Deep beneath the surface of the sun lies a razor-thin transition zone called the tachocline. Despite being only a sliver in size, the tachocline is believed to be the engine room of the un’s magnetic activity.
This boundary divides the sun’s interior into two parts. An inner radiative zone (makes up 70 percent of the Sun by radius), where energy flows smoothly and the whole region spins together like a solid ball, and the outer convective zone (the remaining 30 percent), where hot gases swirl chaotically and spin at different speeds depending on location.
The tachocline is where the seeds of solar flares and coronal mass ejections are thought to form. For decades, scientists have known about it, but could not explain why this boundary is so astonishingly thin or how it stays stable.
Now, researchers from the University of California (UC), Santa Cruz have managed to model this elusive layer in a way that finally makes sense.
Decoding tachocline’s stability
The tachocline was first revealed in the 1980s through helioseismology, a method that uses ripples inside the sun, much like seismologists use earthquakes to probe Earth’s interior. However, translating that discovery into a working physical model proved nearly impossible. The difficulty comes from the nature of the sun itself.
A turbulent plasma sphere with processes that span scales from just 10 meters all the way to a million kilometers, and timescales from seconds to millions of years. Past attempts at modeling could not capture all these extremes at once, and so the tachocline stubbornly refused to appear in simulations.
This is where the new study comes into play. The authors overcome the modeling challenge by attempting what they call “hero” calculations. Using NASA’s Pleiades supercomputer, they spent tens of millions of computing hours over 15 months running simulations large enough and detailed enough to capture the tachocline in a more realistic way.
Previous efforts had given too much importance to viscosity, the sticky property of fluids that resists flow. However, according to the researchers, in reality, viscosity plays a negligible role in the sun’s interior. Instead, they emphasized radiative spreading, the natural tendency of the radiative zone’s energy transport to broaden the tachocline over time.
To their surprise, when the model was allowed to run under these conditions, a tachocline emerged on its own. Even more revealing, the simulations showed that the magnetic fields produced in the convective zone actually help keep the tachocline narrow, not just the other way around.
In other words, the dynamo (the natural process through which the sun produces its magnetic field) that was thought to depend on the tachocline also appears to create and maintain it, a feedback loop that earlier models had missed.
“There’s a synergy here because the tachocline is believed to play a fundamental role in causing the dynamo process. It now seems that the reverse may also be true, in the sense that the magnetic field from the dynamo may cause the tachocline to exist in the first place,” Loren Matilsky, lead study author and a postdoc researcher, notes.
A finding that goes beyond the Sun
Predicting violent solar storms is critical for protecting modern technology, from satellites to global power grids. By reproducing the tachocline, scientists now have a foundation for more reliable forecasts of solar activity.
Beyond Earth, the findings could also guide studies of other stars, since stellar magnetism plays a key role in shaping planetary systems and even in determining whether planets might support life.
“We’re learning a lot about our sun’s dynamics, and in the process, I think we’re also learning about how this works on other stars. The questions of the tachocline become all the more important in light of other stellar systems and exoplanets,” Matilsky said.
However, even with NASA’s second most powerful supercomputer, the simulations cannot yet capture every detail of the sun’s turbulent layers. So the UC Santa Cruz team aims to refine their models further and apply them to other stars.
The study is published in The Astrophysical Journal Letters.