Popular Astronomy

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Form and Structure

No two auroral displays are ever the same, however, most displays follow a fairly predictable pattern of behaviour, developing easily identified structures in the process.

The following notes describe each of the most commonly seen auroral forms.

Glow (N): The first sign of impending auroral activity is normally a faint glow of light lying low on the northern horizon. Auroral glows are similar in appearance to faint twilight conditions, and can easily be mistaken for the light of a distant town or city.
Veil (V): Bright and active displays can produce a background veil of auroral light covering a large area of sky beyond the main parts of the display.
Rayed Arc (RA): Formation of a rayed arc occurs when vertical columns of light project upwards from a homogeneous arc. Rayed activity often follows sudden brightening of the arc. Again, obvious folds or kinks may be present forming a Rayed Band (RB). Auroral Rays (RR) can also be seen in isolation, stretching up from the horizon in the absence of other obvious activity. This represents the uppermost parts of a display, the remainder of which is being masked by the observer's horizon and would be much more impressive from a more northerly location. Auroral rays mark the position of the Earth's magnetic field lines and can extend up to several hundred kilometers above the Earth's surface.
Patches (P): Commonly seen during the declining stages of a display are isolated patches of auroral light. Like arcs and bands, patches can be either homogeneous (HP) or rayed (RP).
Homogenous Arc (HA): As an auroral storm pushes southward it gradually assumes more definite structures. The homogeneous arc is a rainbow-like arch of light – often green in colour – seen only a few degrees above the north point of the horizon. At times the arc may form twists or kinks, in which case the structure is described as a Homogeneous Band (HB)
Corona (C): Very occasionally the aurora may push into the southern half of an observer's sky. When this happens perspective effects cause rayed forms to appear to radiate from a single point close to the zenith. This point represents the magnetic zenith which from UK locations lies 18–25 degrees south of the true zenith. Formation of coronal activity often precedes a decline in activity. Thereafter the aurora will fragment and retreat northward. However, on very rare occasions the aurora can penetrate south of the UK and push over the southern horizon. Recent examples of such unusually intense storms are the displays of 2003 October 29–30–31 and 2003 November 20–21.