Chances of Aurora Interrupting Dinner to Increase

I was eating a cheeseburger at a restaurant in Healy, Alaska, last weekend when a man pounded on the window. He pointed to something he needed to share with a stranger--a purple river of aurora flowing directly over his head.

Soon, the entire restaurant staff and I were shivering outside, looking up at bands of aurora borealis that rippled from east to west. Even the sourdoughs who had seen hundreds of displays started whooping as the show went on. This was no ordinary aurora. People saw it clearly through the light pollution of downtown Anchorage. Those in Juneau saw displays more typically seen in Fairbanks. Bands of aurora flickered over Seattle and upstate New York.

Many scientists compare the aurora to an electrical generator that sputters at times, purrs at others. That night, the generator ran beautifully, said Charles Deehr, the Geophysical Institute's aurora forecaster and a professor emeritus. Deehr's job includes listing weekly predictions of aurora activity and posting them on the Internet at www.gi.alaska.edu.

Because the sun rotates every 28 days, Deehr and others are able to predict the intensity of some auroral displays. Electron and proton particles that create the aurora flow out from dark holes on the sun. Every time the sun rotates, streams of these particles travel toward Earth, speeding up the solar wind and causing auroral displays when the particles transfer their energy to the atmosphere about 65 miles over our heads.

Solar flares, explosions on the surface of the sun, were the ingredients for last weekend's batch of outstanding aurora. Auroras born from great solar flares are often seen much farther equatorward than the more predictable auroras created from dark holes on the sun. Scientists are now able to see solar flares from their desks because satellites that monitor the sun feed information to the Internet as often as every 10 minutes.

Deehr saw the solar flares that caused last weekend's drama three days before the show began over Alaska and the Lower 48. He knew the solar flares had a chance to produce wide-ranging aurora, but he couldn't predict the intensity of the storm. He couldn't make the call because he didn't know the direction of the magnetic field in the solar flare's shock front as it approached Earth. If the magnetic field is pointed south, the circuit from sun to Earth is strong, and the aurora causes people to go outside and make appreciative noises. That's what happened last weekend.

Deehr said people in Anchorage, Juneau, and even the Lower 48 are in for more brilliant displays as we near the year 2,000. That year is what researchers call the "solar maximum," a time of numerous sun spots and solar flares. Because of an abundance of unpredictable solar flares, the solar maximum and the years that precede and follow it make aurora prediction a tough job, but Deehr says that's OK with him. He said it's nice to have a few surprises in this often-predictable world, especially displays of rippling lights in the sky that pleasantly interrupt peoples' routines during the long, dark nights of northern winter.