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On the problem of large-scale magnetic field generation in rotating compressible convection

Lookup NU author(s): Dr Benjamin Favier, Dr Paul Bushby

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Abstract

Mean-field dynamo theory suggests that turbulent convection in a rotating layer of electrically-conducting fluid produces a significant alpha-effect, which is one of the key ingredients in any mean-field dynamo model. Provided that this alpha-effect operates more efficiently than (turbulent) magnetic diffusion, such a system should be capable of sustaining a large-scale dynamo. However, in the Boussinesq model that was considered byCattaneo & Hughes (2006) the dynamo produced small-scale, intermittent magnetic fields with no significant large-scale component. In this paper, we consider the compressible analogue of the rotating convective layer that was considered by Cattaneo & Hughes (2006). Varying the horizontal scale of the computational domain, we investigate the dependence of the dynamo upon the rotation rate. Our simulations indicate that these turbulent compressible flows can drive a small-scale dynamo but, even when the layer is rotating very rapidly(with a mid-layer Taylor number of Ta=10^8), we find no evidence for the generation of a significant large-scale component of the magnetic field on a dynamical timescale. Like Cattaneo & Hughes (2006), we measure a negligible (time-averaged) alpha-effect when a uniform horizontal magnetic field is imposed across the computational domain. Although the total horizontal magnetic flux is a conserved quantity in these simulations, the (depth-dependent) horizontally-averaged magnetic field always exhibits strong fluctuations. If these fluctuations are artificially suppressed within the code, we measure a significant mean electromotive force that is comparable to that found in related calculations in which the alpha-effect is measured using the test-field method, even though we observe no large-scale dynamo action.


Publication metadata

Author(s): Favier B, Bushby PJ

Publication type: Article

Publication status: Published

Journal: Journal of Fluid Mechanics

Year: 2013

Volume: 723

Pages: 529-555

Print publication date: 15/04/2013

Date deposited: 27/03/2013

ISSN (print): 0022-1120

ISSN (electronic): 1469-7645

Publisher: Cambridge University Press

URL: http://dx.doi.org/10.1017/jfm.2013.132

DOI: 10.1017/jfm.2013.132


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