Magnetoconvection and dynamo coefficients: Dependence of the α 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 〈u × b〉. From this electric field, we calculate the α-effect, separately for the stably and the unstably stratified layers, by averaging over time and over suitably defined volumes. From the variation of α we derive an error estimate, and the dependence of α on rotation and magnetic field strength is studied. Evidence is found for rotational quenching of the vertical α effect, and for a monotonic increase of the horizontal α effect with increasing rotation. For Rm ≈ 30, our results for both vertical and horizontal α effect are consistent with magnetic quenching by a factor [1 + Rm(B0/Beq) ]. The signs of the small-scale current helicity and of the vertical component of α are found to be opposite to those for isotropic turbulence.