Storage and equilibrium of toroidal magnetic fields in the solar tachocline: A comparison between MHD shallow-water and full MHD approaches

Recently Dikpati & Gilman have shown, using a shallow-water model of the solar tachocline that allows the top surface to deform, that a tachocline with the observed broad differential rotation and a strong toroidal field is prolate. A strong toroidal field ring requires extra mass on its poleward side to provide a hydrostatic latitudinal pressure gradient to balance the poleward curvature stress. In a parallel study using a different approach, Rempel, Schüssler, & Tóth have shown that such a latitudinal pressure gradient is found in a strongly subadiabatic stratification, whereas a weakly subadiabatic stratification leads to a complementary equilibrium state of the overshoot tachocline in which the magnetic curvature stress is balanced by a prograde rotational jet inside the toroidal ring. We show that the shallow-water model with height deformation is a first-order approach to the equilibrium state found by Rempel, Schüssler, & Tóth for a strongly subadiabatic stratification. We also show that the shallow-water model can be generalized to allow for the equilibrium state found for a weakly subadiabatic stratification by suppressing the shell deformation associated with the toroidal field and allowing the differential rotation to be modified.