National Geographic : 1947 Nov
The National Geographic Magazine C. W. Gartlein Pulsating Arc Auroras Flash Up and Disappear with a Regular Rhythm SThey pulsate with a period of one to 30 seconds. The color is usually bluish green. Short white lines are trails of light made by stars during the long exposure of the photograph. Measurements show that ray forms are higher than arcs. They run from 75 to 100 miles in height and sometimes to 600 miles when the upper atmosphere is sunlit. The corona center is not quite overhead, but about a fifth of the way down the southern sky from the true zenith. The arcs lie along the geo magnetic latitude parallels and have their highest point on the geomagnetic meridian. The pulsating forms appear near the peak of the display and may take the form of an arc which emits other faint arcs that travel upward rapidly. The most exciting form is the flame, where waves of light pass up ray forms to give the appearance of flames. The pulsating surface seems to be a brightening and fading diffuse surface. As the display fades, all forms become more faint and diffuse. Sometimes the arc reforms, and the complicated display begins over. It apparently cannot do this without the arc's reforming. Of course the combinations are varied and striking. Five arcs have been seen at one time at Ithaca. Unusual forms are the high arc, 120 miles high instead of 60, which is widely separated from other auroras. Regu larly spaced rays are very rare, as are certain types of pulsating forms. Twelve Forms of Auroras Our photographs and observations have borne out the reports of other investigators that there are 12 principal forms of auroras: 1. Glow. A faint glow near the horizon, re sembling the dawn, usually white or greenish color but sometimes red. This is often the upper part of an arc whose lower border is below the horizon. 2. Homogeneous Arc. The arc, when seen near the horizon, is usually diffuse above and sharply defined below, as if cut off by a dark segment of cloud (pages 681, 704). Less frequently in our latitude we see the arc isolated high in the sky, both edges being equally defined. Sometimes several parallel arcs occur and may be connected at one end by a sharp curve. The color is usually greenish yellow or nearly white.