Return of the Northern Lights

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The particles—most importantly, electrons—become trapped by the earth's magnetosphere and begin accelerating along the lines of the earth's magnetic field toward the magnetic poles, which are somewhat displaced from the geographical poles. There, the accelerated electrons encounter upper atmospheric gases in a large oval-shaped region around each magnetic pole; those particles excite the gases, causing them to glow in much the same way that electric current in a neon light causes the gas to shine. (The glow is emitted when an electron energized into a higher orbit of its atom falls back into its original orbit.) Unlike in neon lights, however, the prodigious power supply in northern lights is always varying, and the currents play with wild abandon across thousands of miles of the earth's atmosphere.

The gases—mostly oxygen and nitrogen—shine at altitudes ranging from as low as 50 miles up to 500 miles or more. Thus, the north auroral oval (which in satellite photos looks like a fiery necklace) contains an aurora up to hundreds of miles tall. These lights maybe seen towering—as arcs or curtains—in the north sky and may even be visible to observers far south of the location where the display appears directly overhead.

Particles on the solar wind are captured by the earth's mag relic Hold and carried to the polar regions, where they form an "auroral oval"around earth pole.

OBSERVATION AND PHOTOGRAPHIC TIPS

SOMETIMES A MAJOR SOLAR FLARE will be mentioned in the news; if so, you can watch for a possible auroral display about 24 to 72 hours after the flare occurs. Another possible forerunner of a good aurora is a large sunspot group near the central meridian, the central north-south line, on the sun. (Check a good astronomy book for information on how to view sunspots safely by projection with a telescope or binoculars; never look at the sun directly unless you have a suitable solar filter and plenty of experience!)

Additional information about flares, sunspot groups and probable northern lights sightings can sometimes be obtained by calling several non-toll-free recorded phone messages. Sky & Telescope magazine's Skyline (617/497-4168) offers a variety of interesting astronomy and space information, which is usually updated each Friday. The phone message of the Space Environment Services Center in Boulder, Colorado, (at 303/ 497-3235) is rather technical, but you can gather two important things from it. First, you are told whether the earth's magnetic field is—or will be—quiet or active. (A minor storm may produce good northern lights for much of the U.S., and a major storm means we should all be out under the night skies.) Second, every three hours, in Universal time (a 24-hour time period that is five hours ahead of eastern standard time and six hours ahead of central standard), the center gives the Boulder K Index, a measure of the disturbance of the earth's magnetic field. If that measure is four or less, an auroral display for much of the U.S. won't be occurring, but if it's higher than five, the aurora will almost surety be visible well south of Canada.

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