National Geographic : 1947 Nov
Unlocking Secrets of the Northern Lights Wallace J. Trapilo Three Draperies Appear Together in an Aurora Photographed at Narsarssuak, Greenland Predominating colors were yellowish green, with faint traces of a bright cherry red. The red parts were constantly in motion and appeared like bands of gas being ignited, the photographer reported. The picture was made on January 18, 1945. For those interested in the details of auroral displays the following outline is presented. It should be remembered that displays al most always follow a certain sequence, but do not always begin at the same stage of the sequence. Twilight may fall with an aurora already in full blast, while on another night no aurora is seen until after midnight. Three Main Forms of Auroral Displays Auroras are grouped in three main classes ray forms, homogeneous or nonray forms, and pulsating forms. The ray forms are the ray, rayed arc, rayed band, drapery, and corona. The nonray forms are glow, diffuse surface, homogeneous arc, and homogeneous band. The pulsating forms include arcs, pulsating surfaces, and flames. These forms have a tendency to appear in rather definite sequences. The smallest aurora consists only of a glow, a diffuse veil of light extending upward from the northern horizon and fading out gradually above. Often an arc, part of a circle, rises from the north. It is usually sharply bounded below and thus has a definite northern boundary. The usual height of these is 55 to 65 miles above the earth. Presently the glow fades, the arc brightens and may break into isolated parts or into diffuse, cloudlike surfaces which fade away into a glow that gradually disappears. When the lower arc border is bright, the arc usually changes to a rayed arc, a smooth curve with "searchlight" beams diverging from the top. The arc may become wavy and serpentine and is then a band which breaks into a rayed band. The ray parts may form curtains which move as if blown by a breeze. These larger displays become very compli cated, with horseshoe-shaped bands and dra peries (page 679). As they move south of the zenith, the converging rays form a corona. This convergence is a perspective effect, since all the rays are actually parallel.