For 4 minutes and 7 seconds early on the afternoon of March 29, thousands of people who had trekked deep into the southern Sahara Desert saw blazing day turn into night. Wearing turbans to keep the sand out of their hair, the sky watchers in Libya were treated to a picture-perfect view of the sun being blocked by the shadow of the moon. It was also the longest such eclipse ever seen.

As the sun disappeared, a white, lacy halo popped into view. It was the sun's wispy outer atmosphere, or corona, which is rarely seen because it's normally washed out by the sun's glare.

Whether looking through telescopes or special protective glasses, the observers in Libya were jubilant. So were some researchers half a globe away. Their computer model had accurately predicted the appearance and behavior of the corona. In developing the first accurate model of the corona, Zoran Mikie, Jon Linker, and their colleagues at Science Application International Corp. in San Diego have produced the equivalent of a weather map for the sun.

"No other simulation has had the high resolution, physical accuracy, and global coverage" of this model, says solar physicist Craig DeForest of the Southwest Research Institute in Boulder, Colo.

Scientists plan to use the map ultimately to predict the appearance and location of solar flares and coronal-mass ejections. The ejections are billion-ton clouds of hot, electrified gas that are hurled from the corona and can damage orbiting satellites and communications and power systems on Earth. With advance warning, people on Earth might minimize such damage, and astronauts might reduce their exposure to harmful radiation from these turbulent events.

Following on the success of the new map, a pair of NASA spacecraft set for launch on the same rocket on Aug. 31 are expected to greatly advance scientists' knowledge of the corona. As the two nearly identical craft, known as Solar Terrestrial Relations Observatory (STEREO), slowly separate during their mission, they will observe the corona from different perspectives and provide the first three-dimensional views of the sun's outer atmosphere.

The model developed by the San Diego scientists will play a crucial role in interpreting images taken by STEREO, says mission scientist Russ Howard of the Naval Research Laboratory in Washington, D.C.

At the same time, says Linker, he and his collaborators plan to use the STEREO images to hone the model they have developed.

ANATOMY OF AN ERUPTION During a coronal mass ejection, a magnetized cloud of material that has lifted off the sun travels at 1.5 million kilometers per hour. On its 2-to-3-day journey to Earth, the cloud rams into the slower-moving solar wind, the stream of particles continually blown out by the sun. The collision creates a shock wave that in turn sweeps up other charged particles in space, strengthening the moving cloud.

When a coronal mass ejection nears Earth, it can wreak havoc. It can compress the magnetosphere, the magnetic shield that surrounds our planet. Satellites that had been orbiting just inside the magnetosphere may now lie just outside it, where they are no longer protected from an onslaught of energetic charged particles that can harm their sensitive electronics.

Like a bar magnet, Earth's magnetic field has two poles. If the magnetic field of a coronal mass ejection happens to point opposite that of Earth's, the eruption can do further damage. It can connect directly with Earth's field in a catastrophic magnetic handshake that releases vast amounts of energy. Researchers suspect that just such an interaction knocked out power grids, causing the vast power outage that afflicted Quebec in March 1989.

Coronal mass ejections strong enough to cause such damage happen only about twice a year, DeForest says. Depending on solar activity, the sun can launch a coronal mass ejection once every few hours to every few days. About 10 percent of these ejections head toward Earth. With advance warning, engineers can power down satellites and turn them away from the sun, delay launches of craft and make sure astronauts are not out spacewalking, and mitigate widespread damage to electronic systems on Earth.