The importance of the solar tachocline

The solar tachocline, a thin layer containing the strong radial differential rotation at the base of the convection zone has proven to be important for several reasons. Radial shear there is likely to generate the Sun's strongest toroidal fields, which eventually erupt as bipolar spots at the surface, and the tachocline provides a good location for magnetic flux-storage. The subadiabatic stratification of this layer allows storage of strong toroidal field despite its magnetic buoyancy, while toroidal bands are held against poleward slip by either the prolateness of this layer, or by jet-like flows within the band, or both. Global HD and MHD instabilities that are theoretically predicted to occur in this layer produce two major results. One is the production of large-scale non-axisymmetries, by tipping or deforming the toroidal band, and the other is the generation of kinetic helicity. Both have important implications in solar dynamos. The former could be responsible for producing the Sun's "active-longitudes", while the latter produces the extended dipolar poloidal fields that are necessary for magnetically coupling the Sun's N- and S-hemispheres. We will review these theoretical results, and indicate features to look for in the helioseismic data, such as, prolateness, amplitude and location of jets in the tachocline, including their variations with the solar cycle; as well as helical flows and their spatial and temporal dependence.