Auroras Offer Clues to Mysteries of Solar Activity
It's nearly midnight as Tom Hallinan steers his tiny sedan up a winding road flanked by walls of Arctic evergreens flocked with snow.
Earlier in the evening, the University of Alaska geophysicist phoned to say his wife had looked out a window of their home and had seen the northern lights. "Would you like to head up to Poker Flat tonight and view the aurora?" he asks.
At 11:45, he pulls into a darkened parking area outside of one of the world's premier aurora observatories, run by the University of Alaska at Fairbanks' Geophysical Institute, which is built on a hillside at the institute's Poker Flat Research Range.
Tonight's display is breathtaking, as curtains, arcs, and swirls of pale light move across the sky. But it's merely a preview of coming attractions. The sun, which drives the aurora, is beginning to build toward the peak of its 11-year sunspot cycle. Researchers estimate that solar activity will reach its zenith between March 1999 and June 2001.
Auroral displays will grow more dramatic and may play to a wider audience as storms on the sun's surface grow more intense. The warm-ups already are under way. Last April, an auroral display was visible from parts of central Massachusetts, while reports of the northern lights' dance came in from Maine, New Hampshire, and northern Michigan over last Columbus Day weekend.
A tour of the observatory shows the two-story building serves as the control center for the sounding rockets launched at the range, as well as a collection point for data from cameras and other sensors at the range and at similar observatories around the globe. These constantly monitor the night sky for clues that will help scientists unravel the auroras' secrets.
After the tour, Dr. Hallinan settles down in the facility's kitchen, unloads a small grocery bag - standard issue for his frequent nighttime trips from Fairbanks some 30 miles away - and offers the fixings of a peanut butter and jelly sandwich. Between bites, he explains the mighty dynamo that drives aurora and sketches some of the elementary questions about auroral displays that scientists struggle to answer. The information they tease from these enigmatic light shows not only will help deepen their understanding of auroras, it also could help improve forecasts of space weather, of which auroras are the most visible evidence.
Bright lights, big collisions
The story, Hallinan says, starts with the sun, which continuously sheds charged particles as solar wind. Some of these particles penetrate the Earth's magnetosphere, get trapped, and then drawn down the Earth's magnetic field lines toward the North and South magnetic poles. At 250 to 300 kilometers (155 to 186 miles) above the Earth, the show begins as incoming particles slam into various gases in the atmosphere, turning the sky into nature's equivalent of a neon light. Incoming protons capture electrons to form hydrogen, giving off blue light in the process. Electrons collide with nitrogen, emitting a blue or red glow, depending on the nitrogen's form. Electrons also collide with oxygen, which emits green and red light, depending on the oxygen's form.
These latter collisions dominated the Great Red Aurora of March 13, 1989. "People saw that as far south as the Yucatan Peninsula and Florida, and thought it was from a forest fire," Hallinan says. "It caused a regionwide power outage in Quebec."
By the time the particles have dropped to about 100 kilometers above the Earth's surface, they've either been captured or are moving too slowly to elicit a glow from the gasses they hit.
"The curious thing is that as they come from the sun, electrons in the solar wind have energies of only a few electron-volts," Hallinan says, referring to a unit of measure physicists use to describe particles' energy levels, and an amount of that unit that a nitrogen atom hardly finds exciting. "But a few hundred kilometers above the Earth, electrons have been measured with energies of 1,000 to 10,000 electron-volts. What's accelerating them?"
Another question researchers are pursuing involves the almost explosive features found in auroras. "Over the course of a minute or less, energy levels jump by a factor of 1,000," Hallinan continues. "It's as if the energy in the Earth's magnetic field gets dumped in one area. There's a lot of hand-waving" as people try to explain that phenomenon, he says.
Many questions remain
"We also see time variations that can be very dramatic to watch," Hallinan says. "In the early-morning hours you get cloud-like patches of pulsating aurora instead of glowing arcs. Why are things different before and after midnight? And every now and then we see a different vertical profile of luminosity - a bright band across the sky, with a weak band below it and another bright band on the bottom. That doesn't fit the models."
Perhaps these unsolved mysteries allow the scientists to retain their awe. Syun Akasofu, one of the field's pioneers and the Geophysical Institute's director, and his wife are known to head out in the dead of night to watch auroral displays.
Back at the Geophysical Institute, Hallinan's colleague Hans Stenbaek-Nielsen sums up the state of aurora studies. Despite the encyclopedia and textbook articles that seem to explain the phenomenon, "we do not understand what it's all about," he says. "Why do we see a single arc? Why do new arcs form? Why do aurora appear in arcs and curtains? Why do they appear as pulsating patches on the morning side? These are such basic parts of auroral displays that if you don't understand those, you don't understand aurora."
Web Sites Help Make Aurora-Hunting Easy
If you want to see an aurora, but don't have the time, money, or inclination to spend a week in Yellowknife, Northwest Territory - the self-proclaimed aurora capital of the world - don't despair.
As solar activity increases, so does the likelihood of spotting an aurora - even for people who don't live in Alaska or the northern expanses of Canada. On rare occasions, solar storms can be so intense that the aurora they spawn can be seen into the tropics. In 1989, a storm produced a red aurora that could be seen as far south as the Yucatan Peninsula.
The best viewing opportunities are on clear nights during the late fall, winter, and early spring. Plan to look from just before midnight to the predawn hours, although in places like Fairbanks, aurora can be seen as soon as it's dark.
Two Web sites can help you pick dates for aurora-hunting. The University of Alaska at Fairbanks maintains a site (dac3.pfrr.alaska.edu/~pfrr/AURORA) that publishes aurora forecasts, including a map. The Space Weather Operations Center, run by the National Oceanic and Atmospheric Administration, is another source (www.sel.noaa.gov/forecast. html). Look down the forecast for the "GEOMAGNETIC A INDICES," a measure of how active the earth's magnetic field is likely to be. A 25 or 30 means that people living in the northern tier of the lower 48 states have a good chance of seeing an aurora, even if it appears only as a glowing region of the northern sky. If the index exceeds 40, people as far south as Colorado may be able to see one.