Unraveling a long-standing solar mystery: The extreme thinness of the sun’s tachocline layer

Researchers are closer to unraveling a longstanding solar mystery surrounding the extreme thinness of the sun’s tachocline layer of strong shearing motion—a region believed to be critical for creating the violent eruptions of high-energy particles and radiation from the sun known as “space weather.”

University of California, Santa Cruz, professor of applied mathematics Nicholas Brummell and Baskin School of Engineering postdoctoral scholar Loren Matilsky study the sun’s extremely thin tachocline region as part of the NASA center called COFFIES (Consequences Of Fields and Flows in the Interior and Exterior of the sun).

In a new paper published in The Astrophysical Journal, the researchers, with former UCSC graduate student Lydia Korre, reveal new insights into how magnetic fields keep the solar tachocline so thin and, more generally, how tachoclines in other solar-type stars may contribute to stellar “spin down”—the mysterious process by which stars are observed to slow their overall rotation rates, or “spins,” as they slowly evolve. The new simulations suggest a holistic interplay among rotation, magnetism and tachoclines in solar-like stars.

Solar dynamo effects on Earth and beyond

The sun consists of various layers that generate magnetic fields through a process called the solar dynamo. This magnetic engine powers solar activity, sparking solar flares and coronal mass ejections that dictate space weather cycles. Investigating these cycles is vital, as space weather affects astronaut safety, satellite communications and global navigation systems.

The tachocline, an extremely thin layer sandwiched between the sun’s inner radiative zone and outer convective zone, is thought to be a key component of these cycles. Astronomers believe the tachocline serves as the main amplifier of the sun’s magnetic field, storing, organizing and releasing magnetic energy that eventually emerges at the solar surface as sunspots, which trigger space weather events.

Beyond our solar system, the dynamics of a star’s magnetic field are critical to understand because they drive all aspects of the space environment and space weather, including maintaining a habitable atmosphere. A better understanding of these dynamics can help identify criteria for finding potentially life-supporting planets.

“Everyone wants to know what the environment is like between the sun and Earth, because it affects our life here,” Brummell said. “But understanding the origin of space weather is important when we are looking for other habitable worlds, too.”

Insights on synergy

The tachocline’s extreme thinness long remained a mystery, as earlier models failed to replicate its unique dynamics in a self-consistent way. However, the UC Santa Cruz researchers pioneered new simulations that more closely mimic solar dynamics. The team refined state-of-the-art models, powered by hundreds of millions of hours of processing time on NASA’s most powerful supercomputer.

This enabled them to produce a series of simulations that reveal the tachocline is essential in driving the solar magnetic field, while the magnetic field is also essential for keeping the tachocline’s signature thinness—a newly identified synergy between these processes.

“For decades, it was thought that the tachocline was a key component in driving the large-scale solar magnetic field,” Matilsky said. “The picture that’s emerging from our work is that the reverse might also be true: that the large-scale magnetic field might be a key reason why there’s a tachocline to begin with.”

Spinning down

Astronomers have a general theory for the life cycle of stars, but one detail that has long remained a mystery is the “spin down problem”—how stars slow their rotation over time, leading to the eventual collapse of the large-scale magnetic field and possibly bringing to an end extreme space weather events related to the star. Generally, the outer layers of the star are directly slowed by the surface magnetic field, which radiates from the surface like spokes on a wheel. How this surface slowing is transmitted to the deeper interior is unknown.

The UC Santa Cruz researchers’ new simulations show how the tachocline acts to connect the outer and inner regions of the sun via the very dynamo field that is confining the tachocline.

“We know the sun used to rotate much faster and that it has been spinning down for a long time,” Matilsky said. “But this raises a natural question: Why is the radiative zone not just ‘left alone,’ continuing to spin at the rapid rotation rate the sun was born at? In the new simulations, the dynamo magnetic field that penetrates to confine the tachocline also connects the two distinct regions, providing the necessary spin-down self-consistently and thereby supplying a potential resolution to this longstanding conundrum.”

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Lisa Lock

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Andrew Zinin

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