Nonlocal dynamo waves in a turbulent shear flow

Abstract

The turbulent dynamo action in a shear flow is considered by making use of a quasilinear approximation and neglecting the back-reaction of a generated magnetic eld on turbulence. The shear can stretch turbulent magnetic eld lines in such a way that turbulent motions may become suitable for the generation of a large-scale magnetic eld even in the absence of any stratication. There is no -eect present in our computations. The nonlocal integral representation for the mean electromotive force is derived, which is valid even if the turbulent length scale is comparable to that of the mean eld. The basic result is that the presence of shear changes the type of the equation governing the mean magnetic eld so that the latter indeed can be generated even in the absence of rotation or large-scale stratication of turbulence. To this end, however, if the turbulence eld has a monotonously falling (\turbulence-type) spectrum, a rather strong shear is needed. For Kepler disks the instability condition reads corr > 2rot=, which might be fullled in the transition layers between star and disk. A system, on the other hand, consisting of random waves, large-scale magnetic elds and mean-eld shear flow can never be stable.