Effects of anisotropy of turbulent convection in mean-field solar dynamo models

TL;DR

This study investigates how anisotropy in turbulent convection influences magnetic field diffusion and the solar dynamo, revealing that increased anisotropy concentrates magnetic flux and aligns model results with observational data.

Contribution

It introduces a parameter for turbulence anisotropy and demonstrates its effects on magnetic field distribution and dynamo cycle characteristics in solar models.

Findings

Magnetic flux concentrates near convection zone boundaries with increased anisotropy.

Dynamo period and critical number stabilize at high anisotropy levels.

Anisotropic diffusivity models align with helioseismology and surface magnetic field observations.

Abstract

We study how anisotropy of turbulent convection affects diffusion of large-scale magnetic fields and the dynamo process on the Sun. The effect of anisotropy is calculated in a mean-field magneto-hydrodynamics framework using the minimal $\tau$-approximation. We examine two types of mean-field dynamo models: the well-known benchmark flux-transport model, and a distributed-dynamo model with the subsurface rotational shear layer. For both models we investigate effects of the double-cell meridional circulation, recently suggested by helioseismology. We introduce a parameter of anisotropy as a ratio of the radial and horizontal intensity of turbulent mixing, to characterize the anisotropy effects. It is found that the anisotropy of turbulent convection affects the distribution of magnetic fields inside the convection zone. The concentration of the magnetic flux near the bottom and top boundaries of the convection zone is greater when the anisotropy is stronger. It is shown that the critical dynamo number and the dynamo period approach to constant values for the large anisotropy parameter. The anisotropy reduces the overlap of the toroidal fields of subsequent cycles in the time-latitude ``butterfly' diagram. If we assume that sunspots are formed in the vicinity of the subsurface shear layer, then the distributed dynamo model with anisotropic diffusivity satisfies the observational constraints from heloseismology results and is consistent with the value of effective turbulent diffusion, estimated from the dynamics of surface magnetic fields.