The U.S.-led panel charged with predicting the intensity of the next cycle of sunspot activity will have to resolve highly divergent predictions issued this year by two leading solar forecasting modelers, according to solar experts at the American Geophysical Union meeting in San Francisco.

While some scientists are predicting a weak cycle, others are predicting a cycle that would be the most intense solar activity yet recorded.

Sunspots are cool, dark patches on the sun's surface that give rise to solar flares, streams of protons and X-rays that wear down the electricity-generating solar panels on satellites and increase the atmospheric drag on spy satellites, the Hubble Space Telescope and other low-Earth-orbiting spacecraft.

Solar flares also can disrupt communications with airliners traveling on polar routes, degrade the accuracy of GPS satellite signals and cause disruptions in electric power grids on Earth, said Bill Murtagh, a U.S. space weather forecaster.

Satellite manufacturers will review the new prediction, due to be completed in April, to make sure they are planning adequate radiation shielding on electronics and large enough solar arrays to cover the expected wear.

"There's an optimization that's done to try to size that appropriately. You don't want there to be too little, but on the other hand, you don't want to have too much," said Barry Noakes, the chief technology officer for Lockheed Martin Commercial Space Systems, in Newtown, Penn.

NASA is funding the work of the 12-person Solar Cycle 24 Prediction Panel, named for the upcoming sunspot cycle, the 24th since accurate records have been kept. Solar physicist Doug Biesecker of the U.S. National Oceanic and Atmospheric Administration's (NOAA) Space Environment Center in Boulder, Colo., chairs the panel, which met for the first time in October.

The panel's prediction will be the official solar-cycle forecast for NASA, NOAA and the International Space Environment Service, which operates 11 regional space weather warning centers around the world.

The task of forging a "single, clear voice for the user community" will be challenging because of the wide array of predictions that need to be reconciled this time around, Biesecker acknowledged, noting the contrast between those predicting a weak cycle and those predicting a cycle that would nearly rival the record setting Cycle 19 observed in the 1950s.

A similar prediction panel met 10 years ago, before the current solar cycle, and reached a unanimous consensus from predictions that spanned a narrower range. "In the previous cycle there was a tendency to believe it would be a big cycle. It was just a question of how big," Biesecker said.

At the meeting in San Francisco, Biesecker brought together two members of the panel who were scientists representing the two leading techniques for predicting sunspot activity.

Ten years ago, scientists using these techniques arrived at similar predictions. Now, their results are diametrically opposed, and it will be up to Biesecker's panel to reconcile them.

Solar physicist Dean Pesnell of Goddard Space Flight Center in Greenbelt, Md., believes that the best predictor of solar activity is the characteristics of the Sun's polar magnetic field.

"If we look at the current value of the polar fields, they're way down here, about half of the field strength that was measured in the previous solar minimum. This leads us to produce a prediction that the next solar cycle will be relatively weak," he said.

Solar physicist David Hathaway of NASA's Marshall Space Flight Center in Huntsville, Ala., believes a better approach is to study fluctuations or vibrations in the intensity of Earth's own magnetic field as it reacts to streams of particles from the Sun during the current solar cycle.

"It's like listening to a freight train in the distance and trying to estimate the size of the train, but what we're listening to is the Earth's magnetic field," he said. "Listening to what the Earth's magnetic field [was] doing back in 2003, as it turns out, we find that it was a strong peak that suggests that the next cycle ought to be a big cycle," Hathaway said.

Hathaway and Pesnell said the situation facing the panel is interesting because in the past their techniques have pointed to similar results. "The surprising thing is they disagree so fundamentally this time," Pesnell said.

"As scientists, we need to get to the bottom of this as far as understanding how the sunspot cycle works," Hathaway said.

Biesecker said the panel would issue a prediction in April and update it after that, similar to the way hurricane forecasters issue updates.

"A year from April, we will revise our prediction... We [may] have discovered we were wrong spectacularly or we are headed in the right direction," Biesecker said.