Screw dynamo and the generation of nonaxisymmetric magnetic fields

A mechanism is presented here for the amplification of large-scale nonaxisymmetric magnetic fields as a manifestation of the dynamo effect. We generalize a result on restrictions of dynamo actions due to laminar flow originally derived by Zeldovich, Ruzmaikin, and Sokolov [Magnetic Fields in Astrophysics (Gordon and Breach, New York, 1983)]. We show how a screwlike motion having $\phi$ and $z$ components of velocity can help to grow a magnetic field. This model postulates a large-scale flow having $\phi$ and $z$ components with radial dependences (helical flow). Shear in the radial field, because of a near-flux-freezing condition, causes amplification of the $\phi$ component of the magnetic field. The radial and axial components grow due to the presence of turbulent diffusion. The shear in the large-scale flow induces an indefinite growth of magnetic field without the $\alpha$ effect; nevertheless, turbulent diffusion forms an important part in the overall mechanism.