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Turbulent relaxation of a confined magnetofluid to a force-free state

Published online by Cambridge University Press:  13 March 2009

Jill P. Dahlburg
Affiliation:
U.S. Naval Research Laboratory, Washington, DC 20375
David Montgomery
Affiliation:
Dartmouth College, Hanover, NH 03755
Gary D. Doolen
Affiliation:
Los Alamos National Laboratory, University of California, Los Alamos, NM 87545
Leaf Turner
Affiliation:
Los Alamos National Laboratory, University of California, Los Alamos, NM 87545

Abstract

Three-dimensional, pseudo-spectral computation is used to follow the evolution of a resistive, incompressible magnetofluid. The magnetofluid is confined by rigid, free-slip, perfectly-conducting square boundaries in the x, y directions (‘poloidal’ boundaries), and periodic boundary conditions are assumed in the z direction (‘toroidal’ direction). A constant, uniform d.c. magnetic field B0 is assumed in the z direction and a non-uniform current density j flows along it initially. Starting from a non-equilibrium hollow current profile, the evolution is followed for several tens of Alfvén transit times. Considerable small-scale turbulence develops, which causes energy to decay more rapidly than magnetic helicity. The average toroidal magnetic field at the (x, y) boundary reverses sign spontaneously. The near spatial constancy of the ratio jB/(jB) ≡ cos θ, in the relaxed state at late times, suggests that the state is nearly force-free. However, the ratio j. B/B2 ≡ α is considerably less uniform than is cos θ suggesting more residual disorder than a pure minimum-energy state would display.

Type
Research Article
Copyright
Copyright © Cambridge University Press 1987

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