Alpha effect in density-stratified turbulence with large-scale shear in astrophysical clouds and discs

Abstract: A combined effect of the density-stratified turbulence and a large-scale shear for arbitrary Mach numbers causes the alpha effect and the effective pumping velocity of a large-scale magnetic field. This results in generation of the large-scale magnetic field in colliding protogalactic clouds, merging protostellar clouds and protoplanetary discs. Compressibility of the turbulent velocity field (i.e., finite Mach number effects) does not affect the contributions caused by the inhomogeneity of turbulence to the $\alpha$ tensor and effective pumping velocity. The isotropic part of the alpha tensor is independent of the exponent of the turbulent kinetic energy spectrum. There is also an additional contribution to the effective pumping velocity of the mean magnetic field caused by product of the fluid density stratification and the divergence of the mean fluid velocity due to collapsing (or expanding) astrophysical clouds. However, the alpha tensor is independent of the effects of collapsing or expanding of clouds.