Mechanism for Sequestering Magnetic Energy at Large Scales in Shear-Flow Turbulence

TL;DR

This paper reveals that in magnetohydrodynamic turbulence, large-scale shear flows can unexpectedly sequester magnetic energy at large scales, challenging the traditional view that they only generate small-scale magnetic fields.

Contribution

It demonstrates that stable eigenmodes in shear flows play a crucial role in large-scale magnetic energy sequestration, which was previously overlooked.

Findings

Large-scale shear flows can sequester magnetic energy at large scales.

Stable eigenmodes counteract the expected small-scale intensification.

Analysis shows significant dynamics beyond linear instability predictions.

Abstract

Straining of magnetic fields by large-scale shear flow, generally assumed to lead to intensification and generation of small scales, is re-examined in light of the persistent observation of large-scale magnetic fields in astrophysics. It is shown that, in magnetohydrodynamic turbulence, unstable shear flows have the unexpected effect of sequestering magnetic energy at large scales, due to counteracting straining motion of nonlinearly excited large-scale stable eigenmodes. This effect is quantified via dissipation rates, energy transfer rates, and visualizations of magnetic field evolution by artificially removing the stable modes. These analyses show that predictions based upon physics of the linear instability alone miss substantial dynamics, including those of magnetic fluctuations.