Scientists using NASA's RHESSI spacecraft have measured the roundness of the sun with unprecedented precision, finding that it is not a perfect sphere.

During years of high solar activity the sun develops a thin " cantaloupe skin" that significantly increases its apparent oblateness: the sun's equatorial radius becomes slightly larger than its polar radius. Their results appear in the Oct. 2 edition of Science Express.

"The sun is the biggest and therefore smoothest object in the solar system, perfect at the 0.001 percent level because of its extremely strong gravity," says study co-author Hugh Hudson. " Measuring its exact shape is no easy task."

The team accomplished the task by analyzing data from RHESSI, an x-ray/gamma-ray space telescope launched in 2002 on a mission to study solar flares. The spacecraft's rapid rotation and high data sampling rate make it possible for investigators to trace the shape of the sun with systematic errors much less than any previous study.

"We have found that the surface of the sun has rough structure: bright ridges arranged in a network pattern, as on the surface of a cantaloupe but much more subtle," describes Hudson.

During active phases of the solar cycle, these ridges emerge around the sun's equator, brightening and fattening the "stellar waist."

Further analysis of RHESSI oblateness data may help researchers detect a long-sought type of seismic wave echoing through the interior of the sun: the gravitational oscillation or "g-mode." Detecting g-modes would open a new frontier in solar physics -- the study of the sun's internal core.

(Xinhua News Agency October 4, 2008)