Washington, Oct 4 : A new study has revealed that high magnetic activity makes the Sun occasionally appear to have a fatter "waist".
According to a report in National Geographic News, 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.
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," said study co-author Hugh Hudson, of the University of California, Berkeley.
"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," said Hudson.
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, 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.
According to Hudson, "We have six years of data and have only analyzed three months' worth so far."
"The finding that rotation dominates the solar shape is, in a sense, not so surprising. But, it cleans up a messy observational picture and opens the way for really important things," he added.