Inverse transfer of magnetic helicity in direct numerical simulations of compressible isothermal turbulence: helical transfers

TL;DR

This study investigates how magnetic helicity transfers across scales in compressible MHD turbulence, revealing distinct local and non-local inverse transfer mechanisms influenced by helical components and velocity scales.

Contribution

It provides a detailed analysis of helical transfer processes in compressible MHD turbulence using Fourier shell-to-shell transfer analysis, highlighting the roles of different helical components and velocity scales.

Findings

Identification of local and non-local inverse transfer of magnetic helicity.

Role of helical components and velocity scales in transfer processes.

Influence of triad helical geometric factor on transfer strength.

Abstract

The role of the different helical components of the magnetic and velocity fields in the inverse spectral transfer of magnetic helicity is investigated through Fourier shell-to-shell transfer analysis. Both magnetic helicity and energetic transfer analysis are performed on chosen data from direct numerical simulations of homogeneous isothermal compressible magnetohydrodynamic turbulence, subject to both a large-scale mechanical forcing and a small-scale helical electromotive driving. The root mean square Mach number of the hydrodynamic turbulent steady-state taken as initial condition varies from 0.1 to about 11. Three physical phenomena can be distinguished in the general picture of the spectral transfer of magnetic helicity towards larger spatial scales: local inverse transfer, non-local inverse transfer and local direct transfer. A shell decomposition allows to associate these three phenomena with clearly distinct velocity scales while the helical decomposition allows to establish the role of the different helical components and the compressive part of the velocity field on the different transfer processes. The locality and relative strength of the different helical contributions are mainly determined by the triad helical geometric factor.