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Alpha effect due to buoyancy instability of a magnetic layer
Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 534, A 46- p.Article in journal (Refereed) Published
Abstract [en]

Context. A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation. Aims: We calculate the α effect due to helical motions caused by an unstable magnetic layer in a rotating density-stratified system with angular velocity Ω making an angle θ with the vertical. We also study the dependence of the α effect on θ and the strength of the initial magnetic field. Methods: We carry out three-dimensional hydromagnetic simulations in Cartesian geometry. A turbulent electromotive force (EMF) due to the correlations of the small scale velocity and magnetic field is generated. We use the test-field method to calculate the transport coefficients of the inhomogeneous turbulence produced by the layer. Results: We show that the growth rate of the instability and the twist of the magnetic field vary monotonically with the ratio of thermal conductivity to magnetic diffusivity. The resulting α effect is non-uniform and increases with the strength of the initial magnetic field. It is thus an example of an "anti-quenched" α effect. The α effect is also nonlocal, i.e. scale dependent, requiring around 8-16 Fourier modes to reconstruct the actual mean EMF based on the actual mean field.

Place, publisher, year, edition, pages
2011. Vol. 534, A 46- p.
Keyword [en]
magnetohydrodynamics (MHD), magnetic fields, turbulence, dynamo, instabilities
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-70036DOI: 10.1051/0004-6361/201016108ISI: 000296554800016OAI: diva2:478727
authorCount :4Available from: 2012-01-16 Created: 2012-01-16 Last updated: 2012-01-25Bibliographically approved

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Brandenburg, Axel
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