Confinement of the Solar Tachocline by a Non-Axisymmetric Dynamo

Abstract: We recently presented the first 3D numerical simulation of the solar interior for which tachocline confinement was achieved by a dynamo-generated magnetic field. In this followup study, we analyze the degree of confinement as the magnetic field strength changes (controlled by varying the magnetic Prandtl number) in a coupled radiative zone (RZ) and convection zone (CZ) system. We broadly find three solution regimes, corresponding to weak, medium, and strong dynamo magnetic field strengths. In the weak-field regime, the large-scale magnetic field is mostly axisymmetric with regular, periodic polarity reversals (reminiscent of the observed solar cycle), but fails to create a confined tachocline. In the strong-field regime, the large-scale field is mostly non-axisymmetric with irregular, quasi-periodic polarity reversals, and creates a confined tachocline. In the medium-field regime, the large-scale field resembles a strong-field dynamo for extended intervals, but intermittently weakens to allow temporary epochs of strong differential rotation. In all regimes, the amplitude of poloidal field strength in the RZ is very well explained by skin-depth arguments, wherein the oscillating field that gives rise to the skin depth (in the medium- and strong-field cases) is a non-axisymmetric field structure rotating with respect to the RZ. These new simulations reaffirm that tachocline confinement by the solar dynamo (the so-called fast magnetic confinement scenario) is possible, but suggest a new picture in which non-axisymmetric field components rotating with respect to the RZ play the primary role, instead of the regularly reversing axisymmetic field associated with the 22-year cycle.