Sun Gets Fatter "Waist" During Magnetic Peaks

Anne Minard
for National Geographic News

October 2, 2008

High magnetic activity makes the sun occasionally appear to have bigger "love handles," according to a new study that reveals the star's true shape.

Although the sun looks almost perfectly rounded to the naked eye, astronomers have known for decades that it is actually oblate, or slightly flattened at its poles and fatter in its middle.

But recent analysis of data from a NASA satellite called the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) showed that when the sun's magnetic activity is at a peak, the slight bulge in its "waist" increases significantly. The sun goes through a roughly 11-year cycle of activity levels.

(Related: "Sun's Power Hits New Low, May Endanger Earth?" [September 24, 2008].)

Measuring the sun's shape relies on detecting subtle features, and the odd increase in the equatorial bulge could be a misreading based on variations in brightness, according to the research team.

"So we invented a way to screen the data against this brightness effect" using extreme-ultraviolet images from the Solar and Heliospheric Observatory, study co-author Hugh Hudson, of the University of California, Berkeley, said in an email.

"The other and more exciting possibility is that there are 3-D structures in the upper photosphere that just stick up," Hudson told National Geographic News.

Though gaseous bodies such as the sun don't have actual surfaces like Earth's, the photosphere is an analogous region where the sun becomes opaque.

"In that case, the RHESSI would be seeing a real, if tiny, bulge."

As Flat as Predicted

The late Robert Dicke of Princeton University recorded the first modern measurements of the sun's shape in the 1960s.

His data showed that the sun was much more oblate than scientists would predict based on the star's surface rotation.

More recent measurements taken using ground-based telescopes have indicated a rounder shape than Dicke suggested.

The new RHESSI analysis, published online this week in the journal Science Express, offers magnetic activity as an explanation for why the sun occasionally appears more oblate, and bolsters the idea that its true shape is more rounded.

"We found that the sun was about as flat as the surface rotation predicts," Hudson said via email.

Jeffrey Kuhn, associate director of the Institute for Astronomy at the University of Hawaii and a former colleague of Dicke's, said the new study "has a surprising result, which contradicts existing space and ground measurements."

But Kuhn, who was not involved in the new study, said the technique the RHESSI team used is not a reliable measure of the sun's shape.

"So far it is unclear, at least to me, that their ad hoc analysis is telling us what the sun is really doing," he said.

Study co-author Hudson argued that Kuhn's criticism might partly be "based on the fact that if we are right—and we think we are—some of his early work with the legendary Dicke may have been misinterpreted."

But Hudson agrees that more work needs to be done to confirm his team's results.

"We have six years of data and have only analyzed three months' worth so far," Hudson said.

"The finding that rotation dominates the solar shape is, in a sense, not so surprising," he added.

"But it cleans up a messy observational picture and opens the way for really important things."

Starry Yardstick

Martin Asplund of the Max Planck Institute for Astrophysics in Garching, Germany, wrote a commentary for Science about the solar information divide.

Asplund pointed out that uncertainty appears to be the norm in studies of the sun.

Researchers have been retooling estimates of heavy elements in the sun's composition for a number of years, for example, and those data frequently conflict with interpretations of physics in the sun's atmosphere.

That's disappointing for astronomers seeking to rely on the sun as an "astronomical yardstick" for understanding the composition and physics of other stars, he wrote.

"Consensus will only occur when both sides have convincingly demonstrated that their analysis really is reliable and robust, which requires quite some more work."