Mankind's Explanation: Aurora Borealis
Aurora is
a natural display of light in the sky that can be seen with the unaided eye only
at night. An auroral display in the Northern Hemisphere is called the aurora
borealis, or the northern lights. A similar phenomenon in the
Southern Hemisphere is called the aurora australis. Auroras are the most
visible effect of the sun's activity on the earth's atmosphere.
Most auroras occur in far northern and southern regions. They appear chiefly as
arcs, clouds, and streaks. Some move, brighten, or flicker suddenly. The most
common color in an aurora is green. But displays that occur extremely high in
the sky may be red or purple. Most auroras occur about 60 to 620 miles (97 to
1,000 kilometers) above the earth. Some extend lengthwise across the sky for
thousands of miles or kilometers.
Auroral displays are associated with the solar wind, a continuous flow of
electrically charged particles from the sun. When these particles reach the
earth's magnetic field, some get trapped. Many of these particles travel toward
the earth's magnetic poles. When the charged particles strike atoms and
molecules in the atmosphere, energy is released. Some of this energy appears in
the form of auroras.
Auroras occur most frequently during the most intense phase of the 11-year sunspot
cycle. During this phase, dark patches on the sun's surface, called sunspots,
increase in number. Violent eruptions on the sun's surface, known as solar
flares, are associated with sunspots. Electrons and protons released by
solar flares add to the number of solar particles that interact with the earth's
atmosphere. This increased interaction produces extremely bright auroras. It
also results in sharp variations in the earth's magnetic field called magnetic
storms. During these storms, auroras may shift from the polar regions toward
the equator.
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Contributor:
• C. R. O'Dell, Ph.D., Professor of Space Physics and Astronomy, Rice
University.
C.
R. O'Dell, "Aurora," World Book Online Americas Edition,
http://www.aolsvc.worldbook.aol.com/wbol/wbPage/na/ar/co/038160, October 20,
2001.