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Title:
The helicity constraint in turbulent dynamos with shear
Authors:
Brandenburg, Axel; Bigazzi, Alberto; Subramanian, Kandaswamy
Affiliation:
AA(NORDITA, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark; Department of Mathematics, University of Newcastle upon Tyne, NE1 7RU), AB(Department of Mathematics, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy), AC(National Centre for Radio Astrophysics - TIFR, Poona University Campus, Ganeshkhind, Pune 411 007, India)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 325, Issue 2, pp. 685-692. (MNRAS Homepage)
Publication Date:
08/2001
Origin:
MNRAS
Astronomy Keywords:
MAGNETIC FIELDS, MHD, TURBULENCE
DOI:
10.1046/j.1365-8711.2001.04468.x
Bibliographic Code:
2001MNRAS.325..685B

Abstract

The evolution of magnetic fields is studied using simulations of forced helical turbulence with strong imposed shear. After some initial exponential growth, the magnetic field develops a large-scale travelling wave pattern. The resulting field structure possesses magnetic helicity, which is conserved in a periodic box by the ideal magnetohydrodynamics equations and can hence only change on a resistive time-scale. This strongly constrains the growth time of the large-scale magnetic field, but less strongly constrains the length of the cycle period. Comparing this with the case without shear, the time-scale for large-scale field amplification is shortened by a factor Q, which depends on the relative importance of shear and helical turbulence, and which also controls the ratio of toroidal to poloidal field. The results of the simulations can be reproduced qualitatively and quantitatively with a mean-field αΩ-dynamo model with alpha-effect and turbulent magnetic diffusivity coefficients that are less strongly quenched than in the corresponding α2-dynamo.

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