Magnetic helicity dissipation and production in an ideal MHD code

TL;DR

This study compares ideal MHD simulations of turbulent helical dynamos with DNS, revealing limitations in large-scale field generation and helicity behavior at higher resolutions, impacting astrophysical dynamo modeling.

Contribution

It demonstrates the differences between ideal MHD and DNS in magnetic field and helicity evolution, highlighting resolution-dependent discrepancies in dynamo simulations.

Findings

FLASH reproduces DNS large-scale fields at low resolution

Higher resolution ideal MHD fails to generate large-scale magnetic fields

Small-scale current helicity is underestimated in ideal MHD

Abstract

We study a turbulent helical dynamo in a periodic domain by solving the ideal magnetohydrodynamic (MHD) equations with the FLASH code using the divergence-cleaning eight-wave method and compare our results with direct numerical simulations (DNS) using the Pencil Code. At low resolution, FLASH reproduces the DNS results qualitatively by developing the large-scale magnetic field expected from DNS, but at higher resolution, no large-scale magnetic field is obtained. In all those cases in which a large-scale magnetic field is generated, the ideal MHD results yield too little power at small scales. As a consequence, the small-scale current helicity is too small compared with that of the DNS. The resulting net current helicity has then always the wrong sign, and its statistical average also does not approach zero at late times, as expected from the DNS. Our results have implications for astrophysical dynamo simulations of stellar and galactic magnetism using ideal MHD codes.