Sun’s Waves Reveal Core’s Spin

Solar astronomers may have finally detected gravity waves in our star’s core, revealing that the Sun’s central region rotates about four times faster than the outer layers.

The Sun is a swishing, churning, wiggling ball of ionized gas. It’s racked by waves. Astronomers who study these waves are fond of describing our star as a bell that rings, and like bells the Sun resounds at certain frequencies. Scientists can detect acoustic waves on the Sun because their ripples move the visible surface up and down, creating Doppler shifts in the gas’s glow.

cutaway of Sun

This cutaway diagram shows key regions of the Sun. The photosphere is often referred to as the "visible surface." Inside the Sun, there is a turbulent outer convection zone and a more stable inner radiative zone, which surrounds the core. Sound waves ("p-modes," where p is for pressure) reverberate throughout the Sun, but lower-frequency gravity waves ("g-modes") stay deep inside.

ESA / NASA

As acoustic waves move through the Sun, changes in internal structure or density affect how fast they travel. So scientists can use these waves to “see” inside the Sun, backtracking from any anomalies to determine what the waves would have had to encounter to be modified that way.

Astronomers have studied solar oscillations since the 1960s, but they’ve never managed to detect the signature of another, special kind of wave in the Sun’s core, called gravity waves or g modes. Gravity waves are a sloshing motion spurred by the turbulent convection that happens in the Sun’s outermost layer. They don’t survive well in the convective zone, though — instead, they become essentially trapped in the radiative zone beneath it and the core beneath that, where material moves in a way that doesn’t erase the gravity waves.

If solar physicists could detect these gravity waves, then they could learn how fast the core rotates. (Acoustic waves pass through the core too quickly to be sensitive to its spin.) But every detection of gravity waves, going back to the 1970s, has not held up against further investigation. Part of the problem is that scientists aren’t sure how strong g modes are, so it’s hard to verify whether a signal is actually from them.

Eric Fossat (Côte d’Azur Observatory, France) and colleagues now say they’ve unearthed this shy signal. The team dug through more than 16 years of data from the joint ESA/NASA Solar and Heliospheric Observatory (SOHO) spacecraft, launched in 1995 specifically to study the sound waves reverberating through the Sun, a field called helioseismology. The researchers focused on acoustic waves that moved through the core, with minimal effects from other structures within the Sun. Because gravity waves change the Sun’s internal geometry and density, their presence should affect the sound waves’ travel time.

Careful analysis of more than 34,000 acoustic wave patterns uncovered what looks like the imprint of gravity waves, Fossat’s team reports in the August Astronomy & Astrophysics. The team couldn’t tease apart individual waves, Fossat says; what they see is the collective signature of many waves combined.

The gravity waves’ frequency suggests that the core rotates every 7 days. That’s about four times faster than the radiative zone above it, and four to five times faster than the surface. (The surface rotates faster at the equator than at the poles.)

This difference between the core’s spin speed and those of the outer layers is unsurprising, says Thierry Appourchaux (Institute of Space Astrophysics, France). Work using Kepler data, both by him and others, suggests that many stars’ cores rotate at different speeds than their outer layers. Scientists had suspected the same was true for the Sun, but the SOHO result confirms that hypothesis.

If, that is, the effects on the acoustic waves really are from g modes. Appourchaux says the paper looks sound, but he points to the other potential detections over the last several decades that could not be reproduced. Solar gravity waves are notoriously impossible to find. But the team has laid out its analysis in great detail, and Fossat invites other astronomers to dive into the public data for themselves. If the detection proves true, then it’ll be a big deal in helioseismology and one of SOHO’s top discoveries.

 

References:

E. Fossat et al. “Asymptotic g modes: Evidence for a rapid rotation of the solar core.” Astronomy & Astrophysics. August 2017.

T. Appourchaux and P. L. Pallé. “The History of g-mode Detection.” 50 Years of Seismology of the Sun and the Stars, 2013 conference proceedings.


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