Magnetic memory: new model forecasts solar storms.

Even at its quietest, the sun about once a week belches out a billion-ton cloud of charged particles and magnetic fields. When those eruptions are directed toward Earth, they can irradiate astronauts, disable satellites, and knock out power grids on the planet.

This week, scientists announced that they have developed a new computer model for forecasting the frequency and strength of these solar storms, which tend to follow an 11-year cycle. The sun is currently near the low point of its activity. The model predicts that solar activity won't begin rising until late 2008--as much as a year later than the sun's standard cycle would forecast. Furthermore, the next entire cycle of solar activity will be 30 to 50 percent stronger than the current one, according to the new model.

Mausumi Dikpati of the National Center for Atmospheric Research in Boulder, Colo., and her team reported the findings during a March 6 briefing and describe their work in the March Geophysical Research Letters.

Each solar cycle is heralded by the emergence of dark regions, called sunspots, at the sun's midlatitudes. In the sunspots, magnetic fields concentrate and unleash enormous amounts of energy. Changes in the structure of the sun's magnetic fields cause storm activity to wax and wane.

The new model relies on observations of the movement of electrically charged gas, or plasma, as it flows across the sun's visible surface and deep within the roiling interior. Tracking the movement of plasma is pivotal for understanding the solar temperament because the charged gas carries parcels of the sun's magnetic field. As the plasma moves, magnetic fields imprinted on it dissipate and reconcentrate, setting the stage for the next solar cycle.

The plasma exhibits two types of flow. One type acts like a conveyor belt moving plasma--and the magnetic field that it carries--from the equator to the poles, then into the interior of the sun, and finally back to the equatorial surface. While beneath the surface, the remnant magnetic fields gather strength, become buoyant, and eventually tear through the solar surface to generate new sunspots.

The second type of plasma flow also intensifies the remnant magnetic fields. Because the sun's surface rotates faster at the equator than it does at the poles, the magnetic fields stretch and twist.

Observations from the Solar and Heliospheric Observatory, a NASA-European Space Agency mission, indicate that four solar cycles contribute to the sun's magnetic field configuration at any time, Dikpati says. "The next solar cycle depends on characteristics from as far back as 40 years previously--the sun has a magnetic memory;' she notes.

According to Dikpati's team, the conveyor-belt-like motion has slowed over the past few decades, indicating that the next solar cycle will be delayed. But some other models foreeast that the next solar cycle will come a year sooner than average. Predictions also vary about the next cycle's intensity.

"What's exciting is that the next cycle is only a few years away" says solar physicist Leif Svalgaard of the software company Easy Tool Kit in Houston. "We'll know soon which model is pointing us in the right direction, and that will be a real breakthrough."--R. COWEN