SPA Aurora Section

Aurora Conditions, Brightness and Colour

The following terms are used to describe the overall condition and brightness of auroral forms...

Condition

QUIET (q).	Stable auroral structures which only show very slow changes in position or shape.
ACTIVE (a)	The term 'active' is used to describe aurora which starts to move around the sky. Active aurora is normally bright.
MULTIPLE (m)	Particularly strong displays can produce multiple forms. For instance, homogeneous arcs can appear in pairs, one lying above the other.
FRAGMENTARY (f)	At times, auroral arcs, bands and other forms may not fully develop. Arcs may not join in the middle or only occupy the eastern or western half of the sky. Fragmentary aurora is often seen during the declining stages of a display, described by some observers as 'break-up'.

Brightness

Auroral brightness is estimated using a 4 point scale...

1 Faint, comparable to the milky way.

2 Comparable to moonlit cirrus cloud.

3 Comparable to moonlit cumulus cloud.

4 Much brighter than 3. Perhaps able to cast shadows.

Auroral brightness can also vary over short timescales...

PULSATING (p1). Slow, steady changes in brightness and size. This condition is often seen to occur in auroral patches.

FLAMING (p2). One of the most spectacular auroral conditions. Flaming aurora consists of locally intense waves of light passing through the display towards the zenith. The passage of wind through a field of corn is often used to describe flaming. Flaming can herald a subsequent decline in activity, but can precede the formation of a corona.

Colour

The aurora extends upwards of around 100 km above the Earth's surface and occupies any area of varying atmospheric conditions which produce different colours. The most commonly seen colour is green, associated with arcs, bands and patches especially. Auroral rays are often green too, but also show a pink or reddish tint near the top. Aurora seen near dusk or dawn can actually lie in direct sunlight and frequently appears purple as a result. Colour in the aurora is only seen when a display is reasonably bright.

Created by Michael Oates Last modified 3 February, 2001

PULSATING (p1).	Slow, steady changes in brightness and size. This condition is often seen to occur in auroral patches.
FLAMING (p2).	One of the most spectacular auroral conditions. Flaming aurora consists of locally intense waves of light passing through the display towards the zenith. The passage of wind through a field of corn is often used to describe flaming. Flaming can herald a subsequent decline in activity, but can precede the formation of a corona.