Call it a boost to the sun's self image. Researchers have found that our parent star's midsection is considerably slimmer than believed. Not only that, the sun's overall shape doesn’t change as much during the 11-year sunspot cycle as scientists had thought—a hint that the outermost part, a few percents of the sun, may be rotating more slowly than expected.
Rotational forces cause any whirling blob of gas to be flattened at the poles and bulge at the equator, forming a shape called an oblate spheroid. In our solar system, fast-spinning Saturn is the most squished of the planets, with a polar diameter almost 12,000 kilometers, or 10% , less than its average equatorial diameter. Even rocky planets can be oblate; Earth, which rotates once every 24 hours, has an equatorial bulge of almost 43 kilometers.
Even though the sun rotates relatively slowly—only once every 27 days or so—it, too, is oblate, says Jeffrey Kuhn, a solar physicist at the University of Hawaii’s Institute for Astronomy in Pukalani. But the distance of the sun's surface from its center depends on several factors other than its gravitational field, he notes, including the sun’s inner rotation; the convection and turbulence in its outer layers; and magnetic forces, particularly those around sunspots. Previous data have suggested that the sun's oblateness varies with the solar cycle, with the overall shape subtly changing as the number of sunspots waxes and wanes during the 11-year period.
But data recently gathered by sensors onboard NASA’s Solar Dynamics Observatory (SDO), a satellite that continuously watches the sun from geosynchronous orbit, reveals the sun's shape is steadier and more nearly spherical than previously thought, Kuhn and his colleagues report online today in Science. SDO takes more than 15,000 images of the sun each day, but for this new study the team analyzed images taken only twice each year, when the satellite's instruments are calibrated. During these times, SDO continues to stare at the sun but rotates slowly through a full circle—a maneuver that lets scientists analytically remove any distortions caused by the camera’s optics, presumably revealing the sun’s true shape.
Analyses of data gathered during five calibration cycles over the past 2 years—an interval when solar activity has risen from near nil to near maximum, with 90 or more sunspots visible on the surface each day—indicate that the sun's oblateness is remarkably steady despite the large changes in magnetic activity at its surface. Results suggest the sun’s equatorial bulge is only 12 kilometers or so, compared with its average diameter of nearly 1.4 million kilometers. Previous data suggesting that the sun’s shape varies during a solar cycle, which were gathered by ground-based instruments, may have been affected by atmospheric turbulence—the same phenomenon that makes distant stars twinkle, Kuhn notes.
Besides the unexpected steadiness of the sun’s shape, the team found that the sun's oblateness is about 25% smaller than previously estimated, Kuhn says. The oddly low measurement may be caused by misunderstandings about how turbulence or magnetic forces influence the outermost layers of the sun, he notes, "but neither of those adjustments would agree with our current models." Instead, he suggests, it’s most likely that the layers of the sun just below its visible surface are rotating between 3% and 10% more slowly than expected.
"For 50 years we've been working to understand how oblateness changes over the solar cycle," says Philip Goode, a solar physicist at Big Bear Solar Observatory in California who was not involved in the study. The new findings present a strong argument that whatever changes are making oblateness smaller than expected are taking place in the outermost layers of the sun, he says.
The work "is the best attempt so far" at measuring the sun's oblateness, says Douglas Gough, an astrophysicist at University of Cambridge in the United Kingdom. Also, the team’s lower-than-expected estimate for the sun's oblateness "is a fascinating result." However, he adds, "it appears to me that a change in rotation is unlikely to explain this."
An alternate explanation for the result, Gough suggests, might be that the atmosphere over the sun's polar regions is slightly different than that over the sun’s equator. Such a disparity, he notes, could cause distortions in the paths of light headed to Earth from the various regions, thereby skewing the estimate of oblateness lower than expected.