6.2 Models and the tachocline
Even the most generous estimates for the observed tachocline thickness are small enough to pose an
interesting theoretical question: what prevents the shear from spreading further into the radiative interior,
destroying the observed uniform rotation? The literature on tachocline modeling is extensive, far beyond the
scope of this review. In brief, three main candidate mechanisms have been proposed: turbulent flows
(Spiegel and Zahn, 1992); “fossil” magnetic fields (e.g., Gough and McIntyre 1998); and gravity waves,
known to observational helioseismologists as modes (e.g., Zahn et al. 1997), but all these scenarios
have problems when considered as the sole mechanism. Recent advances in computing have made possible
detailed three-dimensional simulations to explore these issues, but these models have not yet been able to
reproduce a self-sustaining tachocline. For a review from a modeler’s perspective, see Miesch (2005).
Also, a variety of discussions of tachocline models are collected in the book edited by Hughes
et al. (2007).