Spacecraft spies hills and valleys of sun

To the ancient Greeks, the sun represented perfection--a notion finally put to rest by Galileo's discovery of sunspots in 1611. However, anyone still looking to Sol for some constancy may be pleased by new findings indicating that a rigid, regular landscape adorns the boiling surface of Earth's star.

"It's pretty surprising. We see hills and valleys," says Jeffrey R. Kuhn of the National Solar Observatory in Tucson, who discussed his group's discovery at last month's meeting of the American Geophysical Union in San Francisco.

 Using the Michelson Doppler Imager aboard the Solar and Heliospheric Observatory (SOHO) spacecraft, the researchers measured motions of gases at the sun's edge to within an accuracy of 3 meters. Kuhn compares this resolution to an Earthbound observer spotting a quarter on the moon. SOHO rests in an orbit where the gravitational force between Earth and the sun is equal.

 By observing the sun as it rotates, Kuhn and his colleagues built up a picture of its entire surface. They saw bumps of bright gas fixed in place on the bubbling outer layer of solar gas. The hills of superheated gas measure 40,000 kilometers across but only half a kilometer high. "They are, more or less, rotating in unison [with the sun]," says Kuhn.

 The astronomers believe that fixed magnetic fields inside the sun guide heated gases from the interior to the surface where they continually renew the hills. In the 7 months of SOHO's operation, the solar landscape has changed little, Kuhn reports.

"It looks like a dimpled golf ball," says Juri Toomre of the University of Colorado at Boulder, who calls the finding part of a renaissance of discovery that has recently been reshaping helioseismography.

 By tracking the motion of powerful sound waves through the sun, the researchers hope to discover the source of its magnetic fields, sunspots, and 11-year sunspot cycle. "We're at the high-adventure stage of the field," says Toomre.

 Despite its dimples, the sun can still stake a claim to excellence. According to Kuhn, it is a perfect orb to within 0.001 percent tolerance, orders of magnitude more spherical than Earth.