Large-scale dynamics of magnetic helicity

TL;DR

This paper studies the large-scale behavior of magnetic helicity in MHD turbulence, revealing a nonlocal inverse cascade directly from forcing scales to the largest magnetic scales, with implications for understanding magnetic field dynamics.

Contribution

It demonstrates the nonlocal inverse cascade of magnetic helicity in MHD turbulence and links the statistical properties of intermediate scales to absolute equilibrium theory.

Findings

Nonlocal inverse cascade of magnetic helicity from forcing to large scales

No significant transfer of helicity or energy to intermediate scales

Intermediate scale properties align with zero flux solutions of equilibrium theory

Abstract

In this paper we investigate the dynamics of magnetic helicity in magnetohydrodynamic (MHD) turbulent flows focusing at scales larger than the forcing scale. Our results show a nonlocal inverse cascade of magnetic helicity, which occurs directly from the forcing scale into the largest scales of the magnetic field. We also observe that no magnetic helicity and no energy is transferred to an intermediate range of scales sufficiently smaller than the container size and larger than the forcing scale. Thus, the statistical properties of this range of scales, which increases with scale separation, is shown to be described to a large extent by the zero flux solutions of the absolute statistical equilibrium theory exhibited by the truncated ideal MHD equations.