"Captain! There's a coronal mass ejection headed straight for us. I dinna think we can outrun it!"

The bulletin issued by the National Oceanic and Atmospheric Administration on July 14 carried this warning in capital letters: MAGNETIC STORM PREDICTED. It went on to say that a NASA spacecraft had just recorded the largest solar flare in a decade, and a coronal mass ejection -- a plume of solar plasma -- was heading earthward at super-sonic speed.

"The coronal mass ejection will impact Earth's magnetic field on Saturday afternoon and will cause a geomagnetic storm that is expected to reach category G3 (strong) to G4 (severe) levels," the bulletin reported. "The radiation and geomagnetic storm are expected to produce adverse effects on spacecraft operations, power systems, high-frequency radio communications and low-frequency navigation signals."

The warning was delivered by e-mail to space scientists at the University of New Hampshire, who have a number of instruments aboard NASA spacecraft -- many of them designed to record just this kind of event. They weren't taken by surprise; they knew it was about time for the sun to start acting up.

Geomagnetic storms occur all the time and usually don't cause too much concern. But every 11 years, the sun reaches a peak of activity called the solar maximum, characterized by lots of sunspots, solar flares and powerful coronal mass ejections.

The result is an increase in solar radiation on Earth, which wasn't much of a problem until we became dependent on electricity, computers, telephones and all kinds of wireless communications devices. Geomagnetic storms interfere with all of those things and can even knock them out of service.

We are in a solar maximum period right now, and it will continue for the next year or two. This unusually active part of the solar cycle is caused by the cyclical tightening and destruction of the sun's magnetic fields, according to UNH astrophysicist Eberhard Möbius. He explains that the sun is a huge plasma ball rotating in space, with its equator turning more rapidly than its poles, winding its magnetic field lines tighter and tighter. Eventually this triggers eruptions of plasma from the surface. The eruptions, in turn, destroy the magnetic field, and the process begins all over again.

A coronal mass ejection is a major event in the solar system, spewing particles and energy from the surface of the sun outward for billions of miles. As this plume of plasma rushes toward Earth and its fellow planets, "it produces a shock wave, where particles get accelerated very efficiently," Möbius explains. "Think of what happens when you bounce a pingpong ball between a paddle and the table, and then you move the paddle closer and closer. The ball bounces faster and faster, and that's what's happening to these particles."

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