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The Polar Crowns
| Article
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13750 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
8 / 1988 |
1,641 Words |
| Author
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Syun-Ichi Akasofu
Syun-Ichi Akasofu is director and professor of geophysics at
the Geophysical Institute of the University of Alaska in
Fairbanks. His studies include the aurora and magnetospheric
disturbances. He is currently involved in a study of solar
flares and the resulting interplanetary disturbances. |
"Who but God can conceive such infinite scenes of glory?" exclaimed polar explorer Charles Hall, with a sigh. William Hoper, another polar explorer, recorded with freezing hands: "Language is vain in the attempt to describe its ever varying and gorgeous phases; no pen nor pencil can portray its fickle hues." These words were written after they gazed upon the magnificent polar auroras.
The auroras--one in the Northern Hemisphere, called the aurora borealis, and another in the Southern Hemisphere, called the aurora australis--look like glowing, moving curtains or thin sheets of light, as they hang in the dark polar sky. Often, there is more than one auroral curtain. At times there are as many as a dozen stretching across the sky from the eastern to the western horizons.
The height of the auroras was one of the most controversial issues among auroral scientists in the nineteenth century, many thinking that they arose from a mountaintop or man-made structure. But scientists have now determined that the height of the bottom of the auroral curtain is about 60 miles above the earth--almost ten times as high as the cruising altitude for jet aircraft.
American astronauts and Soviet cosmonauts have been able to look down at the auroras from the windows of their space vehicles while orbiting the earth at an altitude of a few hundred miles. Also, many auroral images have been taken by satellite-borne devices from an altitude of 10,000 miles. These pictures show that the auroras appear as a ring of light around the poles, or more accurately, the northern and southern magnetic poles. These rings of light are called the auroral ovals.
The auroral ovals are a permanent feature of the earth. They are always present as two crowns of light around the poles, but, for several reasons, they are not always visible from a point on earth. First, in daytime, they cannot be seen because the sun is too bright. Second, the radius of the auroral ovals varies considerably. When an aurora is not active, the oval is located well above the latitude of 700. As an aurora becomes active, the oval expands and the aurora descends to a latitude of about 600, roughly at a line connecting Anchorage, Oslo, Stockholm, and Leningrad. During its most active times, the aurora borealis can be seen overhead in Seattle, Minneapolis, New York, and middle-European cities. On such an occasion, its upper height can exceed 600 miles and the aurora can be seen slightly above the northern horizon in much lower latitudes, such as in Mexico, or Hokkaido,
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