Lorentz force mediation of turbulent dynamo transitions

Abstract

We investigate how the strength of the Lorentz force alters stellar convection zone dynamics in a suite of buoyancy-dominated, three-dimensional, spherical shell convective dynamo models. This is done by varying only the magnetic Prandtl number, $Pm$, the non-dimensional form of the fluid's electrical conductivity $\sigma$. Because the strength of the dynamo magnetic field and the Lorentz force scale with $Pm$, it is found that the fluid motions, the pattern of convective heat transfer, and the mode of dynamo generation all differ across the $0.25 \leq Pm \leq 10$ range investigated here. For example, we show that strong magnetohydrodynamic effects cause a fundamental change in the surface zonal flows: differential rotation switches from solar-like (prograde equatorial zonal flow) for larger electrical conductivities to an anti-solar differential rotation (retrograde equatorial zonal flow) at lower electrical conductivities. This study shows that the value of the bulk electrical conductivity is important not only for sustaining dynamo action, but can also drive first-order changes in the characteristics of the magnetic, velocity, and temperature fields. It is also associated with the relative strength of the Lorentz force in the system as measured by the local magnetic Rossby number, $Ro_\ell^M$, which we show is crucial in setting the characteristics of the large-scale convection regime that generates those dynamo fields.