Petri Käpylä, Maarit Käpylä, Axel Brandenburg, Matthias Rheinhardt

The magnetic field in the Sun undergoes a cyclic modulation with a reversal typically every 11 years due to a dynamo operating under the surface. We simulate a solar-type star, where the interplay between convection and rotation self-consistently drives large-scale magnetic field. We apply the test-field method to characterize the dynamo mechanism acting in this simulation by determining 27 turbulent transport coefficients of the electromotive force, of which 9 are related to the $\alpha$ effect tensor. We find that the alpha-effect has a complex nature and does not follow the profile expected from kinetic helicity. Besides the dominant $\alpha$-$\Omega$ dynamo, also an $\alpha^2$ dynamo is locally enhanced. The turbulent pumping velocities significantly alter the effective mean flows acting on the magnetic field and therefore challenge the flux transport dynamo concept. All coefficients are significantly affected due to dynamically important magnetic fields with quenching as well as enhancement being observed. This leads to a modulation of the coefficients with the activity cycle.

The magnetic field in the Sun undergoes a cyclic modulation with a reversal typically every 11 years due to a dynamo operating under the surface. We simulate slowly to rapidly rotating solar-type stars, where the interplay between convection and rotation self-consistently drives large-scale magnetic field. We apply the test-field method to characterise the dynamo mechanisms acting in this simulations by determining turbulent transport coefficients of the electromotive force. We find that the alphaeffect has a complex nature and does not follow the profile expected from kinetic helicity. However, the alpha effects in these simulations show strong rotational dependency resulting in highly anisotropic tensors and vanish alpha_zz components for rapid rotation. Furthermore, I will present the determination of magnetic helicity fluxes across the equator and through the surface, which are important quantities for the alleviation of catastrophically alpha quenching. Unlike in simulation of forced turbulence of Warnecke et al. (2011), the helicity fluxes across the equator are found to be much weaker in convection simulations. Moreover, I discuss the the relevants of magnetic and current helicity production in the dynamo region for the coronal heating process as well as to understand the activity-rotation-relation of main-sequence stars.