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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
NORTHERN LIGHTS
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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Update: December 2011
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