Helicity inversion in spherical convection as a means for equatorward dynamo wave propagation

Abstract

We discuss here a purely hydrodynamical mechanism to invert the sign of the kinetic helicity, which plays a key role in determining the direction of propagation of cyclical magnetism in most models of dynamo action by rotating convection. Such propagation provides a prominent, and puzzling constraint on dynamo models. In the Sun, active regions emerge first at mid-latitudes, then appear nearer the equator over the course of a cycle, but most previous global-scale dynamo simulations have exhibited poleward propagation (if they were cyclical at all). Here, we highlight some simulations in which the direction of propagation of dynamo waves is altered primarily by an inversion of the kinetic helicity throughout much of the interior, rather than by changes in the differential rotation. This tends to occur in cases with a low Prandtl number and internal heating, in regions where the local density gradient is relatively small. We analyse how this inversion arises, and contrast it to the case of convection that is either highly columnar (i.e., rapidly rotating) or locally very stratified; in both of those situations, the typical profile of kinetic helicity (negative throughout most of the northern hemisphere) instead prevails.

17 pages, 16 figures, 1 table, accepted in MNRAS

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• Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars
• Dynamo cycles in global convection simulations of solar-like stars
• Convection-driven spherical shell dynamos at varying Prandtl numbers
• Anelastic spherical dynamos with radially variable electrical conductivity
• Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulations
• Rotational dependence of turbulent transport coefficients in global convective dynamo simulations of solar-like stars
• The magnitude of viscous dissipation in strongly stratified two-dimensional convection
• Mechanism of mean flow generation in rotating turbulence through inhomogeneous helicity
• Robustness of oscillatory ${\alpha }^2$ dynamos in spherical wedges
• Impact of anti-solar differential rotation in mean-field solar-type dynamos -- Exploring possible magnetic cycles in slowly rotating stars
• Prandtl-Number Effects in High-Rayleigh-Number Spherical Convection
• Mean-field analysis on large-scale magnetic fields at high Reynolds numbers
• Dynamos and Differential Rotation: Advances at the Crossroads of Analytics, Numerics, and Observations
• Solar-like dynamos and rotational scaling of cycles from star-in-a-box simulations