Magnetoconvection and dynamo coefficients III: alpha-effect and magnetic pumping in the rapid rotation regime

TL;DR

This study investigates the alpha-effect and magnetic pumping in rapid rotation regimes of the solar convection zone using 3D MHD simulations, revealing latitudinal shifts in magnetic field generation and confirming analytical models.

Contribution

It provides detailed numerical calculations of dynamo coefficients in rapid rotation, extending understanding of magnetic field generation in the Sun's deep layers.

Findings

Alpha-effect peaks at 30 degrees latitude in rapid rotation.

Magnetic pumping is mainly radially downward and equatorward.

Numerical results align well with first-order smoothing analytical expressions.

Abstract

Aims. The alpha- and gamma-effects, which are responsible for the generation and turbulent pumping of large scale magnetic fields, respectively, due to passive advection by convection are determined in the rapid rotation regime corresponding to the deep layers of the solar convection zone. Methods. A 3D rectangular local model is used for solving the full set of MHD equations in order to compute the electromotive force (emf), E = <u x b>, generated by the interaction of imposed weak gradient-free magnetic fields and turbulent convection with varying rotational influence and latitude. By expanding the emf in terms of the mean magnetic field, E_i = a_ij <B_j>, all nine components of a_ij are computed. The diagonal elements of a_ij describe the alpha-effect, whereas the off-diagonals represent magnetic pumping. The latter is essentially the advection of magnetic fields by means other than the underlying large-scale velocity field. Comparisons are made to analytical expressions of the coefficients derived under the first-order smoothing approximation (FOSA). Results. In the rapid rotation regime the latitudinal dependence of the alpha-components responsible for the generation of the azimuthal and radial fields does not exhibit a peak at the poles, as is the case for slow rotation, but at a latitude of about 30 degrees. The magnetic pumping is predominantly radially down- and latitudinally equatorward as in earlier studies. The numerical results compare surprisingly well with analytical expressions derived under first-order smoothing, although the present calculations are expected to lie near the limits of the validity range of FOSA.