Beautiful sub-auroral arcs across the sky: what we know so far about the phenomenon

While cameras are being more and more performant and astrophotography more affordable and accessible than before, a number of sky phenomena previously thought rare or inexistant are being studied and reported. The sub-auroral arc complex is no exception and brings up much excitement, discussion, enthusiasm and questions within the scientific community but also among amateur astronomers. Here is what we know so far about the mysterious phenomenon.

Source: Anna Michele Mccue©

You might have heard several names given to this strange event that seems to happen only during solar storms: 'Proton aurora', 'Proton arc', 'STEVE' or even 'Sub-auroral arc'... The reason for that is that we actually don't know much about this strange ribbon. However useful research is being conducted on it so that we eventually can know more about it. Despite what a number of recent articles hyping the acronym 'STEVE' have stated (for example: ESA: 'Thanks to social media and the power of citizen scientists chasing the northern lights, a new feature was discovered recently. Nobody knew what this strange ribbon of purple light was, so … it was called Steve'.), the odd arc is not new, and records/sightings of the phenomenon among amateur astronomers and locals have been made for a long time now. I have recently talked to many amateur astronomers that claim to have seen the bright band for years and all around the world. I have seen it myself for the first time three years ago in Denmark, and it definitely matched the characteristics of the sub-auroral arc. Youtube and the internet also offers a variety of images and videos that go way back, just google 'Proton arc'. This article is the occasion for me to urge everyone to have a critical eye on the matter, and on everything else, while being more curious about it and perhaps making a contribution to citizen-science!

Credits: Maciej Winiarczyk© 2016

Sub-auroral arc: likely not a proton-made auroral arc.

Credits: Adrien Mauduit©

For a very long time the aurora community (scientists, amateurs, professionals...) had called the phenomenon proton arc for the simple reason that they occur approximately in the same conditions than proton auroras (high solar activity). You've got to give credit to the entire community and not just a handful of people for this tremendous enterprise because if you look closely, there are countless reports of those 'proton arcs'.

However a large number of aurora scientists now believe that these arcs have LIKELY (meaning MAYBE not, nothing is certain 100%) nothing to do with protons. “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 Jason Ahrns, an aurora researcher at the University of Alaska, Fairbanks. To rephrase it, proton auroras might be too dim and diffuse [Lummerzheim 2001] to be the more or less tight and bright arc that we recognize today. But once again, nothing is an absolute certainty and protons might still have their role to play, that's why some people still call it a proton arc today. Aurora science, space weather, or anything that happens within the 'nomansland' zone (roughly between 50km and 300km of altitude), where no aircraft, balloon nor satellite can reach, are thus complicated things to study. One can use ground observation, ground spectrophotometry, LIDAR, RADAR or other remote ground techniques, and imaging/probing from a low orbit, but not directly into the phenomenon. That's why aurorae (most of it happens from about 100km to 300km), noctilucent clouds, airglow...etc still have their share of mysteries to unravel. More and more efforts to send sub-orbital (low orbit parabola) manned missions are being developed (see Project PoSSUM) to probe noctilucent clouds for example, but I haven't heard of any sub-orbital aurora mission yet (think of what would happen if we send a spacecraft into high-energy plasma and 6000°C electrons!).

'STEVE', though?

Credits:Anna Michele Mccue©

The recent increase in the number of reports and sightings of the phenomenon around the world and especially across Canada thanks to enthusiasm and cheaper/more performant cameras, but also thanks to the citizen-scientist based network Aurorasaurus, or like our sub-auroral arc community (JOIN US ON FACEBOOK or TWITTER), have helped document it with movies, pictures and articles.

Digression: What is a citizen-scientist? While amateur astronomers or astrophotographers send their reports to the organization, its citizen-scientitsts (people employed or volunteering within the organization) help gather and organize the data to be analyzed by scientists who cooperate with them (some of them often work within the organization). I am myself a citizen-scientist (as I work within the Project PoSSUM organization, conducting research on noctilucent clouds) AND an amateur photographer (as I regularly send reports of aurorae, NLC...). I just thought I'd make the distinction here since I recently got lot of questions generating confusion.

Convinced that they were not proton-based aurorae, nor did they (or anyone for that matter) really know what they were, a