Scientists are a step closer to more accurate forecasts of space weather. Data from two satellites have improved their understanding of when a magnetic storm will erupt on the Sun and spew highly charged radiation that can have damaging effects on Earth.

At one time, scientists thought the space between Earth and Sun was a vacuum. But we now know that the sun fills it with gusts of hot, electrically charged atomic particles called the solar wind. Sometimes this wind blows hard. When the Sun's outer layer is very active, it hurls nearly one-third of its gaseous matter outward at supersonic speeds.

The U.S. government's oceans and atmosphere agency NOAA, has a Space Environment Center to monitor these discharges. The center's director, Ernest Hildner, says intense solar emissions are not dangerous to people on the ground, but can be a hazard to airplane occupants and astronauts. They can also shut down satellites, power networks, communications, and other technical systems.

"When the Sun has a storm, an eruption of plasma and magnetic field, it smacks the Earth's magnetic field and it causes difficulties to our technological systems. That is what we think of as space weather," he explained.

Several U.S., European, and Japanese satellites have gone into orbit during the past decade to measure the Sun and its emissions. One of them is at a point where it can warn us that solar particles are just one hour away. Mr. Hildner says these spacecraft have improved short-term space weather predictions.

"We are still not much better than we were on the one day, two day, three day prediction, but on the one hour prediction, we are much better than 90 percent correct now," he said.

However, new research could eventually lengthen the forecasts.

Scientists have long known that solar storms are generated internally by the sudden release of magnetic energy rising to the surface. Like a tightly twisted rubber band, the Sun's magnetic fields can suddenly snap into a new shape. Researchers have tried to predict solar storms by focusing on these magnetic patterns. But this method is not very reliable because they know that electrical currents must also be present. But how are they linked to magnetic fields to power large flares?

New satellite data have provided answers to U.S. government scientists and colleagues from the aerospace firm Lockheed Martin.

They have revealed that magnetic fields merging at different angles to each other are the most likely to produce a flare. Lockheed Martin physicist Karel Schrijver says this interaction creates more electrical current than normally associated with a magnetic field.

"We learned to recognize that it had to come up in the wrong orientation, off with the alignment of what was already there, twisted, and come up, in fact, in such a way that it suggested to us that the field that came up was itself carrying new electrical currents into the atmosphere," he explained.

To discover this, Mr. Schrijver and his government colleagues compared magnetic maps of the sun from the U.S.-European SOHO spacecraft with Sun surface images from the American TRACE satellite.

He says scientists can now predict which magnetic fields will turn into solar flares with 90 percent accuracy about two days before an outburst.

"About 90 percent of the cases we could tell which were carrying currents and which were not. Those that carried the large currents had had two-and-a-half times more flares and the typical flare was at least three times bigger. So ten times more flare energy came out of these regions," he said.

The findings still do not permit forecasts of precisely when a flare will erupt, only whether one is likely to, much as the buildup of snow on a mountain increases the chance for an avalanche. The director of solar studies at the U.S. space agency, Richard Fisher, says refinement of the research will improve predictions.

"This is quite useful for spaceflight operations and flight planning," he noted. "We have been able to understand where it is likely to have a flare and what the size is likely to be. We can also tell when it is not very likely. This has considerable value."

The findings are published in the Astrophysical Journal.