Storm warning. The horizontal "S" appears to presage massive coronal ejections from the solar surface.

A Sinuous Solar Storm Warning

Eruptions on the sun can whip up magnetic storms on Earth, frying communications satellites and jamming aircraft radar. The solar outbursts that cause Earth's magnetic field to convulse in these storms have remained all but unpredictable. But in the 15 March Geophysical Review Letters, and in a press conference today, scientists announced a new method of forecasting the coronal mass ejections that power the most violent storms

A coronal mass ejection is thought to take place when part of the sun's magnetic field snaps, hurling over a billion tons of ionized gas outward in a giant bubble at a million kilometers per hour. Their ultimate power source is the sun's rotation. As the sun spins on its axis, the magnetic fields that thread the solar interior become twisted, like the rubber bands that power balsa wood model airplanes. And just like rubber bands, magnetic field lines become kinked and knotted when they are twisted too much. Knots in the solar magnetic field first appear on the sun's surface as dark, Earth-sized sunspots and then, without warning, the kinky field lines break, releasing a coronal mass ejection.

Computer simulations had suggested that S-shaped features in the sunspot--reflecting the kinked magnetic field lines--are a sign that the lines are about to snap. To test this idea, Richard Canfield, an x-ray astronomer at Montana State University in Bozeman, and his team studied sunspot movies made with the orbiting Yohkoh Soft X-ray Telescope in 1993 and 1997. After watching video of more than a hundred sunspots, and seeing about half of them erupt, they concluded that an S shape does indeed mark the eruptive spot. They also found that larger sunspots are more likely to erupt than smaller spots.

Although the correlation between S-shaped sunspots and mass ejections is good, it won't beat your TV meteorologist for accuracy. "We imagine using these observations like a weather report," says Canfield. Researchers could count the number of sunspots, measure their size, and look for S shapes; then they would issue a "solar weather forecast," alerting the world when the probability of a geomagnetic storm is high. David Rust, a physicist at the Applied Physics Lab in Laurel, Maryland, agrees and adds: "It is important to develop this method now, while we still have the Yohkoh satellite," which is already 8 years old.