Quantifying reconnective activity in braided vector fields.

Abstract

We introduce a technique for evaluating the changing connectivity of a vector field whose integral curves (field lines) form tangled tubular bundles. Applications of such fields include magnetic flux ropes, relativistic plasma jets, stirred two-dimensional fluids, superfluid vortices, and polymer networks. The technique is based on maps of the field line winding—the average entanglement of a given field line with all other field lines. Previously this had been developed for divergence-free vector fields. By extending some previous theoretical results, we show how it can be applied to any vector field that forms a tubular bundle. We demonstrate the efficacy of this technique on data from laboratory plasma experiments with two interacting magnetic flux ropes. Performed in the UCLA Large Plasma Device, the plasma's magnetic field structure is too complex to identify a single dominant current sheet as an expected site of magnetic reconnection. Previously, this complex structure had restricted the ability to analyze the evolving magnetic connectivity, but this is no such restriction to our method. We demonstrate that the plasma establishes a periodically oscillating cycle of magnetic field structure variation which, while triggered by an ideal instability, is dominated by magnetic reconnection. This reconnection leads to periodically varying coherence of a merged central flux rope, a conclusion supported by analysis of the writhing structure of the magnetic field.