Fossat and his team were able to make the detection using helioseismology. Similar to how seismologists study Earth’s interior by listening to how the seismic waves that cause earthquakes travel through rock layers, solar physicists can listen and track oscillations in the sun to see how waves propagate throughout the star.
Helioseismology, which could be considered a type of acoustical spectroscopy, has given scientists the ability to measure the invisible internal structure and dynamics of our sun. Different oscillation modes operate at different depths inside the star, so combining the rate and amplitude of the oscillations can reveal the composition and internal configuration.
But unlike Earth, where seismic events typically happen one at a time, the team said the sun is continuously “ringing’” from sounds waves due to the continuous convection of solar material churning both on — and beneath — the surface, making it difficult to pick out certain low frequencies.
Stellar oscillations are divided into three categories, based on the force that drives them: pressure, surface-gravity, and gravity wave modes. The p-waves have pressure as their force, and can tell us things about the structure and density of regions just below the surface of a star. Surface gravity waves, or f-modes, occur at — or near — the outer layers of stars, so they give us information about the surface conditions of stars.
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Gravity waves, or g-waves, are confined to the interior of a star. They represent oscillations of the deep solar interior that have no clear signature at the surface. They also are at a lower frequency and therefore have been a challenge to detect directly.
Gravity waves are not the same as gravitational waves, which were detected by the LIGO (Laser Interferometer Gravitational-Wave Observatory) team. Gravitational waves are perturbations across space-time at universal scales, while gravity waves are perturbations within solar bodies or planetary atmospheres at smaller scales.
“Over the past 40 years, helioseismology has been enormously successful in the study of the solar interior,” the team wrote in their paper. “A shortcoming has been the lack of a convincing detection of the solar g modes, which are oscillations driven by gravity and are hidden in the deepest part of the solar body – its hydrogen-burning core. The detection of g-modes is expected to dramatically improve our ability to model this core, the rotational characteristics of which have, until now, remained unknown.”
The team used over 16 years of data collected by SOHO, which has been in space since 1995. In particular, they used data from SOHO’s Global Oscillations at Low Frequencies, or GOLF, instrument, which was built to measure oscillations within the sun in a certain frequency range (10-7 to 10-2 Hz.)
The search for g-waves in GOLF measurements has been extremely difficult because of solar and instrumental noise. So they tried a different approach: They looked at all the frequency modulations produced by the sun and figured out how to detect the imprint of the g-waves, which usually ride along with the more easily detected p-waves. They looked at a p-wave parameter that measures how long it takes for an acoustic wave to travel through the sun and back to the surface again, which is known to take four hours and seven minutes. Using various analytical and statistical techniques, they picked out a series of modulations within the p-mode parameter, which they determined was the elusive and very low frequency signature of g-waves pulsating from the structure of the Sun’s core.
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The signature of the g-waves suggests that the solar core is rotating once every week, which is nearly four times faster than the sun’s surface and intermediate layers, which have rotation rates at about 25 days at the equator to 35 days at the poles.
“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,” said Roger Ulrich, from UCLA and co-author of the study. “It’s a surprise, and exciting to think we might have uncovered a relic of what the sun was like when it first formed.”
What can be learned from knowing the rotation rate of the core of the sun? It might provide clues as to how the sun formed and how features form on the sun today, such as sunspots. But Ulrich and Fossat both said the first detection of g-waves now opens up new questions for solar physicists to study, such as how the different layers of the sun interact — even though they are all rotating differently — and learning more about the composition of the core based on its rotation.
“G-modes have been detected in other stars, and now thanks to SOHO we have finally found convincing proof of them in our own star,” Fossat said. “It is really special to see into the core of our own sun to get a first indirect measurement of its rotation speed. But, even though this decades-long search is over, a new window of solar physics now begins.”
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