An auroral form (see aurora) that appears as an arch across the sky, normally with a fairly well-defined lower edge and a more diffuse upper border. The highest point of the arc is towards the magnetic north. An arc is often the first distinct form to become visible during an auroral display, although it may be preceded by an overall veil, a horizon glow, or (occasionally) by diffuse patches. Two varieties are commonly seen: a homogeneous arc, which is largely featureless, and a rayed arc, with distinct vertical striations. See ray, auroral.
Subjects: Meteorology and Climatology.