National Geographic : 1947 Nov
Unlocking Secrets of the Northern Lights BY CARL W. GARTLEIN* EVER since Bible times, and probably much further back in history, people have been awed, thrilled, and often frightened by the beautiful and sometimes dis turbing spectacle of the polar aurora. Men have thought that the glowing, danc ing, rippling arcs and streamers of light in the sky, often called in our hemisphere the "northern lights," were the battles and ma neuvers of spirits or gods, portents of evil tidings, or even warnings of the end of the world. When I was a boy in Indiana, people used to say that the aurora was a reflection of sun light from the faraway ice fields of the Arctic. No one stopped to think that for months at a time during the long night of the polar winter sunlight does not fall upon the ice fields of the Arctic, although the aurora is visible then as much as at other times; so this explanation could not possibly be true. The Causes of the Aurora Today we know what really causes the aurora. Its gorgeous display is produced by a process of Nature so dramatic, vast, and awe inspiring that the old legends and supersti tions seem pale by comparison. Picture to yourself gigantic streams of elec trified particles spouting forth from the far away sun and speeding millions of miles out into space like streams from a gigantic hose. Sometimes they pour out for weeks at a time. As the sun revolves on its axis, these huge streams sweep around through space much as do the jets of water issuing from a rotating lawn sprinkler. Every so often one of them catches the earth in its path, and then for hours or days our planet is drenched with this shower of electrified particles. When one of these streams approaches the earth, it encounters an invisible field of mag netic force that forms a sort of nebulous envelope extending out thousands of miles around the earth. It is exactly the same kind of field of force that exists around an ordinary horseshoe magnet and is what guides compass needles toward the north. This magnetic field diverts most of the on rushing sun particles toward the north and south polar regions (diagram, page 680). There they speed down into the upper part of * Dr. Gartlein is director of the National Geo graphic Society-Cornell University Study of Aurora, carried on since 1938 at Cornell, where he is a member of the Department of Physics. the earth's atmosphere and collide with atoms of the rarefied upper air. The collisions excite the atoms so that they give off light, setting up a glow which can be seen from the earth far below. This glow is the aurora. A Faraway View of the Earth If you could see the aurora from out in space, it would appear like two luminous clouds near both Poles on the dark side of the earth, hundreds or even thousands of miles in length and breadth. At times it may extend 600 miles or more up into the atmosphere. Auroras may consist merely of ill-defined patches of light or may be in the form of streamers, arcs, straight or wavy bands, rays fanning out from a center, or "curtains" of light which seem to hang downward. One of the rarest forms is the corona, which appears close to the zenith (Plate IV and page 694). In most auroral displays the light is greenish white, but in brighter displays it may be yel lowish, greenish, or red. In some cases the light forms are stationary; in others they change slowly or rapidly in position, in bright ness, or in color. A vivid description of an unusually fine aurora in 1866 is given by George Kennan in Tent Life in Siberia, G. P. Putnam's Sons: "The whole universe seemed to be on fire. A broad arch of brilliant prismatic colours spanned the heavens from east to west like a gigantic rainbow, with a long fringe of crimson and yellow streamers stretching up from its convex edge to the very zenith. At intervals of one or two seconds, wide, luminous bands, parallel with the arch, rose suddenly out of the northern horizon and swept with a swift, steady majesty across the whole heavens, like long breakers of phosphorescent light rolling in from some limitless ocean of space." Our primitive ancestors were more right than they knew when they believed that the aurora was a portent of trouble, though it por tends a kind of trouble far different from any they ever imagined. The same showers of particles that produce the aurora also produce what we call "mag netic storms," which can and often do make radio, telephone, and telegraph communi cations impossible or at least highly unreliable for hours. So the communications engineer feels a little unenthusiastic about the beauty of the aurora's lights, for he knows they are a symptom of something that means only dif ficulties for him.