Physics – Plasma Physics
Scientific paper
2011-03-11
J.Phys.A44:265501,2011
Physics
Plasma Physics
17 pages, 11 figures, accepted for publication by J. Phys. A: Math. Theor. (minor revisions)
Scientific paper
10.1088/1751-8113/44/26/265501
A topological constraint on the dynamics of a magnetic field in a flux tube arises from the fixed point indices of its field line mapping. This can explain unexpected behaviour in recent resistive-magnetohydrodynamic simulations of magnetic relaxation. Here we present the theory for a general periodic flux tube, representing, for example, a toroidal confinement device or a solar coronal loop. We show how an ideal dynamics on the side boundary of the tube implies that the sum of indices over all interior fixed points is invariant. This constraint applies to any continuous evolution inside the tube, which may be turbulent and/or dissipative. We also consider the analogous invariants obtained from periodic points (fixed points of the iterated mapping). Although there is a countably infinite family of invariants, we show that they lead to at most two independent dynamical constraints. The second constraint applies only in certain magnetic configurations. Several examples illustrate the theory.
Hornig Gunnar
Yeates Anthony R.
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