Magnetoconvection and dynamo coefficients

III. α-effect and magnetic pumping in the rapid rotation regime

¹ Astronomy Division, Department of Physical Sciences, PO BOX 3000, 90014 University of Oulu, Finland e-mail: petri.kapyla@oulu.fi
² Kiepenheuer – Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany
³ Observatory, PO BOX 14, 00014 University of Helsinki, Finland

Received: 6 February 2006
Accepted: 2 May 2006

Abstract

Aims.The α- and γ-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), , 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, , all nine components of a_ij are computed. The diagonal elements of a_ij describe the α-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 α-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°. 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.