Open and closed boundaries in large-scale convective dynamos

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J. Käpylä , P , Mantere , M & Brandenburg , A 2010 , ' Open and closed boundaries in large-scale convective dynamos ' , Astronomy & Astrophysics , vol. 518 , pp. A22 .

Title: Open and closed boundaries in large-scale convective dynamos
Author: J. Käpylä, P.; Mantere, Maarit; Brandenburg, A.
Contributor organization: Department of Physics
Date: 2010
Language: eng
Number of pages: 9
Belongs to series: Astronomy & Astrophysics
ISSN: 0004-6361
Abstract: Earlier work has suggested that large-scale dynamos can reach and maintain equipartition field strengths on a dynamical time scale only if magnetic helicity of the fluctuating field can be shed from the domain through open boundaries. To test this scenario in convection-driven dynamos by comparing results for open and closed boundary conditions. Three-dimensional numerical simulations of turbulent compressible convection with shear and rotation are used to study the effects of boundary conditions on the excitation and saturation level of large-scale dynamos. Open (vertical field) and closed (perfect conductor) boundary conditions are used for the magnetic field. The contours of shear are vertical, crossing the outer surface, and are thus ideally suited for driving a shear-induced magnetic helicity flux. We find that for given shear and rotation rate, the growth rate of the magnetic field is larger if open boundary conditions are used. The growth rate first increases for small magnetic Reynolds number, Rm, but then levels off at an approximately constant value for intermediate values of Rm. For large enough Rm, a small-scale dynamo is excited and the growth rate in this regime increases proportional to Rm^(1/2). In the nonlinear regime, the saturation level of the energy of the mean magnetic field is independent of Rm when open boundaries are used. In the case of perfect conductor boundaries, the saturation level first increases as a function of Rm, but then decreases proportional to Rm^(-1) for Rm > 30, indicative of catastrophic quenching. These results suggest that the shear-induced magnetic helicity flux is efficient in alleviating catastrophic quenching when open boundaries are used. The horizontally averaged mean field is still weakly decreasing as a function of Rm even for open boundaries.
Description: 9 pages, 7 figures, submitted to Astron. Astrophys
Subject: astro-ph.SR
115 Astronomy, Space science
Peer reviewed: Yes
Usage restriction: restrictedAccess

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