Simulations of the anisotropic kinetic and magnetic alpha effects

Abstract

The validity of a closure called the minimal tau approximation (MTA), is tested in the context of dynamo theory, wherein triple correlations are assumed to provide relaxation of the turbulent electromotive force. Under MTA, the alpha effect in mean field dynamo theory becomes proportional to a relaxation time scale multiplied by the difference between kinetic and current helicities. It is shown that the value of the relaxation time is positive and, in units of the turnover time at the forcing wavenumber, it is of the order of unity. It is quenched by the magnetic field -- roughly independently of the magnetic Reynolds number. However, this independence becomes uncertain at large magnetic Reynolds number. Kinetic and current helicities are shown to be dominated by large scale properties of the flow.

6 pages, 6 figures, accepted by Astron. Nachr

References in corpus (3)

• The case for a distributed solar dynamo shaped by near-surface shear
• Minimal tau approximation and simulations of the alpha effect
• Kinetic and magnetic alpha effects in nonlinear dynamo theory

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• Systematics of the magnetic-Prandtl-number dependence of homogeneous, isotropic magnetohydrodynamic turbulence
• Advances in theory and simulations of large-scale dynamos
• Mean-field dynamo in a turbulence with shear and kinetic helicity fluctuations
• Turbulent processes and mean-field dynamo
• The critical role of magnetic helicity in astrophysical large-scale dynamos
• Effects of turbulence, resistivity and boundary conditions on helicoidal flow collimation: consequences for the Von-Kármán-Sodium dynamo experiment