Types of aurora

Auroras appear in many different forms and no two nights are the same. The British Astronomical Association (BAA) provides some standard guidance to classifying aurorae. In practice, most of these types are not discrete entities. Rather, they transition into each other (sometimes rapidly) and there are many intermediates. It is often the case that more than one form is present at a time. Some of the variation in perceived appearance is also simply the result of the aurora changing in intensity and position relative to the observers' own location, rather than an actual change in structure. Scientific descriptions of aurorae take into account movement, structure, form, brightness and colour, and therefore there are numerous variations to the main forms that I describe below:


An example of more than one type of aurora: a general diffuse veil-type aurora over a wide part of the sky, with some isolated tall rays and some patches.



Glow

  • Description: Wide uniform area of auroral light. Essentially, a glow happens when there is a structured aurora in progress but beyond the horizon or line of sight. See also Veil for a type of auroral glow that occurs higher and wider in the sky.
  • Structure: Diffuse to concentrated uniform area.
  • Movement: None/slow.
  • Location: Usually appearing along the horizon. In Iceland I also sometimes see auroral glows along the tops of mountain ranges, when the main aurora is low in the sky and hidden from my vantage point by the land.
  • Colour (to naked eye): Usually white or pale green.
  • Colour (to camera): Green.

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Aurora glow coming over the top of a mountain range. The aurora itself is low down in the sky, behind the mountains.



Homogeneous arc

  • Description: "Rainbow" type arc of aurora extending east-west across the sky.
  • Structure: Uniform, well-defined upper and lower boundaries, no rays.
  • Movement: Slow. Tends to shift slowly northwards or southwards over time but not obviously to the eye.
  • Location: Can be anywhere from low on the horizon to directly overhead.
  • Colour (to naked eye): Usually either white (when weak) or shades of green (from pale to very vivid).
  • Colour (to camera): Green (shade depending on intensity).

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Homogeneous arc.



Rayed arc

  • Description: "Rainbow" type arc of aurora extending east-west across the sky, with rays.
  • Structure: Presence of vertical rays.
  • Movement: Slow. Tends to shift slowly northwards or southwards over time but not obviously to the eye. Some movement east-west and fluctuations in intensity of the rays.
  • Location: Can be anywhere from low on the horizon to directly overhead.
  • Colour (to naked eye): Usually either white (when weak) or shades of green (from pale to very vivid).
  • Colour (to camera): Green (shade depending on intensity), sometimes with purple ray tops.

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This is definitely not a great example of a rayed arc, as the aurora is becoming more of a band than an arc. However, rayed arcs are a rarity in Iceland in my experience.



Homogeneous band

  • Description: Band of aurora with an irregular shape such as "ribbons."
  • Structure: Uniform, well-defined upper and lower boundaries, no rays.
  • Movement: Slow to moderate. Tend to shift in location and in form, with the curves moving and reforming over time.
  • Location: Can be anywhere from low on the horizon to directly overhead.
  • Colour (to naked eye): Usually either white (when weak) or shades of green (from pale to very vivid).
  • Colour (to camera): Green (shade depending on intensity).

Homogeneous bands.

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Homogeneous bands.



Striated band

  • Description: Well-defined band of aurora, usually vertical and consisting of striated lines of aurora.
  • Structure: Non-uniform. Striated, consisting of multiple distinct narrow vertical stripes of aurora, with clear sky visible between.
  • Movement: Slow. Tends to stay in one area or shift subtly in location very slowly over time.
  • Location: In Iceland I see this type of aurora very regularly, and it usually comprises a thick swathe of aurora extending vertically upwards from the horizon and (usually) diminishing into a narrow diffuse band once overhead. Normally the striated aurora is concentrated either to the east or the west, narrows overhead and appears only weakly in the other direction.
  • Colour (to naked eye): Either white (when weak) or shades of green (usually paler shades).
  • Colour (to camera): Green (shade depending on intensity) and occasionally with pink fringes.



Rayed band

  • Description: "Curtain" of aurora with an irregular shape and with vertical ray structure.
  • Structure: Presence of vertical rays.
  • Movement: Can be fast-changing, with curves and shapes quickly forming and moving over time. Intense forms comprise curtains of vertical rays moving rapidly east or west across the sky.
  • Location: Can be anywhere from low on the horizon to directly overhead.
  • Colour (to naked eye): Usually shades of green to the naked eye, with intense bursts of moving rays sometimes having a pink lower boundary.
  • Colour (to camera): Green or yellow-green, pink lower fringe and/or red ray tops.

Rayed band.

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Rayed bands with tall rays and purple tops.



Rays

  • Description: Vertical shafts of auroral light that can occur singly or in clusters.
  • Structure: Vertical shaft of variable height.
  • Movement: Varies from no or very slow movement, where rays form and hang in one location in the sky, to moderate movement with clusters of rays shifting horizontally or flickering in intensity. Rapid movement of rays normally takes the form of rayed bands.
  • Location: Can be anywhere from low on the horizon to directly overhead, but I most commonly see discrete rays or ray clusters in the mid-sky.
  • Colour (to naked eye): White, yellow-green or green.
  • Colour (to camera): Green or yellow-green, sometimes with red or purple ray tops.

Isolated vertical rays hanging in a moonlit scene.

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Distinct clusters of rays often occur alongside other types of aurora.



Homogeneous patches

  • Description: Scattered, isolated "cloud like" patches of auroral light.
  • Structure: Uniform intensity, often with indistinct boundaries.
  • Movement: Little or no spatial movement, but intensity can alter.
  • Location: Can be anywhere from low on the horizon to directly overhead, but I most commonly see them in the mid-sky to overhead.
  • Colour (to naked eye): Usually white or pale green.
  • Colour (to camera): Green (various shades).

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Veil

  • Description: Wide uniform area of auroral light, without obvious structure.
  • Structure: Diffuse and uniform.
  • Movement: By nature, an auroral veil covers a wide area of sky and there is little or no spatial movement.
  • Location: Generally covers a wide area of sky, sometimes entirely from the horizon to overhead in a particular direction.
  • Colour (to naked eye): Usually a faint white or pale green.
  • Colour (to camera): Green (various shades).

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Veil of diffuse green across the sky.



Pulsating

  • Description: Rhythmic pulsating (appearance and disappearance in the same place) of multiple small patches of auroral light. Seem to occur in the aftermath of a burst of high auroral activity.
  • Structure: Uniform patches.
  • Movement: Very fast, appearing and disappearing at intervals as short as around 1 second.
  • Location: When I see pulsating aurora then they are usually overhead or otherwise high in the sky.
  • Colour (to naked eye): Usually a faint white or pale green.
  • Colour (to camera): Green (various shades) and sometimes with pink fringes.

Rapidly-pulsating aurora, showing pink fringes on camera.

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Corona

  • Description: Coronas are not really a specific form of aurora, but rather the result of perspective. When some types of aurora such as rays or rayed bands move directly overhead of an observer, the perspective looking up causes them to converge to a point to form a crown or "corona."
  • Structure: Converging tall rays.
  • Movement: Usually fast-moving and brief, resulting from active rayed bands of aurora moving directly overhead.
  • Location: Overhead.
  • Colour (to naked eye): The result of intense overhead aurorae, coronas are often bright green and sometimes have purple ray tops to the naked eye.
  • Colour (to camera): Vivid green with purple ray tops.

From the side.



Unclassified

Some auroras simply don't clearly fit the conventional categories. I would probably consider the aurora in this picture to be somewhere between homogeneous patches and a striated band, but sometimes it is difficult to classify them with certainty.