MAGNETIC QUENCHING OF α AND DIFFUSIVITY TENSORS IN HELICAL TURBULENCE

The effect of a dynamo-generated mean magnetic field of Beltrami type on the mean electromotive force is studied. In the absence of the mean magnetic field the turbulence is assumed to be homogeneous and isotropic, but it becomes inhomogeneous and anisotropic with this field. Using the test-field method the dependence of the a and turbulent diffusivity tensors on the magnetic Reynolds number Re(M) is determined for magnetic fields that Re M have reached approximate equipartition with the velocity field. The tensor components are characterized by a pseudoscalar alpha and a scalar turbulent magnetic diffusivity eta(t). Increasing Re(M) from 2 to 600 reduces eta(t) by a factor approximate to 5, suggesting that the quenching of is, in contrast to the two-dimensional case, only weakly dependent on Re(M). Over the same range of Re(M), however, a is reduced by a factor approximate to 14, which can be explained by a corresponding Re Re increase of a magnetic contribution to the alpha-effect with opposite sign. Within this framework, the corresponding kinetic contribution to the alpha-effect turns out to be independent of Re(M) for 2 <= Re(M) <= 600. The level of fluctuations of alpha and eta(t) is only 10% and 20% of the respective kinematic reference values.