The Continuous Aurora

Only during the last decade, when highly sophisticated satellites began to circle the polar regions, have we become aware that the aurora is a truly continuous part of the earth's environment. A person on the ground can see only a comparatively small part of the polar atmosphere wherein the Northern Lights reside, but the properly instrumented satellite can skim across the polar region to look downward and detect whatever aurora exists, wherever it may be. These satellites find that the aurora is always there.

Twin auroral halos crown the two polar regions. Each sits atop the denser part of the atmosphere, rarely penetrating down to altitudes below 70 km (40 miles): more usually the base of the aurora is near altitude 100 km. Called the auroral ovals, the two halos often consist of weak, parallel arcs embedded in a diffuse auroral glow a few hundred kilometers in width. At such times, the auroral ovals typically have diameter of only four or five thousand kilometers. Then, of course, only persons living at very high latitude can see the aurora, and then only if they look closely on a dark, clear night. One living on the north coast of Alaska or western Canada would see aurora only by looking northward.

During times of high auroral activity, the ovals widen and expand toward the equator. Then the aurora might be seen overhead in central Alaska or Canada. On the average, auroras extend this far equatorward perhaps one night in three, or four. Once in a great while, perhaps once in ten years or so, the twin ovals expand so much they cloak practically all of the earth's sky except for that of the tropical region.