The origin of large-scale magnetic fields in various astrophysical bodies still remains a challenging problem. These fields can arise, in principle, by turbulent dynamo generation, from a weak seed field (see, e.g., Moffat 1978, Parker 1979). Turbulent motions showing lack of the reflection symmetry seem to be most suitable for the dynamo action (Krause & Rädler 1980). The conventional alpha-dynamo is an example of such a mechanism where the reflection symmetry of a rotating turbulence is broken by the Coriolis force. Due to rotation, the mean electromotive force, , has a component proportional to the mean magnetic field, and this component may be responsible for the dynamo action. Note, however, that the capacity of this mechanism to produce the observed magnetic fields, at least in some astrophysical objects, has been debated (Cattaneo & Vainstein 1991, Vainstein & Rosner 1991, Kulsrud & Anderson 1992).
Recently, Urpin and Brandenburg (1999) have suggested the qualitatively different turbulent dynamo mechanism which can operate in a differentially rotating fluid. If rotation is non-uniform, the mean electromotive force contains additional terms proportional to the production of shear stresses and derivatives of the mean magnetic field. These additional terms may generally lead to a rapid amplification of the mean field. Obviously this effect cannot be interpreted in terms of the alpha-dynamo. Contrary to the conventional alpha-effect, this mechanism can amplify a large scale magnetic field even if turbulence is homogeneous. It has also been argued that the similar mechanism can operate in a turbulent shear flow (Urpin 1999). Shear breaks the symmetry of turbulence stretching turbulent magnetic field lines and streamlines. This stretching produces a non-zero contribution to the mean electromotive force even in the simplest case of a plane Couette flow.
In the present paper we consider the mean electromotive force induced in a turbulent fluid by shear stresses. The main goal of this paper is to show that a non-uniform mean flow may lead to a large scale dynamo action for a wide class of flows.
© European Southern Observatory (ESO) 1999
Online publication: June 6, 1999