August 8, 2017

The Sun’s core spins four times faster than its surface

For decades, astronomers have wondered if the sun’s core spins faster than its surface, and now, thanks to an international team of scientists, they have the answer: the interior makes a complete rotation in one week, which is 3.8 times faster than the middle and outermost layers.

Researchers from the French National Center for Scientific Research (CNRS), the Côte d'Azur Observatory and elsewhere used observations made by the Solar and Heliospheric Observatory’s (SOHO) Global Oscillations at Low Frequency (GOLF) instrument to measure solar oscillations, then used a new technique to determine the speed at which the solar core was spinning.

What they did, the researchers explained in a statement, was study surface acoustic waves in the sun’s atmosphere. Some of those waves penetrate to the core and interact with gravity waves that have a “sloshing” motion similar to how water splashes around in a container that is half full.

By detecting these sloshing motions, they were able to measure the acoustic waves and figure out how long it took them to travel from the surface to the core and back again. By applying this new method to 16 years worth of GOLF data, the research team determined that the solar core rotated once per week. Their findings have been reported in the journal Astronomy & Astrophysics.

Data brings a decades-long search to a close

Experts have speculated for more than two decades that the sun’s interior may be spinning faster than the surface, but they had never been able to accurately measure its oscillations before, study co-author Roger Ulrich, a professor emeritus of astronomy at UCLA, said in a statement.

“The most likely explanation is that this core rotation is left over from the period when the sun formed, some 4.6 billion years ago,” Ulrich explained. “It’s a surprise, and exciting to think we might have uncovered a relic of what the sun was like when it first formed.”

The SOHO space observatory has been orbiting the sun for more than 20 years, using its GOLF instrument to conduct solar oscillation measurements once every 10 seconds. Thanks to this data, the authors of the new study were able to definitely conclude that earlier speculation was correct, and that the interior portion of the sun does indeed spin more quickly than the outer parts.

“We’ve been searching for these elusive g-waves [gravity waves] in our Sun for over 40 years, and although earlier attempts have hinted at detections, none were definitive. Finally, we have discovered how to unambiguously extract their signature,” lead author Eric Fossat from the Cote d’Azur Observatory explained in a statement released by the European Space Agency (ESA).

“It is really special to see into the core of our own Sun to get a first indirect measurement of its rotation speed,” he added. “But, even though this decades long search is over, a new window of solar physics now begins.”


Image credit: JAXA/NASA