Forms

(Section 3.2.4.3.3)

Polar aurorae display several distinct forms and structure. These can be classified as follows:

• Glow: A dawn-like glow along the horizon
• Homogeneous patches: Diffuse patches of auroral light without distinct boundaries and of no particular shape
• Veil: A general auroral light covering most of the sky, but with little structure
• Homogeneous arc: An arc extending across the sky as a uniformly curved arch of light, without any vertical ray structure; there is a relatively well-defined lower edge that is more clearly defined than the upper edge
• Homogeneous band: A band across the sky without the regular shape of an arc and without any vertical ray structure − a twisted arc; the lower edge will be more irregular and less well defined than that of an arc
• Rays: Rays or shafts of vertical auroral light, like a searchlight beam stretching upwards into the sky; may occur singly, scattered or in bundles
• Rayed arc: An arc with vertical ray structure; usually exhibits moderate activity of small movements and irregular brightness variations
• Rayed band: A band with vertical ray structure; if the rays are long it may show kinks and folds and resemble a curtain or drapery waving in the sky; in a strong display, multiple bands may be present
• Corona: Rays or other forms overhead converging to a point to form an auroral crown or corona