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Issue A&A
Volume 472, Number 3, September IV 2007
Page(s) 715 - 726
Section Astrophysical processes
DOI http://dx.doi.org/10.1051/0004-6361:20065087



A&A 472, 715-726 (2007)
DOI: 10.1051/0004-6361:20065087

Nonlinear MHD dynamo operating at equipartition

V. Archontis1, S. B. F. Dorch2, 3, and Å. Nordlund2

1  School of Mathematics and Statistics, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, UK
    e-mail: vasilis@ll.iac.es
2  The Niels Bohr Institute, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
3  Copenhagen University Library, The Royal Library, Nørre allé 49, 2200 Copenhagen N, Denmark

(Received 24 February 2006 / Accepted 2 July 2007)

Abstract
Context.We present results from non linear MHD dynamo experiments with a three-dimensional steady and smooth flow that drives fast dynamo action in the kinematic regime. In the saturation regime, the system yields strong magnetic fields, which undergo transitions between an energy-equipartition and a turbulent state. The generation and evolution of such strong magnetic fields is relevant for the understanding of dynamo action that occurs in stars and other astrophysical objects.
Aims.We study the mode of operation of this dynamo, in the linear and non-linear saturation regimes. We also consider the effect of varying the magnetic and fluid Reymolds number on the non-linear behaviour of the system.
Methods.We perform three-dimensional non-linear MHD simulations and visualization using a high resolution numerical scheme.
Results.We find that this dynamo has a high growth rate in the linear regime, and that it can saturate at a level significantly higher than intermittent turbulent dynamos, namely at energy equipartition, for high values of the magnetic and fluid Reynolds numbers. The equipartition solution however does not remain time-independent during the simulation but exhibits a much more intricate behaviour than previously thought. There are periods in time where the solution is smooth and close to energy-equipartition and others where it becomes turbulent. Similarities and differences in the way the magnetic field is amplified and sustained for experiments with varying Reynolds numbers are discussed.
Conclusions.Strong magnetic fields, in near equipartition, can be generated also by a non-turbulent dynamo. A striking result is that the saturation state of this dynamo reveals interesting transitions between turbulent and laminar states.


Key words: magnetic fields -- magnetohydrodynamics (MHD) -- plasmas -- turbulence



© ESO 2007


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