Types of Aurora

A quiet arc is a smooth band of green stretching across the northern horizon, roughly east to west, with a sharp lower edge and a soft upper one. It barely moves. Most people who have seen the aurora once have seen this.

The sharp bottom edge is where the incoming electrons run out of energy, around 100 km up. The fuzzy top is where the atmosphere thins out and there is less oxygen left to excite. That is why the boundary is crisp below and vague above, and it holds for almost every form here.

An arc means the system is loaded but has not released yet. It can sit like that for hours. It can also be the ten minutes before everything happens, because an arc is what breaks up into a substorm.

A band is an arc that has developed folds and kinks. Instead of a straight line it curls, sometimes doubling back on itself in loops that look like a ribbon caught in wind.

The wind image is wrong but useful. Nothing is blowing. The folds trace the shape of the magnetic field lines the electrons are riding down, and when the field gets disturbed the pattern in the sky follows within seconds.

Bands are where the aurora becomes obviously alive. If you are watching an arc and it starts to kink, stay out. This is usually the beginning of a substorm.

Rays are the vertical stripes running up out of an arc or band, sometimes reaching hundreds of kilometres. They are the classic curtain shape and the reason "curtain" is the word everyone reaches for.

Each ray follows a magnetic field line. Because those lines run nearly vertical at these latitudes, so do the rays. What looks like a hanging curtain is really a sheet of light standing on edge, and you are seeing it from the side.

Rays mean the electrons arriving are more energetic and better organised than during a quiet arc. When rays appear, the display is building.

A corona is the moment the rays appear to radiate outward from a single point overhead, like looking up the inside of a dome. It is the most striking thing the aurora does, and it is a trick of perspective.

The rays are still parallel. You are simply standing underneath them, looking straight up their length, and parallel lines converge at a vanishing point. It is the same reason railway tracks meet on the horizon. The convergence point sits at your magnetic zenith.

You only get a corona when the aurora is directly overhead, which means you have to be under the oval rather than looking at it from the south. That is the strongest argument for going to 68 to 70°N rather than watching from further down. From Scotland or Maine you see arcs on the horizon. From Tromsø you can be inside it.

After a substorm burns out, the sky often settles into large, diffuse patches that blink on and off over a few seconds, out of step with each other. It looks less like curtains and more like faintly glowing clouds switching themselves on and off.

This is pulsating aurora, and it usually runs in the small hours after midnight. It is the recovery phase, when the system is dumping its remaining energy in a slower, less organised way.

It is easy to miss because it is dim and slow, and people who have packed up after the substorm never see it. If you are already out and the show seems over, give it another twenty minutes.

Red aurora sits above roughly 200 km, where oxygen is thin enough that a slow, faint red emission has time to happen before anything interferes with it. Because it is high, it is visible from much further south than green, which is why extreme storms produce reports of red skies over places that never see aurora. Red usually means a strong event.

STEVE is the interesting one. It appears as a narrow mauve or purple-white ribbon running east to west, often with a green picket-fence structure beneath it, and it sits noticeably further south than normal aurora. Citizen scientists in Alberta documented it and named it, half as a joke, before the science caught up.

It then turned out not to be an aurora at all. Aurora is caused by particles hitting the atmosphere. STEVE is caused by a river of extremely hot gas flowing through the upper atmosphere at a few kilometres a second, glowing from the heat and friction rather than from particle bombardment. It is a different phenomenon that happens to look similar and to appear during the same conditions.

If you are at a mid latitude and see a thin purple ribbon running east to west rather than a green arc to the north, you may have found one. Photograph it, note the time and location, and it is genuinely useful data.