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

October 14, 2017

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 group of people composed of scientists (notably E. Donavan from the U of Calgary) and administrators of the Facebook Alberta Aurora Chaser group decided to call it 'STEVE'. The name was found over a joke at bar during a reunion, as a reference to the cartoon Over The Hedge. In the animated film, the characters are facing something they don't know and call it 'Steve'. The people behind the name proposed to match the letters with the phenomenon to make it an acronym: 'Strong Thermal Emission Velocity Enhancement'. The name got hyped within the community and caught on in the AAC group, as well as on a myriad of space-based platforms like,, APOD, Earthsky, but also internationally on TV and the internet. Needless to say that 'STEVE' has now taken the internet by storm. Now however entertaining and anecdotic the name can be, calling a scientific phenomenon that might be used for centuries using a cartoon character as a joke is far from being the most suitable. As a matter of fact the name itself has nothing to do with the phenomenon (even if it became an acronym afterwards), whereas science usually utilizes well thought-over and adequate nomenclature. Science should not be fun, it should be solemn, factual and objective. Its communication and education on the other hand, should be fun, innovative, pedagogical and subjective (use of arts)...


Credits: Helene Savard©



Strong Thermal Emission Velocity Enhancement?


According to Dr. Donavan in a recent SWARM Science meeting in Canada: 'As the SWARM (ESA) satellite flew straight though Steve, data from the electric field instrument showed very clear changes. The temperature 300 km above Earth’s surface jumped by 3000°C and the data revealed a 25 km-wide ribbon of gas flowing westwards at about 6 km/s compared to a speed of about 10 m/s either side of the ribbon.' Prof. Donavan's neat findings would not have been possible without the help of ground observations and reports. He managed to triangulate SWARM data with GPS coordinates and sightings from the ground.


However my critical analysis of Dr. Donavan's statement as it is in ESA's article raises questions and considerations. The SWARM satellite orbits the Earth at an altitude of 450 to 530 km ( and collects data from electrons (temperature, density...) precipitating along the magnetic lines of force and into a zone of high ionization (interaction between the atmosphere's gas molecules and the electrons), so it can only measure for an altitude within its polar orbit range and not 300km. Then how can the satellite draw precise (numbered) conclusions on the glowing gas' properties (temperature, speed, width) when it can only detect and measure the flow of precipitating electrons? ( Another information to also take with caution is that the satellite flew 'straight through it'. Since we have no idea of the exact altitude of the phenomenon (maybe just an approximation), one can only safely say that the satellite flew through a system (the flux of electrons) supposedly correlated with the incident ribbon. But even if we assume that the measurements have been made inside the SAA and that its gas properties can be deduced, the measurements have been done relatively high in altitude, whereas the literature tends to show that the higher the energy the particles have, the further they dive into our atmosphere (sometimes down to 80km during powerful and high-energy events: The arc could be an exception to the rule if it really happened at 400-500km of altitude, however ground observations have excavated another argument against it being a very strong temperature and velocity enhancement. Recent reports and discussion threads (that I can also corroborate myself, see the video below) tend to show that these sub-auroral arcs develop when the main auroral body is receding or prior to growing again (perhaps corresponding with a flip in Bz from southward to northward or conversely, or a quick decrease in density/field intensity), while solar wind data remain relatively high (at active/storm levels). I have observed 4 sub-auroral arcs myself from Denmark, and one that I captured in March 2017 was faint but visible with the naked eye. As the following video suggests (and the data), it matches the possible description above, although I can definitely recognize the high velocity of the white wavy arc. But hey, I've seen 'normal' aurora go faster than this, at least visually! 


For all these reasons, I am definitely reluctant to using the name Strong Thermal Emission Velocity Enhancement, and even more STEVE, now used as a name 'Steve' and not even an acronym. Donavan's findings have been made comparing a zone with and without ribbon, and at an unfamiliar altitude that does not match the main auroral altitude and only measure electrons, not gas properties. Comparing the ribbon to the main auroral body's temperature and velocity would be more suitable if we are to call it temperature and velocity enhancement: enhancement yes, but compared to what? I'd love to see all of Donavan's present and future published research on it, and while it sounds like I'm being hostile, I am definitely not, I am just being critical of the info I get, and I ask myself a lot of questions (I just don't agree with the name, how it has been found and how it's being used erroneously in the media).   I'd rather call it resurgence instead of enhancement and you will understand why in the later in the article. ... On a side note, I also try avoiding the term proton, as its use generates a lot of controversy.


