STEVE, Airglow, and Other Things That Are Not the Aurora

Anyone who spends enough nights watching the sky at high latitudes eventually sees something that isn’t quite the aurora, but looks close enough to cause genuine confusion — especially in a long-exposure photo, where cameras reveal far more colour and detail than the naked eye ever does. Knowing the most common look-alikes helps you read your own photos honestly, and avoid either false disappointment or false excitement.

STEVE: a real phenomenon, but not aurora

STEVE — Strong Thermal Emission Velocity Enhancement, a name backronymed onto an existing nickname coined by citizen scientists before researchers understood what they were seeing — is a narrow, glowing ribbon of mauve-to-white light, often accompanied by a green picket-fence-like structure beneath it, that can appear at lower latitudes than typical aurora and sometimes alongside a genuine display. For years, observers assumed it was simply an unusual form of aurora, but research since around 2018 has shown STEVE is physically distinct from ordinary aurora.

The leading explanation centres on a phenomenon called subauroral ion drift (SAID): a narrow channel in the ionosphere, positioned equatorward of (below, in geomagnetic latitude terms) the main auroral oval, where charged particles are driven to move extremely fast — sometimes several kilometres per second — relative to the surrounding, more slowly moving ionosphere. The intense friction and heating generated within that fast-moving channel is believed to produce STEVE’s mauve glow through a different physical process than the particle-precipitation-driven collisional excitation described in why aurora is green, red and pink — STEVE’s colour comes from heating and thermal emission dynamics in the channel itself, not (primarily) from charged particles raining down and striking atmospheric gas the way ordinary aurora does.

The green, vertically striped “picket fence” pattern that sometimes accompanies STEVE is a separate puzzle again — current research suggests it may involve genuine particle precipitation, unlike the mauve ribbon itself, but the details remain an active area of study and are considerably less settled than the explanation for STEVE’s main ribbon. STEVE is genuinely exciting to see and photograph, and it does tend to appear during geomagnetically active periods (making destinations like Edmonton, Calgary and Saskatoon — sitting at the kind of sub-auroral latitude where SAID channels are most often reported — reasonable places to watch for it), making it a legitimate bonus for aurora chasers — just not, technically, the aurora itself.

Airglow: the sky’s constant, faint background glow

Airglow is a much subtler and far more common phenomenon: a very faint, roughly uniform emission from the upper atmosphere, caused by ongoing (not aurora-related) photochemical reactions rather than particles arriving from space. A major contributor is the Chapman mechanism, named after the same Sydney Chapman whose atmospheric research underpins much of modern space physics: during the day, sunlight splits molecular oxygen (O₂) into individual oxygen atoms; overnight, as those atoms slowly recombine back into O₂ and O₃ (ozone), a fraction of the energy released in that recombination escapes as a faint glow of light, mostly in the green and near-infrared. Separate photochemical processes involving sodium atoms and the hydroxyl radical (OH) contribute additional airglow emission at other wavelengths, high in the mesosphere. Unlike aurora, none of this is tied to geomagnetic activity or particle precipitation from space, and it happens every clear, dark night, everywhere on Earth, aurora or no aurora.

To the naked eye, airglow is essentially invisible except under exceptional dark-sky conditions, far from any light pollution. To a camera doing a multi-second exposure, however, it commonly shows up as a soft green or orange band low on the horizon, and it is one of the most frequent sources of “is this aurora?” confusion among people reviewing their own night-sky photos for the first time — especially from mid-latitude, storm-only locations where a similarly faint green tinge from actual aurora would be a much rarer, more newsworthy event. Astrophotographers who image the night sky regularly from mid-latitudes are often more familiar with airglow than most casual aurora watchers, simply because it shows up in almost every long-exposure sky photo they take, aurora or not.

Light pollution and light domes

A diffuse, warm-coloured glow low on the horizon is very often simply light pollution from a distant town or city — what’s sometimes called a light dome. It can look superficially similar to a faint aurora glow in a long-exposure photo, but two things distinguish it: it stays essentially fixed in position, colour and intensity over the course of an evening (since it’s coming from streetlights and buildings that don’t move or fade), and its colour (usually orange, amber or yellowish-white from sodium or LED streetlighting) differs from the greens and reds of genuine aurora emission. A useful practical test is simply to wait and watch: aurora shifts, brightens, dims, or moves within minutes; a light dome from the nearest town does not.

Noctilucent clouds

At high latitudes in summer, an entirely different and much rarer phenomenon — noctilucent (“night-shining”) clouds — can appear as delicate, electric-blue, wave-like structures low on the horizon shortly after sunset or before sunrise. These are ice-crystal clouds at extremely high altitude (around 80 km, higher than any weather cloud) catching sunlight from below the horizon; they’re unrelated to geomagnetic activity and only occur in a narrow window around the summer solstice at high latitudes — coincidentally, often the same season when many flagship aurora destinations have too much natural light for aurora viewing at all, making noctilucent clouds one of the few night-sky phenomena worth watching for specifically during a high-latitude summer visit.

Moving objects: satellites and aircraft

A separate, much simpler category of confusion involves things that move in a straight line at constant speed: satellites and aircraft. A bright pass of the International Space Station, or a “satellite train” — a line of dozens of newly launched satellites still flying in tight formation shortly after launch, a phenomenon that has become considerably more common and more widely photographed in recent years — can appear in a long-exposure night photo as a series of dots or a continuous streak. Distant aircraft navigation lights produce a similar effect, sometimes with a distinctive blinking pattern. None of these show the diffuse, irregularly shifting structure of real aurora, airglow, or STEVE; they move at a constant, predictable rate in a straight line, which is usually enough on its own to identify them once you know to look for it.

The practical takeaway

If what you’re seeing (or photographing) shows real structure — distinct arcs, rays, or curtains that visibly shift position or brightness over a few minutes — you’re very likely looking at genuine aurora. If it’s a smooth, static, colourless-to-faintly-coloured haze that doesn’t move much, it’s more likely airglow or light pollution. If it moves in a straight line at constant speed, it’s a satellite or aircraft, not any atmospheric phenomenon at all. When in doubt, check the actual geomagnetic conditions: a Kp index and OVATION probability well below your location’s threshold (see how aurora forecasts work) is a good reason to suspect you’ve caught one of these look-alikes rather than the real thing — and if you’re somewhere like Lerwick or Thunder Bay on a quiet night, a faint green photo is statistically more likely to be airglow than a genuine, if modest, aurora.