A solar forecast.

Solar activity, which waxes and wanes in an 11-year cycle, will most likely begin its next round in March 2008 and peak sometime between late 2011 and mid 2012, predicts the National Oceanic & Atmospheric Administration's (NOAA) Space Environment Center in Boulder, Colo.

During the peak of activity, eruptions such as solar flares and coronal mass ejections hurl X rays, ultraviolet light, and billion-ton clouds of charged particles into space. If these outbursts head toward Earth, they can disrupt the planet's ionosphere and pierce Earth's protective magnetic bubble, potentially knocking out powers grids, disturbing satellites, and harming space-walking astronauts. An especially active solar cycle shows up as abundant sunspots--relatively cool blotches of concentrated magnetic activity.

A panel of solar scientists convened by NOAA initially expected that the next solar cycle, to be the 24th on record, would already have started by last fall. However, the current solar cycle has faded more slowly than predicted.

Although the panelists now concur on when the next cycle will begin, they disagree on how intense it will be. Half the panel, basing its prediction primarily on the intensity of the sun's polar magnetic field during the past few months of the current cycle, forecasts a moderately weak next cycle. The panel's other half, considering properties of the past few cycles such as the surface flow of the sun's magentic field, predicts a moderately strong cycle. The panel announced its split decision April 26 in Boulder at NOAA'S annual workshop on space weather.--R.C.