Dynamical constraints from field line topology in magnetic flux tubes

TL;DR

This paper develops a topological theory linking fixed point indices of magnetic field line mappings to dynamical constraints in flux tubes, applicable to turbulent and dissipative regimes, with implications for magnetic relaxation phenomena.

Contribution

It introduces a general theoretical framework for topological invariants in magnetic flux tubes, revealing at most two independent constraints on their dynamics.

Findings

Sum of fixed point indices is invariant during ideal evolution.

There are at most two independent topological constraints.

Examples illustrate the application of the theory.

Abstract

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.