Lagrangian chaos in an ABC--forced nonlinear dynamo

TL;DR

This study investigates how magnetic energy influences fluid mixing in a magnetohydrodynamic dynamo, revealing that increased magnetic energy reduces chaotic mixing, with implications for understanding solar magnetic phenomena.

Contribution

It introduces a detailed analysis of Lagrangian coherent structures in dynamo simulations, linking magnetic energy levels to flow mixing properties, which is a novel approach in dynamo research.

Findings

Lagrangian coherent structures delineate transport barriers in the flow.

Magnetic energy inversely affects flow stirring and mixing.

Flow mixing properties decay linearly with magnetic energy.

Abstract

The Lagrangian properties of the velocity field in a magnetized fluid are studied using three-dimensional simulations of a helical magnetohydrodynamic dynamo. We compute the attracting and repelling Lagrangian coherent structures, which are dynamic lines and surfaces in the velocity field that delineate particle transport in flows with chaotic streamlines and act as transport barriers. Two dynamo regimes are explored, one with a robust coherent mean magnetic field and one with intermittent bursts of magnetic energy. The Lagrangian coherent structures and the statistics of finite--time Lyapunov exponents indicate that the stirring/mixing properties of the velocity field decay as a linear function of the magnetic energy. The relevance of this study for the solar dynamo problem is discussed.