aurora

aurora, a display of lights high in the atmosphere, known as the aurora borealis in the northern hemisphere and the aurora australis in the southern hemisphere. In Scotland the former is also known as the Northern Lights. Both auroras are most frequently seen at latitudes higher than 60°. A display consists of rapidly moving patches or columns of different coloured lights. They are most dramatic during disturbances in the earth's magnetic field and the sun is on the equinoctial, when there are periods of extensive interference with radio and telephone transmissions. They are less intense when the sunspot cycle is at its peak, when the sun emits more ultraviolet light. The UV light alters the ionization of the upper atmosphere so that it absorbs the particles being emitted from the sunspots more efficiently. But once the sun's activity has peaked, the atomic particles are able to penetrate the Van Allen radiation belt, particularly within 20° of the magnetic poles. In the upper atmosphere the particles collide with the gas molecules, exciting them to emit light in the visible spectrum. Occasionally the sun emits huge pulses of particles known as solar winds, which result in particularly spectacular displays of auroras that are visible at lower latitudes than normal.

M. V. Angel