The alpha effect in rotating convection with sinusoidal shear

TL;DR

This study demonstrates that sinusoidal shear in rotating convection induces a sinusoidal lpha effect and large-scale magnetic fields, emphasizing the importance of inhomogeneity and boundary effects in magnetic field generation.

Contribution

It reveals the lpha effect's sinusoidal nature in rotating convection with shear and highlights the impact of inhomogeneity and boundaries on magnetic field dynamics.

Findings

Sinusoidal shear induces a matching lpha effect.

Large-scale magnetic fields exhibit sinusoidal variation.

Non-uniform mean fields affect magnetic diffusion and lpha balance.

Abstract

Using three-dimensional convection simulations it is shown that a sinusoidal variation of horizontal shear leads to a kinematic \alpha effect with a similar sinusoidal variation. The effect exists even for weak stratification and arises owing to the inhomogeneity of turbulence and the presence of impenetrable vertical boundaries. This system produces large-scale magnetic fields that also show a sinusoidal variation in the streamwise direction. It is argued that earlier investigations overlooked these phenomena partly because of the use of horizontal averaging and also because measurements of \alpha using an imposed field combined with long time averages give erroneous results. It is demonstrated that in such cases the actual horizontally averaged mean field becomes non-uniform. The turbulent magnetic diffusion term resulting from such non-uniform fields can then no longer be neglected and begins to balance the \alpha effect.