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

TL;DR

This paper explores a hydrodynamical mechanism that inverts kinetic helicity in spherical convection, potentially explaining the equatorward propagation of dynamo waves in the Sun, contrasting with previous models showing poleward propagation.

Contribution

It introduces a new mechanism for reversing kinetic helicity in dynamo models, emphasizing the role of internal heating and low Prandtl number in spherical convection.

Findings

Inversion of kinetic helicity can alter dynamo wave propagation direction.

Low Prandtl number and internal heating facilitate helicity inversion.

Typical convection profiles with negative helicity are contrasted with inverted cases.

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.