This is the simple reason why I am calling it by the neutral and adequate name of 'SUB-AURORAL ARC' (SAA) while the scientific community decides and approves of a more scientific name than Steve as more discoveries are being made. Sub-auroral arc directly refers to the fact that it looks like an arc (despite a very few times where it looks like some isolated patches) that is often happening in the sub-auroral zone (at the edge of of the solar storm at mid-latitudes like Canada, US, Ireland, UK, Denmark, Estonia...) during a solar sub-storm. You also have to understand the double meaning, as in its direct translation from Latin (‘sub’ = under or secondary). Indeed the phenomenon has always been observed ‘next to and equator ward of the main auroral body’ to this date, hence this very neutral and descriptive adjective. I believe this name describes the phenomenon much better scientifically for now. But more importantly than a battle on names (even though I know it bears meaning to a lot of people), Prof. Donavan also underlines the importance of all-sky imagers and amateur photography boom in helping science, and adds that it wouldn't have been possible 20 years ago. Citizen science is a relatively new discipline that connects the public and scientists in a concerted effort to achieve new discoveries, and I am also convinced that it should keep developing around the world in communities, as literally millions of eyes armed with technology can collect significant bulk data in record time. 



How to recognize a sub-auroral arc?


The following video is from Ian Stewart and was taken in 2015 in Tasmania, Australia. It shows the life cycle and characteristics of a beautiful sub-auroral arc. After looking through tons of pictures, videos and reports, I have recapped a list of information about how to recognize the astronomical phenomenon. 


Many people say it is not an auroral phenomenon, and whether or not it is connected to the main auroral body remains to be studied, however reports and pictures suggest that the arc always occurs during a solar sub-storm, or least when the solar activity is high. It is thus very likely to be made of glowing gas ionized by electrons, much like an aurora. It seems to be composed of a system of three/four different connected but seemingly detached features:


- a green part at the base apparently lower in altitude. It often appears as a horizontal band of individual 'flames' or 'pickets', which people often call 'picket fence aurora'. This features often seems to stay in place but somehow of flickers back and forth, showing that the whole structure is connected, exactly like in a curtain. It may or may not be present. 
- a white part on top of (perhaps higher altitude) the green picket, but seemingly detached. It is sometimes accompanied by a green base which might be connected or part of the picket fence (that's why I wrote 4 features). It is probably the most intriguing part. While the picket fence doesn't appear every time, the white part seems to appear more often. The white part looks like a band, but sometimes bends and wiggles into transverse snake-like wave patterns and sometimes beams. 

- a pinkish-red part seems to be the continuity of the white part at higher altitude, much like nitrogen signatures on top of the main auroral body. The color varies from pink to red and sometimes even purple. This part seems to follow the white part and makes similar patterns. this part is also often observed.


Credits:Niel Moroney©


The image above shows that the sub-auroral arc can display one (here obviously white-pinkish), two or all three features together, probably depending on the intensity of the particle precipitation. It always seems to pop out detached from the main auroral body. Some displays appear way detached, while some (like this one) are rather close by. Consequently, in the former case, the SAA can spawn in the south while the aurora is barely visible in the north, causing it to often cross paths with the milky way. One property that appears to be true though, is that the SAA occurs following the auroral oval, merely at the edge of the solar storm (equatorward of the main electron aurora). Some people have reported to have experienced the phenomenon for up to an hour, while some others (most cases, including mine) were only minutes, even seconds long, making it very fleeting and elusive.

The systems looks quite detached but seems to act as a whole in the same direction of the main auroral body (visibly westward). It generally looks faded in colors but more 'stable' than the fast-moving pillars of the main body, and in some ways it looks like a scattered aurora. By scattered, I mean disorganized, not behaving like a normal one, very detached. What's really peculiar about the ribbon is its behavior. In some cases, it just remains a simple and continuous band across the sky. In some other instances, it acts as a disorganized but stable wavy arc that moves North-South as a whole. The following amazing footage (by Vanexus Photography) showcases the latter instance very well, as well as the fact that when the aurora body seems to recede northwards, the SAA quickly moves on the other direction (southward), depicting a possible connection between the two (perhaps connected to Bz, density, field strength...). My hypothesis is that the SAA evolves at the edge of the solar storm, perhaps in a zone where the precipitation of particles is very different- unstable from the receding main auroral body, thus creating the uneven patterns and weird evolution that we see. It could be due to an accumulation of particles in this region followed by resurgence of particles ignited by the flip of the magnetic field direction? That's why more research has to be done on it, and more reports to be made by amateur astronomers and citizen-scientists! 


Here is another great video to help you understand what it looks like and how it behaves: 




Where do I have the best chances of witnessing a sub-auroral arc?


There is no evidence to date that the SAA spawns in some place more than others, and the proof is that the reports come from all around the world, and the southern hemisphere is no exception (on the contrary!). However it might seem like some areas are luckier than others, and that is probably because of several factors.


