Tachocline

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The tachocline is a thin transition layer inside stars with more than about 0.3 solar masses. It sits between the inner radiative zone, where energy moves mainly by radiation, and the outer convective zone, where hot gas swirls and the rotation varies with latitude. In this region, the rotation rate changes rapidly with depth, creating a large shear.

In the outer convective zone, the gas rotates faster at the equator and slower at the poles, like a spinning sheet with differential rotation. The inner radiative zone rotates more like a solid body, possibly kept in that state by a fossil magnetic field. The rotation inside is roughly between the pole’s slow pace and the equator’s fast pace, similar to mid-latitudes.

Helioseismology shows that the Sun’s tachocline lies at about 0.70 solar radii from the center and is about 0.04 solar radii thick. This strong shear is thought to help wind up weak magnetic fields into stronger toroidal fields, which plays a key role in the stellar dynamo that drives magnetic activity.

The shape and width of the tachocline are important for models of stellar dynamos. Interestingly, cooler stars and brown dwarfs that lack a radiative core still show large-scale magnetic fields and solar-like activity, suggesting that the convective zone alone can power dynamos in some stars.

The term tachocline was coined in 1992 by Edward Spiegel and Jean-Paul Zahn, by analogy to the ocean’s thermocline.