Spacecraft sounds out the sun's hidden half.

The solar merry-go-round never stops. Carried by the sun's rotation, intense magnetic activity that suddenly appears at the eastern edge of the solar disk will disappear in just 14 days. Space-weather forecasters get little advance notice of solar storms that may erupt and head toward Earth when these hidden regions emerge, potentially harming satellites and knocking out electric-power grids.

By detecting sound waves that have traveled through the sun, two physicists have found a way to view disturbances on the sun's hidden half. The technique provides a glimpse of stormy weather patterns a week to 10 days before they come into view.

That feat, if it can be performed routinely, "has a lot to offer," comments Ernest Hildner, director of the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder, Colo. "Every time an active region rotates off the [visible] disk, our forecasters wonder whether it is growing or declining. Will it come back ... like a lion or a lamb?"

Seething gas just beneath the sun's visible surface generates a cacophony of sound waves that ring the sun like a giant bell. Analyzing the vibrations of the solar surface, Charles Lindsey of Solar Physics Research Corp. in Tucson and Douglas C. Braun of NorthWest Research Associates in Boulder used a technique called seismic holography to reconstruct images of the sun's far side.

Sound waves travel freely through the sun, taking about 3.5 hours to cross from the far side to the near. But when the waves encounter regions of high magnetic activity, which pulls in gas and shortens the travel path by a few hundred kilometers, they take 6 seconds less.

Because of the tiny difference in travel time, sound waves that pass through magnetically active regions are slightly out of step with waves that don't. The mismatch is enough to create an interference pattern that enabled the researchers, with the aid of a computer, to trace the path of the sound waves back in time and image magnetic activity on the sun's far side.

Although Lindsey and Braun first proposed the acoustic technique more than a decade ago, they needed a device that would record the undulations of the visible solar surface with high accuracy. The Michelson Doppler Imager on the Solar and Heliospheric Observatory (SOHO) spacecraft fills the bill. The researchers describe their work in the March 10 SCIENCE.

SOHO can't routinely transmit the data required to continuously track storms on the sun's hidden half, Braun notes. So, full-time monitoring isn't possible during the current maximum of the solar activity cycle but may begin as soon as 2006, when NASA plans to launch a successor to SOHO.