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Aurora Season

As soon as the August rains let up, people living in interior Alaska could see the aurora (or northern lights) again---at least the nighthawks could. The best shows come late, it seems.

Aurora occurs all year, as the faithful electronic measuring devices here at the Geophysical Institute inform us (though institute director Syun-ichi Akasofu has told me they are least common near the solstices, for reasons no one yet properly understands). Images captured by high-flying satellites prove that the instruments are honest. Summer aurora is invisible to us because the long daylight keeps the sky too bright for the dancing curtains to be seen.

In fact, part of the reason that aurora displays in late August and September seem to occur only late at night is that twilight still lingers. It takes a few hours for the sky to darken sufficiently for the aurora to show up. Later in the year, the dancing lights may appear early enough to distract people driving home after work. That's a good sign that the aurora will be especially good all night, with displays of varying strength coming and going throughout the evening. Usually, though, the hours from about 11 p.m. to 1 a.m. offer the best chance of seeing the bright arcs that challenge photographers and captivate observers.

The time may not be ideal, but the place is nearly perfect. Alaska is probably the best location in the world for seeing auroras. That comment isn't based on loyalty to the state, but on statistics. In fact, statistically speaking, Ft. Yukon may be the aurora capital of the world. The northern hemisphere's auroral oval, an area shaped like a skewed doughnut and centered about the geomagnetic North Pole, behaves so as to favor our part of the world. The tourist commission had nothing to do with it; it's all physics. The interaction of particles streaming from the sun, the earth's magnetic field, and the gases high in the atmosphere combine to give us the best seats for watching auroras.

Since the auroral light is emitted high in the atmosphere---the lower edge of an auroral arc is about 80 kilometers (over 50 miles) above the ground---Fairbanksans can see Ft. Yukon's auroras. A display directly overhead at Ft. Yukon would appear to be 20 degrees above the northern horizon to someone watching in Fairbanks. If the aurora seems to meet the eastern horizon at Fairbanks, it is nearly overhead in Whitehorse.

When the aurora is directly overhead, auroral rays look like glowing needles in some gigantic game of pick-up-sticks, except they are all tidily oriented. The apparent convergence of the rays as they extend upward is only a trick of perspective. The rays are actually parallel. The point toward which they seem to converge is the magnetic zenith---a nice detail to know for impressing newcomers to the north.

Nowadays there's one aurora-watching platform the old-timers couldn't have dreamed of: jet aircraft. Commercial planes can offer terrific views to passengers on late-night flights. Try for a window seat so you can see east-northeast---the left side if you're flying south, right side heading north---and use a blanket or jacket to block out cabin light reflections. Thanks to the earth's curvature, auroras over 1000 miles away are visible from a plane flying at 30,000 feet. Since most of the earth's atmosphere is below cruising altitude on long flights, the air muffles little of the aurora's light; the aurora will actually be brighter seen from the airplane than from the ground. According to Neil Davis, who is presently writing the Aurora Watcher's Handbook, at times of good visibility and auroral activity, it's possible to see almost one-quarter of all aurora occurring in the Northern Hemisphere from a flight between Seattle and Alaska.