"The visual crimson at twilight was absolutely stunning against the still-blue sky," says Carol Lakomiak, who watched the display from Tomahawk, Wisconsin.

It was amazing," agrees sky watcher Brett Clapper. He was at a star party in North Carolina when the sky unexpectedly turned red. "A local Boy Scout troop was there and many of the boys had never even seen the aurora borealis before--what a treat!"

Across the Atlantic Ocean in Finland, photographer Jorma Koski says the "the auroras were so intense, they cast shadows on the ground."

It was a good time to be outside.

More good times are coming, say researchers, because autumn (which begins today in the northern hemisphere) is "aurora season." Autumn is special in part because lengthening nights and crisp pleasant evenings tempt stargazers outside; they see things they ordinarily wouldn't. But there's more to it than that: autumn really does produce a surplus of geomagnetic storms--almost twice the annual average.

In fact, both spring and autumn are good aurora seasons. Winter and summer are poor. This is a puzzle for researchers because auroras are triggered by solar activity. The Sun doesn't know what season it is on Earth--so how could one season yield more auroras than another? To understand the answer, we must first understand what causes auroras themselves.

Auroras appear during geomagnetic storms--that is, when Earth's magnetic field is vibrating in response to a solar wind gust. Such gusts pose no danger to people on the ground because our magnetic field forms a bubble around Earth called the magnetosphere, which protects us. The magnetosphere is filled with electrons and protons. "When a solar wind gust hits the magnetosphere, the impact knocks loose some of those trapped particles," explains space physicist Tony Lui of Johns Hopkins University. "They rain down on Earth's atmosphere and cause the air to glow where they hit--like the picture tube of a color TV."

Some solar wind gusts ("coronal mass ejections") are caused by explosions near sunspots, others are caused by holes in the Sun's atmosphere ("coronal holes") that spew solar wind streams into interplanetary space. These gusts sweep past Earth year-round, which returns us to the original question: why do auroras appear more often during spring and autumn?

The answer probably involves the Sun's magnetic field near Earth. The Sun is a huge magnet, and all the planets in the solar system orbit within the Sun's cavernous magnetosphere. Earth's magnetosphere, which spans about 50,000 km from side to side, is tiny compared to the Sun's.

The outer boundary of Earth's magnetosphere is called the magnetopause--that's where Earth's magnetic field bumps into the Sun's and fends off the solar wind. Earth's magnetic field points north at the magnetopause. If the Sun's magnetic field tilts south near the magnetopause, it can partially cancel Earth's magnetic field at the point of contact.

"At such times the two fields (Earth's and the Sun's) link up," says Christopher Russell, a Professor of Geophysics and Space Physics at UCLA. "You can then follow a magnetic field line from Earth directly into the solar wind." Researchers call the north-south component of the Sun's nearby magnetic field "Bz" (pronounced "Bee-sub-Zee"). Negative (south-pointing) Bz's open a door through which energy from the solar wind can reach Earth's inner magnetosphere. Positive (north-pointing) Bz's close the door.

In the early 1970's Russell and colleague R. L. McPherron recognized a connection between Bz and Earth's changing seasons. "It's a matter of geometry," explains Russell. Bz is the component of the Sun's magnetic field near Earth which is parallel to Earth's magnetic axis. As viewed from the Sun, Earth's tilted axis seem to wobble slowly back and forth with a one-year period. The wobbling motion is what makes Bz wax and wane in synch with the seasons.

In fact, Bz is always fluttering back and forth between north and south as tangled knots of solar magnetic field drift by Earth. What Russell and McPherron realized is that the average size of the flutter is greatest in spring and fall. When Bz turns south during one of those two seasons, it really turns south and "opens the door wide" for the solar wind.

Mystery solved? Not yet. In a recent Geophysical Research Letter (28, 2353-2356, June15, 2001), Lyatsky et al argued that Bz and other known effects account for less than one-third of the seasonal ups-and-downs of geomagnetic storms. "This is an area of active research," remarks Lui. "We still don't have all the answers because it's a complicated problem."

But not too complicated to enjoy. Dark nights, bright stars, an occasional meteor--and the promise of Northern Lights. Perhaps scientists haven't figured out why auroras prefer autumn, but it's easy to understand why sky watchers do....

Editor's note:Seasons are reversed in Earth's two hemispheres. Today is the beginning of both northern autumn and southern spring. Because geomagnetic activity is higher during spring and autumn, aurora season is therefore beginning in both hemispheres.

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