Viscosity-alpha and dynamo-alpha for magnetically driven compressible turbulence in Kepler disks

For a given isotropic and homogeneous field of magnetic fluctuations both t he viscosity-alpha as well as the dynamo-alpha have been computed for accre tion disks on the basis of a quasilinear approximation with shear flow and density fluctuations (i.e. magnetic buoyancy) included. The resulting visco sity-alpha proves to be positive for sufficiently strong sheer (i.e. the an gular momentum transport is outwards) while the sign of the dynamo-alpha de pends on the hemisphere. Again, for sufficiently strong shear it changes it s sign, it is no vv negative for the upper disk plane and positive for the lower one. The current helicity [j' . B'] also changes its si,on with increasing shear . For a Kepler flow in the upper (lower) disk plane, the sign is positive ( negative). In our turbulence model the current helicity of the fluctuations and the alpha -effect of dynamo theory are almost always out of phase: the signs of all the quantities are in perfect correspondence to the numerical simulations of Brandenburg (1998, 2000). The kinetic helicity has the same sign as the alpha -effect - not, as often assumed, the opposite one. The resulting ratio between the dynamo-alpha and the viscosity-alpha reveal s the dynamo-alpha amplitude as rather small compared with the turbulence i ntensity. This is in contrast to earlier results on the basis of a quantita tive approximation but again is in agreement with recent results of numeric al simulations.