STEVE — Strong Thermal Emission Velocity Enhancement — is a narrow ribbon of mauve and white light that runs east to west across the sky, often for hundreds or thousands of kilometres, and appears at latitudes noticeably closer to the equator than the normal auroral oval. Despite being photographed alongside auroras for decades and routinely mislabelled as a proton arc, it is not an aurora. An aurora is produced by charged particles precipitating down magnetic field lines into the atmosphere; STEVE is produced by a river of exceptionally hot, fast-moving plasma flowing horizontally through the upper atmosphere at around 450 km altitude, heating the gas around it until it glows. The distinction was established in 2018 using data from the European Space Agency's Swarm satellites, after a group of amateur aurora photographers in Alberta pushed the phenomenon to the attention of professional researchers. The name began as a deliberate joke and was subsequently reverse-engineered into a technical acronym.
An Alberta Facebook Group and a Joke That Stuck
The Alberta Aurora Chasers, an online community of amateur night-sky photographers in western Canada, had been collecting images of a distinctive feature that did not behave like an aurora: a thin, sharply defined purple arc, often with a green picket-fence structure hanging beneath it, appearing well south of where the aurora normally sat. The prevailing folk explanation among aurora photographers was that it was a proton arc.
That explanation was wrong, and demonstrably so. Proton auroras exist, but they are diffuse and almost entirely invisible to the naked eye, emitting mainly at wavelengths that cameras do not usefully record. Nothing about a sharp, bright, structured purple ribbon fits. When the physicist Eric Donovan of the University of Calgary encountered the group's photographs at a public talk around 2016, he recognised both that the identification was wrong and that nobody knew what the correct one was.
Lacking a name and unwilling to keep calling it a proton arc, the group adopted Steve — a borrowing from the animated film Over the Hedge, in which characters confronted with an unidentified creature decide to call it Steve to make it less frightening. When researchers began publishing on it, the name was retrofitted into the backronym Strong Thermal Emission Velocity Enhancement, which is both an accurate description and a rare instance of a scientific term formed by working backwards from a joke.
What the Swarm Satellites Found
The decisive evidence came from the European Space Agency's Swarm constellation, three satellites launched in 2013 to map Earth's magnetic field, whose orbits carry them straight through the region where STEVE appears. In July 2016 one of them flew directly through an active STEVE while ground observers photographed it from below, and the instrument record was unambiguous.
Inside the ribbon, the satellite measured a gas temperature of about 3,000 °C against roughly 300 °C in the air immediately alongside, and a horizontal flow of plasma moving westward at about 6 km per second — some 600 times faster than the surrounding atmosphere. This is a known structure in space physics, a subauroral ion drift, which had been measured by instruments for years without anyone connecting it to a visible sky feature. The 2018 paper reporting the result, led by Elizabeth MacDonald of NASA Goddard with Donovan and colleagues, established the identification.
A follow-up analysis the next year settled the mechanism. Simultaneous satellite measurements found no evidence of particles precipitating into the atmosphere along the STEVE ribbon — the defining signature of an aurora and the thing that would have to be present for the term to apply. The light is instead produced by the heating itself, as the fast-flowing plasma collides with and excites the ambient gas. Researchers coined the term skyglow to mark the distinction from true aurora, and it is a genuinely new category of atmospheric emission rather than a variant of a known one.
The Picket Fence Is a Separate Problem
The green vertical structures that often hang below the mauve ribbon — universally called the picket fence — turn out not to share the same explanation. Unlike the ribbon, the picket fence does show signs of energetic electrons striking the atmosphere, which makes it aurora-like in a way the ribbon is not.
The complication is that it appears at latitudes far too low for the ordinary auroral mechanism, and it can occur in both hemispheres simultaneously, which points to a source in the magnetic equatorial plane rather than in the auroral zone. The green emission is at the familiar 557.7 nm oxygen line, but the picket fence lacks the blue nitrogen emission that normally accompanies electron precipitation vigorous enough to produce that green — evidence that the electrons involved have a different, lower energy distribution than auroral ones. STEVE is therefore not a single phenomenon at all but two co-located ones, and the picket fence remains the less well understood of the pair.
A Band Where Nobody Was Looking
STEVE appears equatorward of the auroral oval, typically several degrees of latitude south of it in the Northern Hemisphere. That places it over southern Canada, the northern continental United States, the United Kingdom, southern Scandinavia and northern continental Europe — well-populated country, and precisely the band where auroras are rare enough to be an event.
This is a large part of why it took so long to identify. The specialist observatories and all-sky camera networks were sited under the oval, where the aurora is; STEVE was passing over the suburbs. Meanwhile the people underneath it were photographing it constantly, filing it under a wrong label, and, until the arrival of large online photography communities, having no route by which their observations reached anyone who could evaluate them. It is one of the cleanest examples in recent atmospheric science of amateur observation identifying something the professional infrastructure had structurally arranged not to see.
What Is Still Unresolved
Identifying STEVE did not close the subject; it opened a set of questions that remain live. The most basic is why the emission is mauve at all. The colour does not correspond neatly to any of the standard atmospheric emission lines that account for auroral greens and reds, and the leading explanations involve either a continuum produced by the collisional heating itself or emission from excited nitrogen under conditions not previously studied. Laboratory work and spectroscopy from the ground are still narrowing this down.
A second question is why subauroral ion drifts, which instruments detect routinely, only occasionally produce a visible ribbon. Whatever additional condition is required — a threshold in flow speed, a particular density profile, some interaction with the picket-fence electrons — has not been pinned down. And the ribbon's remarkable narrowness, often only 25 km wide while extending for thousands of kilometres, is itself unexplained by any simple heating model.
These are not marginal curiosities. Subauroral ion drifts represent a substantial transfer of energy into the upper atmosphere at latitudes where most satellites in low Earth orbit spend their time, and heating on this scale changes atmospheric density enough to affect orbital drag. A phenomenon that turns out to be visible from a suburban back garden gives researchers a cheap, distributed way of observing when and where that heating occurs.
The observing effort has been formalised accordingly. Aurorasaurus, a citizen-science platform founded by MacDonald, collects timestamped and geolocated public reports of both auroras and STEVE, and those reports have been used directly in the published literature. The pipeline that identified STEVE in the first place — amateurs photographing, a researcher noticing, satellites confirming — has effectively been institutionalised.
Seeing One
STEVE requires a geomagnetically active night, so the underlying forecast is the same one used for the aurora borealis. It is most often reported in the equinox months, and it appears most frequently during the recovery phase of a substorm — often about half an hour after a bright auroral display has peaked and started to fade, which is exactly when many observers pack up and go inside. The single most useful piece of advice is to stay out for the hour after the aurora dies down, and to keep an eye on the sky to the south rather than the north.
What to look for is unmistakable once known: a narrow ribbon, far narrower than any auroral band, running east to west, often passing close to overhead, and holding its shape for anywhere from twenty minutes to an hour before fading. To the eye it is usually a faint grey-white streak; the mauve colour is a photographic result, and cameras record it far more strongly than the eye does — the same discrepancy that governs aurora viewing, and one reason STEVE went unrecognised through the entire era of film photography.
It has also been confirmed in the Southern Hemisphere, photographed from New Zealand and Tasmania, where it faces the same geographic constraints as the aurora australis — with the important difference that STEVE's lower latitude actually puts it over inhabited land more often than the southern oval manages.