What Lies Beneath the Sun’s Surface? Solar Physicists Uncover Hidden Depths of Supergranules

Artistic Impression of the Sun’s supergranules. Supergranules transport heat near the surface of the Sun and are approximately 3 times wider than the Earth. Hot material from inside the sun rises to the surface, cools and turns over before sinking back into the interior. Scientists use sound waves to see below the surface, which appear as ripples on the surface. Credit: Melissa Weiss

Using sound waves, scientists uncover findings that challenge standard theories of solar convection.

A team of solar physicists has made significant discoveries about the sun’s supergranules, using data from the Solar Dynamics Observatory. Their research shows weaker downflows compared to upflows in these supergranules, indicating potential unseen components like small-scale plumes. This challenges the conventional understanding of solar convection.

Researchers have revealed the interior structure of the sun’s supergranules, a flow structure that transports heat from the sun’s hidden interior to its surface. Led by Research Scientist Chris S. Hanson, Ph.D.,  the team of solar physicists at NYU Abu Dhabi’s Center for Astrophysics and Space Science (CASS) conducted an analysis of the supergranules that presents a challenge to the current understanding of solar convection.

Breakthrough in Solar Convection Understanding

The sun generates energy in its core through nuclear fusion; that energy is then transported to the surface, where it escapes as sunlight. In the study “Supergranular-scale solar convection not explained by mixing-length theory” published today (June 25) in the journal Nature Astronomy, the researchers explain how they utilized Doppler, intensity, and magnetic images from the helioseismic and magnetic imager (HMI) onboard NASA’s Solar Dynamics Observatory (SDO) satellite to identify and characterize approximately 23,000 supergranules.

Since the sun’s surface is opaque to light, the NYUAD scientists used sound waves to probe the interior structure of the supergranules. These sound waves, which are generated by the smaller granules and are everywhere in the sun, have been successfully used in the past in a field known as Helioseismology.

Artist’s concept of the Solar Dynamics Observatory (SDO). Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

Methodology and Discoveries

By analyzing such a large dataset of supergranules, which were estimated to extend 20,000 Km (~3% into the interior) below the surface of the sun, the scientists were able to determine the up and down flows associated with supergranular heat transport with unprecedented accuracy. In addition to inferring how deep the supergranules extend, the scientists also discovered that the downflows appeared ~40 percent weaker than the upflows, which suggests that some component was missing from the downflows.

Implications for Solar Physics

Through extensive testing and theoretical arguments, the authors theorize that the “missing” or unseen component could consist of small-scale (~100 km) plumes that transport cooler plasma down into the sun’s interior. The sound waves in the sun would be too big to sense these plumes, making the downflows appear weaker. These findings cannot be explained by the widely used mixing-length description of solar convection.

“Supergranules are a significant component of the heat transport mechanisms of the sun, but they present a serious challenge for scientists to understand,” said Shravan Hanasoge, Ph.D., research professor, co-author of the paper and co-Principal Investigator of CASS. “Our findings counter assumptions that are central to the current understanding of solar convection, and should inspire further investigation of the sun’s supergranules.”

Reference: “Supergranular-scale solar convection not explained by mixing-length theory” 25 June 2024, Nature Astronomy.

The research was conducted within CASS at NYUAD in collaboration with Tata Institute of Fundamental Research, Princeton University, and New York University, using NYUAD’s high-performance computing resources.