Morphology of Auroras

Auroral Morphology

Auroral morphology describes the forms/types of visible auroras that occur within our atmosphere.

Forms without Rays

Homogeneous Quiet Arcs (H.A.): can appear near the horizon; can be narrow or broad and very often diffuse along the upper border but sharp along the lower one; several parallel arcs can appear at the same time and can merge together to form a large arc or zone across the heavens.

Homogeneous Bands (H.B.): don't have regular shape as do the arcs and are more rapidly moving. The lower border is irregular and sharp; sometimes consists of a semi-circle and ellipsoidal shape.

Pulsating Arcs (P.A.): parts of an arc can flash up and disappear regularly with a period of about 20 seconds and often stands isolated in the sky without other auroras; can often be more rapid and much more intense in such a way that whole arcs appear and disappear one after another almost at the same place.

Diffuse Luminous Surfaces (D.S.): appear like a diffuse veil or glow over great parts of the heavens without distinct boundaries and often appear after intense displays of rays and curtains or as more isolated feeble luminous streaks which sometimes bear a striking resemblance to clouds. May appear as green, violet or red, diffuse light.

Pulsating Surfaces (P.S.): diffuse patches of light can appear and disappear rhythmically at the same place, retaining the same irregular shape; forms often appear in connection with flaming auroras.

Feeble Glow Near the Horizon Resembling the Dawn: of white or red color; often upper part of an arc whose lower border is below the horizon.

Forms with Rays

Arcs with Ray-Structure (R.A.): a homogeneous quiet arc that becomes sharp and luminous along the lower border and then rapidly changes into an arc of rays.

Bands with Ray-Structure (R.B.): bands that are made up of a series of rays which can be arranged close to each other along the band or can appear more scattered; when near magnetic zenith may have the form of a corona.

Draperies (D): rays become very long and the band appears like a curtain or drapery whose lower border is often more luminous.

Rays (R): can be isolated, narrow, or broad, short or long; can appear in great segments or like masses of rays.

Corona (C): by perspective, forms when rays approach the magnetic zenith because they seem to converge at that point: can also be formed by bands or by draperies near the magnetic zenith (or by more diffuse forms).

Other Types

Flaming Auroras (F): characteristic rapidly moving form consisting of strong waves of light which move upwards one after the other in the direction of the magnetic zenith; waves can have the form of detached arcs which move upward normally to the direction of the arcs or they can be compared to invisible waves which, in their passage, illuminate broad rays and patches which appear and reappear rhythmically when the waves pass them.

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