Not Much Helicity is Needed to Drive Large Scale Dynamos

TL;DR

This paper investigates the minimal fractional kinetic helicity needed to drive large scale dynamos, revealing that very small helicity fractions can significantly influence magnetic field amplification in turbulent astrophysical environments.

Contribution

The study quantifies how the critical helicity fraction decreases with increased scale separation, supported by direct numerical simulations and a developed theoretical framework.

Findings

Critical helicity fraction decreases with scale separation.

For scale separation >8, helicity fraction <3%.

Small helicity fractions can drive large scale magnetic fields.

Abstract

Understanding the in situ amplification of large scale magnetic fields in turbulent astrophysical rotators has been a core subject of dynamo theory. When turbulent velocities are helical, large scale dynamos that substantially amplify fields on scales that exceed the turbulent forcing scale arise, but the minimum sufficient fractional kinetic helicity f_h,C has not been previously well quantified. Using direct numerical simulations for a simple helical dynamo, we show that f_h,C decreases as the ratio of forcing to large scale wave numbers k_F/k_min increases. From the condition that a large scale helical dynamo must overcome the backreaction from any non-helical field on the large scales, we develop a theory that can explain the simulations. For k_F/k_min>8 we find f_h,C< 3%, implying that very small helicity fractions strongly influence magnetic spectra for even moderate scale separation.