Description
The solar tachocline is a narrow shear layer (no thicker than ~5% the solar radius), in which strong differential rotation in the convection zone (CZ) transitions to solid-body rotation in the adjacent radiative zone (RZ). How the tachocline can remain so thin at the current age of the Sun, despite the effect of 'radiative spread' (where latitudinal entropy gradients in the CZ spread inward via radiative diffusion and carry along a meridional circulation and differential rotation) remains a long-standing unresolved issue of stellar astrophysics. In the parameter regimes most accessible to global simulations, the simulated tachocline usually spreads viscously instead of radiatively (i.e. the CZ drags the RZ into differential rotation via a higher-than-realistic viscosity). In 2022, we explored a 3-D MHD simulation of a CZ-RZ system in which a dynamo was self-excited in the CZ, spread into the RZ, and confined a tachocline against viscous spread via magnetic torque. As a whole, this result was reminiscent of the 1-D 'fast magnetic confinement scenario', in which a poloidal dynamo-generated field (cycling with a period of 22 years) penetrates below the CZ to a magnetic skin depth, determined by the single cycle frequency. We have recently shown more carefully that the skin effect indeed well describes the amplitude of poloidal field strength in the RZ for several CZ-RZ dynamo simulations at different magnetic Prandtl numbers, even when the cycle is aperiodic (i.e. composed of many distinct frequencies). Overall, these results suggest a much wider range of operation for a general 'dynamo confinement scenario.' To assess if this scenario is realistic for the Sun, we also discuss our latest dynamo simulations for a tachocline in the radiative spreading regime.
