Spacecraft probes beneath sun's surface

Glimpsed by the naked eye, the sun's surface appears smooth and uniform. Solar astronomers, however, see it as a witch's cauldron, belching blobs of hot gas and energetic radiation far into space. Data from a spacecraft now allow scientists to dig deep under the surface, probing the turbulent motions that power these eruptions.

 By watching the rise and fall of gas at the surface, motion generated by sound waves emerging from the deep interior, researchers over the past 2 decades have begun mapping the flow of gases thousands of kilometers below. The latest maps, compiled from data collected by an instrument aboard the Solar and Heliospheric Observatory (SOHO) spacecraft, may provide new insight into the origin of sunspots, concentrated magnetic fields that appear from Earth as dark blemishes.

 The data, presented last week at a NASA press briefing in Washington, D.C., highlight the sun's diversity. Ground-based observations taken more than a decade ago had indicated that the sun is ringed by bands of gas moving at different speeds. The observations from SOHO, launched in late 1995 and now poised 1.5 million kilometers sunward from Earth, confirm the existence of these bands and reveal that they extend far beneath the surface, penetrating at least 19,000 km.

 Each belt is a swath, more than 64,000 km wide, of charged gas that travels about 16 km per hour relative to its surroundings. Researchers liken the belts to the trade winds circling Earth's equator or the colorful bands of gas ringing Jupiter. Intriguingly, note SOHO investigators Craig DeForest and Jesper Schou of Stanford University, the boundaries between neighboring belts are precisely where sunspots form.

 The change of speed across the boundaries could twist and intensify magnetic field activity inside the sun, suggests solar astronomer Douglas O. Gough of the University of Cambridge in England. At the surface, such activity could generate a sunspot. Alternatively, notes SOHO researcher Philip H. Scherrer of Stanford, cause and effect may work the other way around: The intense magnetic field may present an obstacle to the flow of gas, giving rise to the bands.

 In either case, the link between the bands and magnetic field activity provides "an inroad to understanding the 11-year sunspot cycle, which has been puzzling us for centuries," DeForest says. As the sunspot cycle progresses, the sites where these short-lived features originate move from the north and the south toward the equator. Like the illusion of sinking stripes on a barber pole, "the bands that we see are also slowly migrating toward the equator, at just the same rate as the sunspots," says Gough.

 This movement, he adds, is complicated by another global pattern detected by SOHO. Data taken over the past year show that the sun's outer layer, to a depth of at least 24,000 km, displays an overall flow of gas from the equator to the poles of about 80 km per hour.

"That's quite different from Earth, and that is something we have yet to understand," says Gough.

 Another SOHO discovery ranks as an even bigger surprise, says Schou. Data compiled by the craft's Michelson Doppler imager show evidence of jet streams near the poles. Within several flattened oval regions that ring the sun at 75@ latitude, gaseous material moves about 10 percent faster than the surrounding gases. Moreover, these streams are entirely submerged. Gough theorizes that the feature may be temporary, a signpost of the new solar cycle that began about 1.5 years ago.