As a tangible prerequisite to observing it, the first factor is your geographical location according to the auroral oval. Since the magnetic poles tilt (the North magnetic pole tilts towards North-America and the South, Oceania), the sub-auroral zone (equator ward of the auroral zone, in yellow in the picture below) descends to lower latitudes too, at these locations, enabling people to see these SAA further south. It wouldn't really matter if it was ocean, but when it is covered by a large continent like North-America or Siberia (Russia), that increases the number of people being able to witness such a phenomenon. (SN: the yellow line is actually absent, as the sub auroral zone is doesn't have a real southern boundary; it goes as far south as the event is powerful). 


Source: PressReader©



Source: Space Weather Canada©



The second factor is the weather. Since the light from aurorae cannot penetrate through clouds, you would generally need decent weather conditions to view them. In the northern hemisphere northern Europe is under the constant influence of the Jet Stream hauling mild temperatures and humidity. In North America, most places are often under a cloud cover because of the cold temperatures created by the polar currents (it's actually more complicated than that of course), but there is one place where the clouds tend to stay at bay more often: the region situated directly east of the rockies: Alberta, Saskatchewan, Manitoba (apparently less) and the northern US benefit from the mountains that more often stop the humid south-west influence, and give impeccable sighting conditions. For having lived 1.5 years in southern Alberta and also conducted research on NLC in northern Alberta, I can confirm. Other places (Scotland, Denmark, Southern Sweden, northern Ireland, Estonia, Lithuania, Russia) never seem to get the same number of observing hours because of bad weather conditions (Look at this comparison of the total hours of sun for European cities (, and then compare it to the Canadian cities: ( 

Lastly, the third factor is a human one: the variable access to technology, to a source of internet or report platforms, or even the level of enthusiasm or interest for this phenomenon. For example, I have seen very few reports from Russia during my investigation, possibly caused by a lack of information (people might not know what they are looking at) or interest... Russia's Siberia has such a big potential surface area to observe sub-auroral arcs and I am very surprised there aren't more reports (Maybe because of language, policies...). The major problems of northern Europe appear to be the weather and the very small surface area in the sub-auroral zone, although the concentration of people should maybe compensate?  Eventually south-west Canada and the north-western US dispose of great weather conditions compared to other places, a large surface area under the sub-auroral zone, and large population of amateurs (especially within the aurora community), perhaps explaining this surge in photos, videos and reports lately. Plus, since the phenomenon got famous only recently thanks to that localized community (Alberta Aurora Chasers), more and more people know about it and report it. That does not mean that you cannot see it anywhere else. In fact a beautiful SAA has been sighted in the UK last week during a substorm, here is a beautiful picture from Maciej Winiarczyk:


Credits: Maciej Winiarczyk©


The southern hemisphere appears to have only one possible area where SAA can be viewed because the rest of the sub-auroral zone is over the ocean. According to the number of sightings and reports already made, one can conclude that south-east Australia and New-Zeland are actually good places to spot the arc. The beautiful videos you watched above were shot there during powerful storms and constitute evidence that the phenomenon didn't just pop out of the blue. Because of its privileged position and weather conditions, this area of the world still has the best videos of picket fence aurora I have ever seen (And there are plenty more on the net for you to watch!).


Credits: Service Aurora / NOAA©




Sub-auroral arcs are beautiful but ephemeral auroral phenomena that may make their appearance in your night sky during a solar sub-storm, so that alone should encourage you to go out and experience it. It is a very special moment, also for those who are used to watching and chasing aurorae. However very little is known about them and even today, scientists need to study them more, helped by citizen-scientists and a fortiori amateur photographers. Every one is free to call them what they want, since the scientific community has not yet agreed upon a definite name, but I generally urge you to use your sense of criticism and reasoning when you read, watch or hear information like this one. While I have a deep respect for everyone's hard work on the matter, since it contributes to science, the 'Steve' case is, to me, an example of citizen-science faux-pas where the contribution from the non-scientific party takes the upper hand over the purely scientific one. What would I know about it? Well I do own a scientific background/rigor/approach as well as publications, and I now have 5 years and countless hours of astronomy observation behind me. Being an citizen-scientist myself and working with scientists and the general public, I am always very cautious about the words I use and the things I do, because hey, no one ever knows all (Errare humanum est!). We need to work together on this huge enterprise, and the work should not be mistakenly credited only for a specific community, rather the whole world's aurora community. That's probably the downside of how we process and share information through social media and the general media today.  Be critical and curious, ask questions, use 'maybe, perhaps, might', but most important of all: go out and experience nature, something that more than one could benefit from!


Credits: Maciej Winiarczyk© 2016








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