Nonhelical mean-field dynamos in a sheared turbulence

Mechanisms of nonhelical large-scale dynamos (shear-current dynamo and effect of homogeneous kinetic helicity fluctuations with zero mean) in a homogeneous turbulence with large-scale shear are discussed. We have found that the shear-current dynamo can act even in random flows with small Reynolds numbers. However, in this case mean-field dynamo requires small magnetic Prandtl numbers (i.e., ${\rm Pm} < {\rm Pm}^{\rm cr}<1$). The threshold in the magnetic Prandtl number, ${\rm Pm}^{\rm cr} = 0.24$, is determined using second order correlation approximation (or first-order smoothing approximation) for a background random flow with a scale-dependent viscous correlation time $\tau_c=(\nu k^2)^{-1}$ (where $\nu$ is the kinematic viscosity of the fluid and $k$ is the wave number). For turbulent flows with large Reynolds numbers shear-current dynamo occurs for arbitrary magnetic Prandtl numbers. This dynamo effect represents a very generic mechanism for generating large-scale magnetic fields in a broad class of astrophysical turbulent systems with large-scale shear. On the other hand, mean-field dynamo due to homogeneous kinetic helicity fluctuations alone in a sheared turbulence is not realistic for a broad class of astrophysical systems because it requires a very specific random forcing of kinetic helicity fluctuations that contains, e.g., low-frequency oscillations.