing the sky from east to west, pulsing and dancing with the Northern lights,” says photographer Catalin Tapardel. “Quite a show….”
Aurora photographers see these structures from time to time–tight ribbons of light, sometimes red, sometimes green, writhing across the night sky. They are commonly called “proton arcs.”
Yet aurora scientists say they probably have nothing to do with protons.
“My opinion, and I believe the consensus of most aurora scientists, is that these arcs are not proton related, ” says Jason Ahrns, a researcher at the University of Alaska Fairbanks, “but I don’t know what does cause them.”
“Ordinary auroras we see from the ground and space are caused by electrons precipitating down into the atmosphere,” says Dennis Gallagher of the NASA Marshall Space Flight Center. “Protons can cause auroras, too, but they are different. For one thing, proton auroras are brightest in the UV part of the spectrum, invisible to the human eye.”
There is some visible light from proton auroras, but the structures they make are not tight and filamentary, but rather broad and diffuse–“in part because the gyroradius of protons is large,” says Ahrns. In other words, massive protons circle around magnetic fields in broad lazy arcs unlike lightweight electrons, which can tightly circle magnetic fields to form narrow structures.
Ahrns photographed an authentic proton aurora in February 2014: photo. “It appearance matched the description of proton arcs in the scientific literature – ‘a dim and diffuse glow’ with ‘very little structure in the observed brightness’ with a total brightness of only a few kiloRayleighs, which is just on the verge of visual threshold (Lummerzheim 2001).”
So what are the “proton arcs” often photographed by amateur aurora chasers? “I don’t know,” says Ahrns, “but it is something many of us would like to get to the bottom of!” For more examples of this mystery in the sky, browse the Proton Arc Photo Gallery.
Spaceweather.com 
