## Abstract

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., when Pm < Pm^{cr} < 1). The threshold in the magnetic Prandtl number, Pm^{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 τc = (?k^{2})^{-1} (where ? 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.

Original language | English |
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Pages (from-to) | 732-736 |

Number of pages | 5 |

Journal | Astronomische Nachrichten |

Volume | 329 |

Issue number | 7 |

DOIs | |

State | Published - 1 Sep 2008 |

## Keywords

- Magnetic fields
- Magnetohydrodynamics (MHD)
- Turbulence

## ASJC Scopus subject areas

- Astronomy and Astrophysics
- Space and Planetary Science