A proton arc is oftentimes described as a broad band of diffuse aurora. If you do a Google Image search for “Proton Arc” a plethora of beautiful images depicting a purple, red, green, or pale band of aurora will greet your eyes. Go ahead, really, search it, I can wait. Or, you can visit this website at Spaceweathergallery.com.
I had the pleasure of seeing this pale phenomenon in Juneau on September 20th, 2016 for the first time ever. In the scene, the aurora swirled to the north in front of me over mountains. However, a pale, confined, band of aurora ran perpendicular to the northern display, and stretched far to the south past a large, brilliant moon. In my camera it was cool blue/white in color and was in stark contrast to the green aurora that played on the northern horizon over the mountains of Juneau. I posted the image to an aurora group on Facebook and labeled it a “proton arc” as so many before me had done. However, I received an interesting response from renowned aurora researcher Neal Brown – a true “proton aurora” is nearly undetectable by the human eye and the concept of a “proton arc” is a widespread misconception. The disagreement between the science and the public perception set my wheels turning, and even though I am not an aurora scientist, I would like to dissect why proton arcs are not truly visible.
There are two ways that auroras may be formed. Most auroras are formed when excited electrons collide with oxygen or nitrogen or if protons collide with nitrogen or oxygen. Electrons which are lighter and have a lot of energy result in the traditional, dancing auroras. Electron auroras emit light at many wavelengths including 630nm (red) and 427.9nm (blue). The second way that auroras can form is when protons collide with nitrogen and oxygen. The proton collisions result in emissions of 656.3nm (red) and 486.1nm (blue) (Lummerzheim et al. 2001). Separation of these light bands are difficult because at 656.3 the emissions require a precise instrument to differentiate them from the electron aurora. The same can be said of the emissions at 486.1 which are nearly indiscernible from the electron emissions. To quote Neal Brown’s response in the aurora group, “To prove it is a true proton arc one would have to use some sort of spectral discrimination to see if it contained only 656.3 and 486.1 nm emissions”. Aurora researcher Jason Ahrms had this to say in a detailed Facebook post – “We don’t use color, location in the sky, how long it’s been there, or anything like that to identify a proton aurora.”. This means that simply looking at an aurora with your eyes is not enough to determine if it is a proton arc – so why is it so commonly mislabeled. The mistake is likely an innocent use of scientific jargon; those posting the images (like me) simply did a brief search to confirm what they saw before spreading the lie themselves.
Although it is impossible to detect a proton aurora with your eyes, they have been successfully photographed once identified with instrumentation. Tony Phillips of Spaceweather.com discussed the phenomena with University of Alaska Fairbanks Researcher Jason Arhns. His image below shows how difficult true differentiation between electron and proton aurora is. Where the proton arc has been identified is barely discernible from the aurora.
It was interesting to realize that my perception of what a proton arc was had been so wrongfully influenced by what I saw online. However, if the pale auroras being captured by photographers (like the photos below) are not truly proton arcs, what are they? Incredibly, as Jason Ahrns explains, to date there are is no known explanation for these pale, elusive aurora displays! They are a new opportunity for scientific exploration in the aurora research arena. I hope they keep us posted.
Lummerzheim, D., M. Galand, and M. Kubota. “Optical emissions from proton aurora.” Proc. of Atmospheric Studies by Optical Methods 1 (2001): 6.