Magnetoconvection and dynamo coefficients: Dependence of the alpha-effect on rotation and magnetic field

We present numerical simulations of three-dimensional compressible magnetoconvection in a rotating rectangular box that represents a section of the solar convection zone. The box contains a convectively unstable layer, surrounded by stably stratified layers with overshooting convection. The magnetic Reynolds number, Rm, is chosen subcritical, thus excluding spontaneous growth of the magnetic field through dynamo action, and the magnetic energy is maintained by introducing a constant magnetic field into the box, once convection has attained a statistically stationary state. Under the influence of the Coriolis force, the advection of the magnetic field results in a non-vanishing contribution to the mean electric field, given by uxb. From this electric field, we calculate the alpha-effect, separately for the stably and the unstably stratified layers, by averaging over time and over suitably defined volumes. From the variation of alpha we derive an error estimate, and the dependence of alpha on rotation and magnetic field strength is studied. Evidence is found for rotational quenching of the vertical alpha-effect, and for a monotonic increase of the horizontal alpha-effect with increasing rotation. For Rm~30, our results for both vertical and horizontal alpha-effect are consistent with magnetic quenching by a factor 1/[1+Rm(B_0/B_eq)^2]. The signs of the small-scale current helicity and of the vertical component of alpha are found to be opposite to those for isotropic turbulence. ; Comment: 14 pages, 11 figures; to appear in Astronomy & Astrophysics (accepted)