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where ${\bf B}$ is the magnetic field, $\eta$ is the magnetic diffusivity and ${\bf u}$ is the large-scale flow field. This flow filed can be expressed as a sum of an axisymmetric azimuthal component originating from the differential rotation ans a axisymmetric poloidal component originating from the meridional circulation:  \begin{equation}  {\rm \bf u} = {\rm \bf u_p}\left( r,\theta \right) + r ~{\rm sin}\theta sin}~\theta  ~ \Omega \left( r, \theta \right) \hat {\rm e}_{\phi} \end{equation}  This field includes a great variety of different subjects, but for this review we have make a selective of what we think are the most important subjects: