Sun's Ripples Reveal Clues to the Core

The core of the Sun holds secrets into how it and the planets formed billions of years ago, but the bright solar surface obscures the view of its heart.

Now after a 30-year search, astrophysicists may have detected hints of ripples on the surface, just a few yards high, that could finally help shed light on the mysterious core.

The results suggest the Sun's core rotates more slowly than theorists have predicted.

Long search

Astronomers first detected waves on the solar surface 30 years ago. Soaring miles high, those "p-mode" waves are generated by sound running through the star. They hinted at the existence of far more subtle ripples driven by gravity. These "g-mode" waves are believed to occur when gas churning below the surface plunges even deeper and collides with denser material, sending ripples propagating through the interior and up to the surface--the equivalent of dropping a stone in a pond.

Scientists wanted to detect g-mode waves since they pass through the Sun's mysterious heart and carry vital information concerning internal activity. For instance, the speed at which the solar core rotates is uncertain. Knowing this detail could shed light on the birth of the entire solar system, because it represents the hub of rotation for the dusty cloud of matter that eventually formed the sun and planets.

Unfortunately, g-modes are badly degraded during their passage to the solar surface, and by the time they reach the exterior, they are little more than ripples a few yards high.

To make matters harder, the g-modes take between two and seven hours to rise and fall just once, which means astronomers are faced with having to detect a swell that moves just a few millimeters per second at most.

Finally, success

Now, however, an international team of astronomers employing the ESA-NASA Solar and Heliospheric Observatory (SOHO) may have finally caught glimpses of these long-sought waves, findings detailed online May 3 in the journal Science.

The key was the telescope's Global Oscillation at Low Frequency (GOLF) instrument. This device looks for bright, distinctive signals given off by sodium vapor, allowing researchers to measure ripples on the Sun with great sensitivity. The g-modes are very distinct from the p-modes in terms of both size and the speed at which they rise and fall.

The instrument cannot distinguish lone g-modes. Instead, astrophysicist Rafael Garcia at DSM/DAPNIA/Service d'Astrophysique in France and his colleagues looked for the signature of a large number of these oscillations from 10 years of GOLF data.

"By analogy, imagine that the Sun was an enormous piano playing all the notes simultaneously. Instead of looking for a particular note--middle C, for instance--it would be easier to search for all the C's, from all the octaves together," Garcia said. "So that's what we looked for, the cumulative effect of several g-modes."

What was learned

The way the g-modes got distorted as they passed through the Sun have given the first hints of the core's rotation speed. The surface of the Sun rotates at different rates depending on location, with the equator spinning faster (about 25 days) than the poles (roughly 36 days).

"The core of the Sun seems to rotate about three to five times faster on average," Garcia told LiveScience.

Current theories of solar formation suggest the original cloud of matter that gave rise to the solar system had a high rate of rotation, a remnant of which "could exist in the deepest regions of the Sun," Garcia said. "It seems that the solar core rotation is slower than expected by those theories," he added.

Garcia noted a magnetic field leftover from the solar system's formation could have contributed to slowing down the solar core. A new generation of instruments now being built that can measure individual g-modes--for instance, "the GOLF New-Generation prototype that could fly in the DynaMICCS spacecraft presented inside the Cosmic-Vision 2015-2025 ESA program"--could help provide the insight needed to explain the mysteries of the solar core, he said.

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