Nonlocal dynamo waves in a turbulent shear flow

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 field on turbulence. The shear can stretch turbulent magnetic fie ld lines in such a way that turbulent motions may become suitable for the g eneration of a large-scale magnetic field even in the absence of any strati fication. There is no ct-effect present in our computations. The nonlocal i ntegral representation for the mean electromotive force is derived, which i s valid even if the turbulent length scale is comparable to that of the mea n field. The basic result is that the presence of shear changes the type of the equation governing the mean magnetic field so that the latter indeed c an be generated even in the absence of rotation or large-scale stratificati on of turbulence. To this end, however, if the turbulence field has a monot onously falling ("turbulence-type") spectrum, a rather strong shear is need ed. For Kepler disks the instability condition reads tau (corr) > 2 tau (ro t)/pi, which might be fulfilled in the transition layers between star and d isk. A system, on the other hand, consisting of random waves, large-scale m agnetic fields and mean-field shear flow can never be stable.