Global MHD theory of tachocline and the current status of large-scale solar dynamo

Helioseismic inversions have shown the existence of the solar tachocline, a thin layer containing strong radial differential rotation, at or below the convection zone base. Occurrence of latitudinal instabilities has been argued to be necessary for explaining the thinness of tachocline. Global MHD instability theory and computations applied to the tachocline yield three major results: (i) production of large-scale non-axisymmetry through "tipping" or "deformation" of a toroidal magnetic band, (ii) generation of kinetic helicity and (iii) achievement of stability through formation of a prograde jet imbedded in the tachocline latitudinal differential rotation. After briefly reviewing these theoretical tachocline features, we discuss their implications for large-scale solar dynamo models. The advent of helioseismology has significantly changed the status of large-scale solar dynamo models by putting observational constraints on the dynamo ingredients as well as their output. Currently, the most widely accepted large-scale solar dynamo is of flux-transport type, which is not only able to reproduce many solar cycle features, but is also proving itself to be a powerful tool for predicting cycle features. The dynamo cycle period in such models is primarily governed by the meridional flow speed. To reproduce the correct periodicity of solar cycle, this class of dynamos require about 1 ms^-1 equatonvard return-flow at the bottom of convection zone. We will show how the knowledge gained from recent theoretical studies exploring certain meridional flow properties help improve our physical understanding of the solar dynamo mechanism. We close by indicating features to look for in helioseismic data, such as prolateness of the tachocline, amplitude and location of tachocline jets, amplitude and pattern of helical flow there, amplitude and depth of the subsurface meridional circulation counterflow, and solar cycle variation of all these features.