tachocline

The tachocline is a thin shear layer in the Sun that marks the transition between the radiative interior and the differentially rotating outer convection zone. It sits at about 0.69–0.71 solar radii from the center, just below the base of the convection zone, and has a thickness of only a few percent of the solar radius. In the tachocline, the rotation changes rapidly with radius: the radiative interior rotates roughly as a solid body, while the convection zone shows latitude-dependent angular velocity.

Helioseismology, the study of solar oscillations, reveals the presence of this thin shear layer. Analyses of measurements from projects such as GONG and the SOHO and SDO space missions indicate a tachocline thickness of about 0.02–0.05 solar radii (roughly 14,000–35,000 km). The thickness may vary with latitude and over the solar cycle.

Because of its strong shear, the tachocline is regarded as a likely site for the solar dynamo, where poloidal magnetic fields are stretched into toroidal configurations and stored against buoyant loss. The mechanism by which the tachocline is confined and maintained—whether by magnetic fields in the radiative interior, rotationally constrained flows, or overshoot from the convection zone—is an active area of research.

Analogues of the tachocline are predicted in other solar-type stars with convective envelopes, and its properties depend on rotation rate and internal structure.

Open questions remain about its precise confinement, latitudinal and temporal variation, and the details of its interaction with the convection zone and radiative interior.