Technology
NASA's COFFIES team solves a 50-year solar mystery — why the Sun's tachocline is so thin
Our Sun is a giant magnetic engine, but for decades scientists could not explain why one of its most critical internal layers — a thin region called the tachocline — remains so remarkably narrow. Now, researchers working with NASA's COFFIES Science Center have finally solved the puzzle, publishing their findings in The Astrophysical Journal in July 2026.
What is the tachocline and why does it matter?
The tachocline is a transition zone deep inside the Sun, sandwiched between the radiative interior (which rotates like a solid body) and the convective outer layers (which rotate at different speeds depending on latitude, like the surface of Jupiter). Think of it as the Sun's internal gearbox — a paper-thin layer where the slow, steady rotation of the deep interior meets the chaotic, differential rotation of the outer convection zone.
This layer matters enormously because scientists believe it is the primary site of the solar dynamo — the process that generates the Sun's magnetic field. That magnetic field, in turn, produces solar flares and coronal mass ejections that drive space weather, which can disrupt satellite communications, GPS navigation, power grids, and endanger astronauts. Understanding the tachocline means understanding — and potentially predicting — space weather.
The mystery: why is it so thin?
Computer models of the Sun's interior consistently predicted that the tachocline should be much thicker than observations show. The region where the radiative interior transitions to convective motion ought to be spread across a broad zone, yet the real tachocline is remarkably sharp — less than 4% of the Sun's radius in thickness. This discrepancy persisted for over five decades, becoming one of the most stubborn puzzles in solar physics.
How the COFFIES team cracked it
The COFFIES (Consequences Of Fields and Flows in the Interior and Exterior of the Sun) team, part of NASA's DRIVE Science Center program, used a novel approach: they built high-resolution magnetohydrodynamic simulations that allowed the magnetic field to evolve dynamically rather than imposing preset conditions. Previous models had treated the magnetic field as a fixed background, which obscured the crucial feedback loop.
The team discovered that the Sun's magnetic field itself is what keeps the tachocline thin. As the tachocline attempts to spread due to fluid motions, the magnetic field lines become stretched and intensified, creating a restoring force that compresses the layer back into its narrow configuration. It is a self-regulating system: the same magnetic dynamo that the tachocline enables also keeps the tachocline confined.
Broader implications beyond the Sun
The findings extend well beyond our own star. Many Sun-like stars exhibit unexplained "spin-down" — a gradual slowing of their rotation over billions of years. The tachocline dynamics discovered by the COFFIES team offer a potential mechanism for this process. As the magnetic field transports angular momentum from the interior to the surface, it gradually slows the star's rotation over evolutionary timescales. This could help astronomers understand why stars of similar mass and age can have dramatically different rotation rates, a question that has puzzled stellar astrophysicists for years.
Former UCSC graduate student Lydia Korre, the lead author of the study, noted that the findings also apply to exoplanet-hosting stars. A star's magnetic activity directly affects the habitability of orbiting planets, and understanding how internal magnetic fields evolve over time is essential for assessing which exoplanets might truly be Earth-like.