The tachocline is the transition region of the Sun between the radiative interior and the differentially rotating outer convective zone. It is in the outer third of the Sun (by radius). This causes the region to have a very large shear as the rotation rate changes very rapidly. The convective exterior rotates as a normal fluid with differential rotation with the poles rotating slowly and the equator rotating quickly. The radiative interior exhibits solid-body rotation, possibly due to a fossil field. The rotation rate through the interior is roughly equal to the rotation rate at mid-latitudes, i.e. in-between the rate at the slow poles and the fast equator. Recent results from helioseismology indicate that the tachocline is located at a radius of at most 0.70 times the Solar radius (measured from the core, i.e., the surface is at 1 solar radius), with a thickness of 0.04 times the solar radius. This would mean the area has a very large shear profile that is one way that large scale magnetic fields can be formed. The geometry and width of the tachocline plays an important role in models of the solar dynamo by winding up weaker poloidal field to create a much stronger toroidal field. The term tachocline was coined in a paper by Edward Spiegel and Jean-Paul Zahn in 1992 by analogy to the oceanic thermocline.
- Spiegel, E.~A., & Zahn, J.-P., 1992, Astronomy and Astrophysics, 265, 106 
- Section 3.2 from Living Reviews in Solar Physics
- Charbonneau, P., Christensen-Dalsgaard, J., Henning, R., Larsen, R.M., Schou, J., Thompson, M.J., Tomczyk, S., 1999a, “Helioseismic Constraints on the Structure of the Solar Tachocline”, Astrophys. J., 527, 445-460, .
- Basu, S., Antia, H.M., Narasimha, D., 1994, “Helioseismic Measurement of the Extent of Overshoot Below the Solar Convection Zone”, Mon. Not. R. Astron. Soc., 267, 209-224, 
- Hughes, D.W., Rosner, R., Weiss, N.O. 2007 The Solar Tachocline, 382pp (Cambridge University Press).