On effects of surface bipolar magnetic regions on the convection zone dynamo

TL;DR

This study uses a 3D mean-field dynamo model to analyze how surface bipolar magnetic regions influence the solar-like dynamo process, highlighting the dominant role of the mean-field alpha effect and the impact of BMR tilt fluctuations.

Contribution

It introduces a nonlinear 3D mean-field dynamo model that incorporates surface BMR emergence via magnetic buoyancy, revealing their limited global impact but significant local effects.

Findings

The BMR's effect on the global dynamo is less than that of the convective zone dynamo.

The mean-field alpha effect from BMRs contributes more to the dynamo than BMR tilt.

Tilt fluctuations of BMRs cause parity braking in the global dynamo.

Abstract

We investigate the effect of the surface bipolar magnetic regions (BMR) on the large-scale dynamo distributed in the bulk of the convection zone. The study employs the nonlinear 3D mean-field dynamo model. We model the emergence of the BMR on the surface through the nonaxisymmetric magnetic buoyancy effect, which acts on the large-scale toroidal magnetic field in the upper half of the convection zone. The nonaxisymmetric magnetic field which results from this mechanism is shallow. On the surface, the effect of the BMR on the magnetic field generation is dominant. {However, because of the shallow BMR distribution, its effect on the global dynamo is less compared to the convective zone dynamo.} We find that the mean-field $\alpha$ effect, which acts on the nonaxisymmetric magnetic field of the BMRs, provides the greater contribution to the dynamo process than the BMR's tilt does. Even so, the fluctuations of the BMR's tilt lead to the parity braking in the global dynamo. At the surface the nonaxisymmetric magnetic field, which are generated because of the BMR's activity, shows a tendency for the bihelical spectrum with the positive sign for the low $\ell$ modes during the maximum of the magnetic activity cycle.