Observing The Visible Aurora
Form And Types of Aurora
Typical details of the visible aurora are given in the following illustrations. The code for describing the form and behaviour of the aurora together with methods of measuring and recording the activity on a standard form are detailed below.
Typical auroral forms and structures
|Aurora Borealis means ‘northern dawn’. This name was given because from middle latitudes it most frequently appears as dawn-like GLOW (N) along the northern horizon. The word ‘glow’ is to be used only when the auroral light is down on the horizon.
|Sometimes there are patches of auroral light without distinct boundaries, in clear sky (so that it is not the effect of obscuring clouds) and well up from the horizon (so that it is not a glow). These forms are called PATCHES (HP).
|Aurora often takes the form of an ARC (A) extending east-west across the sky. There is an area of clear below the lower edge, which is usually more clearly defined than the upper edge. When an arc has no vertical ray structure it is called a HOMOGENEOUS ARC (HA).
|Sometimes the auroral light forms a BAND (B) without the regular shape of an arc. If there is no ray-structure in it this is called a HOMOGENEOUS BAND (HB)
|When an arc shows vertical ray-structure it is called a RAYED ARC (RA). This form usually exhibits moderate activity, that is, small movements and irregular brightness variations
|When a band shows ray-structure it is called a RAYED BAND (RB). If the rays are long it may resemble a curtain or drapery waving in the sky.
|A common form assumed by the auroral light is that of a RAY (R), which is like a searchlight beam, usually nearly vertical. Rays may occur singly or in bundles.
|When rays or other forms pass overhead, perspective causes them to to converge to a point to form a crown or CORONA (c).