Ulysses pictures the sun's magnetic field.

Fanning out into interplanetary space like a pinwheel, the magnetic field streaming from the sun's equatorial latitudes has a distinctly spiral pattern. That's the finding of the Ulysses spacecraft, which has taken the first snapshot of this field.

A year ago, from its unique vantage point high above the south pole, Ulysses scanned the sun's equatorial plane as far as Earth's orbit and recorded at radio wavelengths the pinwheel structure predicted by theory. Maryland-based astronomers Michael J. Reiner of the Hughes STX Corp. in Lanham and Joseph Fainberg and Robert G. Stone of NASA's Goddard Space Flight Center in Greenbelt describe the study in the Oct. 20 Science.

Two properties of the sun conspire to create the pinwheel shape: the billowing solar wind, which carries the sun's magnetic field outward, and the sun's rotation, which winds it up. Other spacecraft had failed to make a full portrait of the interplanetary magnetic field because they took only spot measurements in or near the plane in which Earth orbits the sun.

"What we needed was the equivalent of an aerial photo," says Stone.

Detectors can't image the field directly. But high-speed electrons ejected from the sun's equatorial regions during outbursts trace its structure. These fast electrons move outward along the low-latitude magnetic field lines, causing ionized gas in their path to emit radio waves. Like iron filings sprinkled around a bar magnet, the radio waves reveal the pinwheel pattern of the interplanetary field.

Because the electrons take only about 20 minutes to travel from the sun to Earth's orbit, the radio waves they induce provide a virtually instantaneous snapshot of the field over millions of kilometers.

The interplanetary field doesn't always maintain its spiral shape, Reiner adds. Ulysses found that after particularly violent eruptions, when the sun ejected blobs of gas from its outer atmosphere, the field had a much more distorted, less organized pattern. The ability to detect major solar disturbances by recording the field pattern may ultimately provide a new way for spacecraft to predict geomagnetic storms before they strike Earth, Reiner says.

Ulysses completed its pass over the north pole last summer, and the team plans to use the new data to make additional maps. From its position in the plane of Earth's orbit, another craft, called Wind, provides a simultaneous, though different, perspective on the interplanetary field. Combining the data from each craft, the researchers plan to make the first three-dimensional map of the field.

In related work, scientists reported this week at a Ulysses workshop in Dana Point, Calif., that the interplanetary field appears to be wound more tightly in the sun's southern hemisphere than in the northern. Andre Balogh of the Imperial College of Science, Technology & Medicine in London and his colleagues base their finding on measurements taken by the craft's magnetometer during the polar passes.