How many times have you heard -- after the fact -- about an absolutely fabulous display of northern lights that danced across the sky the previous night? Unless you are a night owl, catching the best light shows can be a matter of chance since they tend to occur between 10 pm and 3 am.

But you can improve your chances for seeing one of the North's greatest winter attractions by going online. The Aurora Forecast, produced by the Geophysical Institute of the University of Alaska at Fairbanks, has been posted on the Internet since 1997. The forecast predicts the expected location of the most active light displays, and the accompanying map shows most of the Yukon as well as Alaska.

Auroras occur because the Earth's magnetic field interacts with the solar wind, a superhot stream of charged particles blowing away from the sun. The Earth's magnetic field protects the planet from most of the effects of the solar wind, but some of the charged particles break through into the Earth's upper atmosphere.

When the charged particles rain down along the Earth's magnetic field lines, they create the phenomenon we know as northern lights -- the aurora borealis in the northern hemisphere. The colour of the aurora depends on the type of atom or molecule struck by the charged particles.

Charles Deehr, a professor emeritus at the university, is responsible for the daily aurora forecasts. He says that when the Institute started making these predictions in 1994, auroral activity was so predictable that they were able to put out month-long forecasts with reasonable accuracy. At that time they sent the forecast out by fax, sometimes transmitting as many as 200 forecasts a week to various newspapers.

But since 1998, solar activity has been on an upswing as it nears the apex of an 11-year cycle. This increase in solar eruptions makes long-term forecasts ineffective. Now forecasts are updated each day by 3 pm Alaska Time, and are considered to be valid for the next three nights.

Satellites that continually monitor solar activity supply information for the forecasts, which are based on two main factors: the predictable rotation of the sun on its axis every 28 days, and the highly sporadic activity of sun spots.

Solar flares, eruptions of plasma bursting out of the sun, cause sun spots. In a matter of minutes these flares can produce a million times more energy than the largest earthquakes.

If the flare is aimed towards the Earth, the planet's magnetic field guides the plasma towards regions called auroral ovals, which are located over the magnetic north and south poles. Since electrons from the same sources create both the aurora borealis in the North and the aurora australis in the South, the southern aurora is often a mirror-image of the northern aurora.

The solar wind can take anywhere from 10 hours to five days to reach Earth, with 43 hours being the average time. Improvements in technology mean that the forecasters sometimes know several days ahead of time when energy from a solar flare will hit the Earth's atmosphere.

"When we started doing the forecasts, we would sometimes get the information from the satellite and say 'Oh, oh, it's going to be here in an hour.' But now when we see a flare or some type of solar event, we can establish a velocity and a magnetic field for it and have time to run our computer model," says Deehr.

Even though the ability to predict auroral activity keeps improving, Deehr says that they still get surprises. For example, on Saturday, December 11, northern lights were seen as far south as Seattle and South Dakota.

"We did not see that one coming," he says.

Deehr says that there are many similarities between weather predictions and aurora predictions. "We use the same statistical tests as weather forecasting. Space weather is just like terrestrial weather during the Cold War. We consider pressure, temperature, wind and then there is the bomb, the solar flare," he says.

The Aurora forecasters will be kept busy for the next few years by what is called a solar maximum, a time of increased solar activity. For more on the solar maximum, look at yourYukon column 161.

The Aurora Forecast is available at www.gi.alaska.edu.