Aurora forms and names: what you are looking at

There is a standard vocabulary for auroral forms, agreed for international observing programmes and used consistently in the research literature. It is worth learning for a practical reason: the names describe stages of a process, so identifying what you are looking at tells you what is likely to happen next.

An auroral form is the shape the emission takes in the sky, classified by structure, by outline and by how it is moving. The classification below follows the scheme set out in the International Auroral Atlas, published under IAGA in 1963 and still the basis of how forms are described.

The three-part classification

Auroral forms are described by combining three attributes rather than by picking a single name from a list. The atlas notation writes structure first, then form, so a rayed band is RB and a homogeneous arc is HA.

Arc

An arc is a smooth strip of auroral light stretching roughly east to west across the sky, with a sharply defined lower edge and an upper edge that fades gradually. It is the most common form and usually the first thing to appear.

The asymmetry between the edges has a physical cause. The lower boundary is where incoming electrons lose the last of their energy and stop, which happens over a narrow altitude range around 100 kilometres and therefore looks like a floor. The upper boundary is set by the thinning of the atmosphere, which is gradual, so the top fades out.

A quiet arc sitting low on the poleward horizon and drifting slowly toward the equator is the signature of a substorm growth phase. It is the least dramatic thing in the sky and the best available warning that something is coming — see what a substorm is.

Band

A band is an arc that has developed folds, kinks or curls along its length. The transition from a smooth arc to a folded band is a visible marker that the display is intensifying.

Folds develop at several scales, and the atlas distinguishes them: small folds, large folds, and tight curls, in roughly increasing order of activity. In practice, if a smooth strip starts to ripple, the useful response is to stop adjusting the camera and watch.

Rays

Rays are vertical striations aligned with Earth’s magnetic field lines. They can appear within an arc or band, turning it into a rayed arc or rayed band, or occur as isolated bundles.

Their verticality is not an accident of perspective — the emitting particles travel along field lines, which are close to vertical at auroral latitudes. This is also why rays appear to move sideways rather than up and down: the structure shifts across the sky while each ray keeps its orientation.

Corona

A corona is rays appearing to converge on a point overhead. It is the appearance most people describe afterwards as the sky opening up.

It is a perspective effect rather than a separate form. The rays are parallel, and when you are directly beneath them you are looking along their length, so they seem to radiate from a single point near the magnetic zenith. Seeing a corona is informative: it means the emitting region is directly above you rather than toward the horizon, which normally happens only during a substorm expansion.

Patches and veils

A patch is an irregular region of emission with no clear form, often pulsating with a period of several seconds. Patches are characteristic of the recovery phase, after the structured display has broken up.

A veil is a broad, even, structureless glow covering a large area of sky. It is usually faint and is frequently the background over which other forms appear.

Both are easy to underrate. Pulsating patches during recovery are a sign that the magnetotail is reloading, and a second substorm may follow within a couple of hours.

Diffuse and discrete: the split that matters most

Underneath the form vocabulary is a more fundamental distinction, and it explains more disappointment than any other single fact in this subject.

Discrete aurora is the structured aurora of arcs, bands, rays and coronae. It is produced by electrons accelerated along magnetic field lines through a potential drop, which concentrates energy into narrow regions. It is bright, sharp-edged, and it is what people mean by the northern lights.

Diffuse aurora is a broad, structureless glow produced by electrons scattered into the atmosphere over a wide region. It is much fainter and it frequently sits below the threshold of unaided human vision while appearing clearly in a photograph.

This is why “the camera saw it and I did not” is such a common experience. It is not a failure of technique or of eyesight; it is diffuse aurora being recorded by an instrument that integrates light over seconds by an observer whose dark-adapted vision is nearly colourblind. The diagnostic table covers the practical consequences.

How bright is it, really

The International Brightness Coefficient rates visual auroral brightness on four steps, defined by comparison with familiar sources rather than in physical units.

The scale describes what an eye sees, not the energy deposited. A camera can produce a striking image of an IBC I display, which is the source of a great deal of mismatched expectation. Camera settings for the aurora and photographing it with a phone cover the other side of that gap.

Colour, and what it tells you about altitude

Colour is not part of the form classification, but it carries information about where the emission is happening.

Green at around 557.7 nanometres comes from atomic oxygen and dominates between roughly 100 and 150 kilometres. Red at 630.0 nanometres comes from atomic oxygen too, but from a transition with a long lifetime that only survives where collisions are rare — above roughly 200 kilometres. Blue and purple below about 100 kilometres come from ionised molecular nitrogen, and they appear when unusually energetic particles penetrate deeper than usual.

A display showing red above green is therefore telling you it extends high; a purple lower border is telling you it reaches unusually low. Why the aurora is green covers the emission physics.

STEVE and the picket fence: outside the classification

STEVE, short for Strong Thermal Emission Velocity Enhancement, is a narrow mauve or purple arc running roughly east to west, seen at lower geomagnetic latitudes than typical aurora. It does not fit anywhere in the atlas classification, and that is not an oversight — it is a different phenomenon.

STEVE is produced by a fast-flowing, heated stream of plasma in the subauroral ionosphere rather than by the particle precipitation that causes aurora. Its emission mechanism and its location both differ from auroral forms. Calling it an aurora is a convenient shorthand rather than a correct classification.

The name has an unusual history. It was coined by aurora photographers in Alberta, who had been recording the feature for years without a scientific name, and was later adopted as a backronym when researchers characterised it using satellite measurements alongside citizen-science reports. It is one of the clearest cases of amateur observation producing a scientific result.

The picket fence is a row of evenly spaced vertical green structures that sometimes appears below or alongside a STEVE arc. Unlike the mauve arc, the picket fence does appear to involve particle precipitation, which places it closer to aurora proper — and its frequent co-occurrence with STEVE is part of why the whole feature was initially assumed to be an unusual aurora.

If you photograph a narrow mauve arc at a latitude where aurora would be surprising, you have very likely seen STEVE, and it is rarer than the aurora you went out for.

What is not aurora

Two things are regularly reported as aurora and are not.

Airglow is faint continuous emission from the upper atmosphere, present on every clear night regardless of geomagnetic activity. In long exposures it appears as a mottled green or reddish wash. The distinguishing tests are motion and direction: aurora changes structure over seconds to minutes and is concentrated toward the pole, while airglow is effectively static across an exposure sequence and appears in all directions.

Light pollution produces a coloured glow above a horizon that can look convincing in a photograph, particularly where sodium or LED lighting dominates. It does not move, it sits over a town, and it is brightest at the horizon rather than showing vertical structure.

Bottom line

Auroral forms are named by combining structure — homogeneous, striated or rayed — with outline — arc, band, ray, patch or veil — following the classification set out in the International Auroral Atlas of 1963. An arc is a smooth east-west strip with a sharp lower edge around 100 kilometres, where electrons stop, and a diffuse top. A band is an arc that has developed folds, which signals an intensifying display. Rays are vertical because they follow magnetic field lines, and a corona is rays seen end-on from directly beneath, which means the emission is overhead. Patches and veils are typical of the recovery phase.

The deeper distinction is between discrete aurora, structured and bright, and diffuse aurora, broad and frequently below the visual threshold while still photographing well. Visual brightness is rated on the four-step International Brightness Coefficient, from IBC I at Milky Way brightness to IBC IV bright enough to cast shadows; most displays are IBC I or II.

STEVE is not aurora: it is a mauve subauroral arc produced by heated fast-flowing plasma, named by photographers and later characterised scientifically, and the green picket fence often seen with it is a separate structure that does involve precipitation.