Why the Aurora Is Green (and When It Isn't)

Almost every explanation of aurora colour says something like "different gases produce different colours". That is true and it hides the more interesting fact, which is that the two most common aurora colours come from the same element.

Green at 557.7 nanometres and deep red at 630.0 nanometres are both atomic oxygen. What separates them is time.

When a particle from the magnetosphere hits an oxygen atom, it knocks an electron into a higher energy state. The atom then emits a photon as the electron falls back. But it does not do this instantly, and how long it waits depends on which state it landed in.

The state that produces green light has a lifetime of roughly one second. The state that produces red light has a lifetime of about two minutes.

Two minutes is a very long time for an atom. Down where the air is denser, an excited oxygen atom will collide with something else long before those two minutes are up, and that collision carries the energy away as motion rather than light. The physicists call it quenching. The atom was ready to glow red and something bumped into it first.

So a display showing a strong red top and a green body is telling you it extends over a couple of hundred kilometres of altitude. A purple or pink lower fringe means the particles are getting deeper than usual, which generally means the event is a strong one.

The colour is a readout of where in the atmosphere the energy landed. It is one of the few things you can diagnose standing in a field with no instruments.

This is the part that catches most first-time visitors, and the industry does not go out of its way to warn them.

Human colour vision runs on cone cells, which need a reasonable amount of light. In the dark you switch to rods, which are far more sensitive but cannot distinguish colour at all. So an ordinary aurora, which is not very bright, registers as a pale grey or grey-green band. The colour is there. Your eyes cannot get at it.

A camera has no such limit. Open the shutter for ten seconds and it accumulates light until the green is obvious. That is why the photograph that persuaded you to book looks nothing like the thing you will stand under, unless you get a genuinely strong display, at which point the cones do fire and the colour appears to the naked eye as well.

A small extra irony: human vision peaks in sensitivity at around 555 nanometres, which is almost exactly the 557.7 nm of the green oxygen line. Of all the colours the sky could have chosen, it picked the one we are best equipped to notice. We are still bad at it in the dark.

None of this is a reason not to go. It is a reason to arrive with the right expectation, and it is covered properly on our page about what a phone can capture.

In January 2016 members of a Facebook group called the Alberta Aurora Chasers started sharing photographs of something that did not behave like aurora: a narrow mauve ribbon running east to west, sometimes with a green picket-fence structure beside it, appearing well south of the oval.

They had been calling it Steve, a joke borrowed from the animated film Over the Hedge, where the characters name something they do not understand. When researchers at the University of Calgary and NASA looked into it with them, the name stuck and was reverse-engineered into Strong Thermal Emission Velocity Enhancement.

It turned out to correspond to a subauroral ion drift, a fast river of charged particles that had been measured by instruments for decades but never knowingly seen. It is not produced by particles raining down the way the aurora is, which is why it is not, strictly, an aurora at all.

The reason to tell this story is not the trivia. It is that a group of amateurs with cameras found something the professional literature had missed, in a sky that people have been photographing for a century.

Beyond the standard palette there are a few emissions that turn up rarely enough that most people never knowingly see one, and that photograph better than they look.

SAR arcs, short for stable auroral red arcs, are wide, faint bands of pure red light that appear well south of the main oval during and after strong storms. They are not produced by particles raining down at all: the energy arrives by heat conducted from the ring current far out in the magnetosphere. They are usually invisible to the naked eye and show up in long exposures as a broad red band where the photographer expected nothing.

Proton aurora comes from incoming protons rather than electrons. It is diffuse, structureless and very faint, which is why it rarely features in photographs. It matters more to researchers than to visitors.

The pink or magenta lower fringe that appears along the bottom edge of an active display is molecular nitrogen, and it needs particles to penetrate unusually deep. When you see it, the event is a strong one.

And STEVE, covered below, which for years was photographed by amateurs and ignored by the literature.

The common thread is that the sky at high latitude is doing more than one thing at a time, and our eyes are equipped for almost none of it. Which is the honest reason photography has become so central to this hobby: the camera is not embellishing, it is the instrument.

Since the colours in a genuine photograph already look unreal, a certain amount of outright fabrication circulates alongside them, especially in tour marketing. A few things give it away.

• The Milky Way and a bright aurora in the same sharp frame. Possible in principle, and often composited in practice. The giveaway is when both are unnaturally crisp: the exposures that suit each are different.
• Aurora reflected in water with no ripple, on a night with visible wind elsewhere in the frame.
• Colours outside the physical palette. Green, red, blue, violet and their blends are real. Saturated orange or yellow curtains are not.
• A full moon and a faint Milky Way together. Moonlight drowns the Milky Way. Having both means two frames.
• The same sky over different landscapes across an operator's gallery. Worth a reverse image search if you are choosing a company on the strength of its photographs.
• Foreground lit evenly with no visible light source, on a moonless night.

None of this means processed images are dishonest. Every serious aurora photograph has been through raw conversion and adjustment, and the green really is there. The line worth caring about is between bringing out what the sensor recorded and assembling something that did not happen, and the second kind is most common exactly where it does the most damage: in the listings people book from.

• NOAA Space Weather Prediction Center — emission physics and the aurora tutorial: swpc.noaa.gov.
• Canadian Space Agency — accessible reference on auroral colours and the altitudes they occur at.
• STEVE: MacDonald, E. et al. (2018), New science in plain sight: Citizen scientists lead to the discovery of optical structure in the upper atmosphere, Science Advances, doi 10.1126/sciadv.aaq0030. The initial observations came from the Alberta Aurora Chasers group in January 2016, working with researchers at the University of Calgary and NASA.
• Emission wavelengths: atomic oxygen at 557.7 nm (green) and 630.0 nm (red), molecular nitrogen for the blue and violet components, with altitude ranges following standard references.
• Human night vision: rod cells dominate at low light levels and do not discriminate colour, which is why a faint aurora reads as grey to the eye and green to a sensor.