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Generation of coherent magnetic fields in sheared inhomogeneous turbulence: No need for rotation?

Leprovost, N. and Kim, E.J. (2011) Generation of coherent magnetic fields in sheared inhomogeneous turbulence: No need for rotation? Physics of Plasmas, 18 (2). Art no.022307 . ISSN 1070-664X

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Abstract

Coherent magnetic fields are often believed to be generated by the combination of stretching by differential rotation and turbulent amplification of magnetic field, via the so-called alpha effect. The latter is known to exist in helical turbulence, which is envisioned to arise due to both rotation and convection in solar-type stars. In this contribution, a turbulent flow driven by a nonhelical inhomogeneous forcing and its kinematic dynamo action are studied for a uniform magnetic field in the background of a linear shear flow. By using a quasilinear analysis and a nonperturbative method utilizing a time-dependent wave number, turbulence property and electromotive force are computed for arbitrary shear strength. Due to the large-scale shear flow, the turbulence is highly anisotropic, as a consequence, so is the electromotive force. The latter is found to exist even without rotation due to the combined effect of shear flow and inhomogeneous forcing, containing not only the alpha effect but also magnetic pumping (the gamma effect representing a transport of magnetic flux by turbulence). Specifically, without shear, only the magnetic pumping exists, aligned with the direction of inhomogeneity. For a weak but nonzero shear, the combined effects of shear and inhomogeneous forcing modify the structure of the magnetic pumping when the inhomogeneity is in the plane of the shear flow, the magnetic pumping becoming bidimensional in that plane. It also induces an alpha tensor which has nondiagonal components. When the inhomogeneity is perpendicular to the plane of the shear flow, the alpha effect has three nonzero diagonal components and one off-diagonal component. However, for a sufficiently strong shear, the gamma and alpha effects are suppressed due to shear stabilization which damps turbulence. A simplified dynamo model is then proposed where a large-scale dynamo arises due to the combined effect of shear flow and inhomogeneous forcing. In particular, the growth of a large-scale axisymmetric magnetic field is demonstrated in case of an inhomogeneity which is perpendicular to the plane of the shear flow. Interesting implications of these results for the structure of magnetic fields in star with slow rotation are discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3551700]

Item Type: Article
Copyright, Publisher and Additional Information: © 2011 American Institute of Physics. This is an author produced version of a paper subsequently published in Physics of Plasmas. Uploaded in accordance with the publisher's self-archiving policy.
Keywords: Solar Tachocline; MHD Turbulence; Dynamo Models; Convection; Transport; Helicity
Academic Units: The University of Sheffield > Faculty of Science (Sheffield) > School of Mathematics and Statistics (Sheffield)
Depositing User: Miss Anthea Tucker
Date Deposited: 25 Mar 2011 11:57
Last Modified: 08 Feb 2013 17:31
Published Version: http://dx.doi.org/10.1063/1.3551700
Status: Published
Publisher: American Institute of Physics
Refereed: Yes
Identification Number: 10.1063/1.3551700
URI: http://eprints.whiterose.ac.uk/id/eprint/42919

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