Solar weather whips up Northern Lights show

By Jim Downing
Seattle Times staff reporter

JAMES BRANAMAN / THE SEATTLE TIMES The Northern Lights shimmered for about eight hours Sunday night above the Puget Sound region and may again tonight or tomorrow, forecasters say. This view was from Hamilton Viewpoint in West Seattle.

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For about eight hours, an "extreme geomagnetic storm" raged in the upper atmosphere, as high-energy particles emitted by the sun three days ago swept over Earth. Hence, Sunday evening's throbbing Northern Lights, visible as far south as Oklahoma.

The National Oceanic and Atmospheric Administration's "solar weather" forecasters are predicting a 30 percent chance of similarly severe cosmic weather through tonight, decreasing to a 15 percent chance tomorrow.

While the light show, known as aurora borealis in the Northern Hemisphere, came as a surprise to most in Seattle, astrophysicists had seen it coming for days, said John Sahr, an associate professor of electrical engineering at the University of Washington.

A storm this size is "a once or twice a year — or less — event, at this time in the solar cycle," Sahr said. Solar activity varies according to an 11-year cycle; the cycle peaked last year with some of the heaviest solar activity recorded. The current storm, Sahr said, is the strongest in months, but only about one-tenth the intensity of last year's events.

Scanning the sky Check the forecast The NASA-sponsored Web site www.spaceweather.com gives solar-wind forecasts and provides alerts about upcoming Northern Lights events and fun facts, such as the current speed of the solar wind. Last night, the forecast was for a 30 percent chance of severe geomagnetic storms through tonight, decreasing to 15 percent tomorrow. Get to someplace dark and wide Find a viewing spot away from city lights and where you can get a wide view of the sky. Check the sky Clouds, obviously, block the view; moonlight will make the lights harder to see. Source: National Aeronautics and Space Administration

Predicting the "weather" in space, and thus the intensity of the Northern Lights, is in some ways similar, and about as accurate as predicting weather on Earth, said Robert Winglee, a UW earth-and-space sciences professor.

Spacecraft "weather stations" between the sun and Earth transmit information on the energy and velocity of particles streaming toward the Earth. Still, Winglee said, we can't know how those particles will interact with Earth's magnetic field until the storm is almost on top of us. "We typically have only 40 minutes' warning," Winglee said.

Once a strong solar wind begins to buffet Earth, things get interesting. "At some point, the magnetic field can no longer support all the energetic solar-wind particles, and the magnetic-field lines become unstable," said Alfred Hanssen, a physics professor at the University of Tromso, Norway.

"This instability literally means that the magnetic field starts shaking, and the result is that the charged particles are dumped into the atmosphere in a [ring] close to the magnetic North and the magnetic South Pole," Hanssen said via e-mail.

These charged particles collide with molecules of oxygen and nitrogen in the atmosphere 20 to 200 miles above Earth. The collisions give off different colors depending on the altitude of the collision and the gas involved.

Collisions with oxygen at 150 miles or more above Earth produce red light; collisions with oxygen up to 150 miles produce green; nitrogen collisions higher than 60 miles emit purple-violet light, while nitrogen collisions lower than 60 miles emit blue.

	FRED NELSON / THE SEATTLE TIMES The aurora borealis as viewed from Golden Gardens Park in Seattle late Sunday. The 30-second exposure was taken on a tripod.

Since charged particles are dumped around the magnetic South Pole as well, Hanssen said, a Southern Light is formed simultaneously. "The Northern and Southern Light formed at any time are mirror images of each other," he said.

The Northern Lights appear in the sky above Seattle several times each year but are often difficult to see due to cloud cover. "I've never seen it this bright in Seattle," said Winglee of UW.

Rather than following the cosmic-storm online, he said, "I went outside and looked at the aurora instead."

Jim Downing: 206-515-5627 or jdowning@seattletimes.com

Copyright © 2004 The Seattle Times Company

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