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.
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| HOMOGENEOUS 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).
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| 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.
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| 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.
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| 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).
|
| 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.
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