Forest Preserve District of Cook County, Illinois
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Northern Lights
Nature Bulletin No. 178-A   February 6, 1965
Forest Preserve District of Cook County
Seymour Simon, President
Roland F. Eisenbeis, Supt. of Conservation

To a person seeing the Aurora Borealis or "northern lights" for the first time, it is an uncanny awe-inspiring spectacle. Sometimes it begins as a glow of red on the northern horizon, ominously suggesting a great fire, gradually changing to a curtain of violet-white, or greenish-yellow light extending from east to west. Some times this may be transformed to appear as fold upon fold of luminous draperies that march majestically across the sky; sometimes as a vast multitude of gigantic flaming swords furiously slashing at the heavens; sometimes as a flowing crown with long undulating colored streamers fanning downward and outward.

The brighter auroras are commonly yellowish-green, often tipped with red along the lower edge, but there are many other colors including silver-white, pink, pale to deep reds, blue, violet and violet-gray. There are several light-forms varying from diffuse glows to homogeneous horizontal bands; or to homogeneous arcs, rays, bands and arcs with rays, draperies, coronas; and the pulsating forms which only occur at the peak of a display. Rarely, the latter may appear as waves of light rhythmically flashing upward along colored rays to disappear like flames. Some light forms are stationary; others may change slowly or rapidly in position, in brightness, or in color. A bright aurora lights up the landscape about half as much as a full moon, and the displays always follow a certain sequence, but do not always begin at the same stage of the sequence, because auroras also occur in daytime when they are not seen, and twilight may fall with one of those in full blast.

A few scientists, who have photographed, measured and studied auroras for many years, tell us that the most brilliant displays coincide with the periods of greatest sunspot activity of the sun, as in 1937 and 1946, appearing more frequently and more strongly in September and March when the earth is most nearly opposite a huge sunspot facing the earth.

Sunspots are tremendous whirlpools or cyclones in the molten or gaseous surface of the sun, increasing and decreasing thru a regular cycle of approximately 11 years, featured by terrific flare-ups of hot gases spouting forth like streams from a gigantic hose, sometimes for weeks. As the sun revolves, these streams sweep thru space, widening as they travel at hundreds or thousands of miles per second, like water from a rotating lawn sprinkler. If the earth happens to get in the path of such a stream it gets a bath of the electrified particles of gas -- recently discovered to be hydrogen. Most of these particles, encountering the invisible magnetic field which enveloped the earth, are diverted toward our north and south magnetic poles -- which are now about 12 degrees away from the geographic poles of the earth's axis. This explains why the region where northern lights most frequently appear is an elliptical ring passing thru northern Norway, central Hudson Bay and Point Barrow, Alaska; a ring centered about the north magnetic pole and extending much farther south on the American side than in Siberia on the Asiatic side.

The particles of nitrogen and oxygen gases in the ionosphere -- which begins at about 50 miles above the earth's surface and extends upward 250 miles or more -- bombarded by the electrified particles of solar hydrogen, become luminous and produce the aurora. Never approaching closer than 35 miles, this glow may extend as much as 600 miles above the earth. It is at such times that it is seen in Texas and Florida and may extend across the continent. At such times, too, we have "magnetic storms" that disrupt radio, telegraph and telephone communication.

Few people ever see the southern light: the aurora australis. Same difference.

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