Mean electromotive force and dynamo action in a turbulent flow

We derive the expression for the mean electromotive force in a turbulent fl ow taking into account stretching of turbulent magnetic field lines and str eamlines by the mean flow. Shear stresses break the symmetry of turbulence in such a way that turbulent motions become suitable for the dynamo action. The contribution of this effect to the mean electromotive force is proport ional to the production of spatial derivatives of the mean magnetic field a nd the mean velocity thus it cannot be described, in principle, in terms of the conventional alpha-effect.