Magnetic helicity fluxes in dynamos from rotating inhomogeneous turbulence

TL;DR

This study uses numerical simulations to explore how shear and turbulence influence magnetic helicity fluxes and dynamo efficiency, revealing that shear can mitigate quenching effects and sustain magnetic fields.

Contribution

It demonstrates the role of shear-induced hemispheric magnetic helicity fluxes in large-scale dynamos, showing they can alleviate catastrophic quenching and are nearly Reynolds number independent.

Findings

Hemispheric magnetic helicity fluxes are nearly Reynolds number independent.

Shear can cause small-scale fluxes to overcompensate large-scale fluxes.

Mean magnetic field remains stable with increasing Reynolds number in shearing dynamos.

Abstract

We analyze direct numerical simulations of large-scale dynamos in inhomogeneous nonhelically driven rotating turbulence with and without shear. The forcing is modulated so that the turbulent intensity peaks in the middle of the computational domain and drops to nearly zero at the two ends above and below the midplane. A large-scale dynamo is driven by an $\alpha$ effect of opposite signs in the two hemispheres. In the presence of shear, the hemispheric magnetic helicity flux from small-scale fields becomes important and can even overcompensate for the magnetic helicity transferred by the $\alpha$ effect between large and small scales. This effect has not previously been observed in nonshearing simulations. Our numerical simulations show that the hemispheric magnetic helicity fluxes are nearly independent of the magnetic Reynolds number, but those between large and small scales, and the consequent dynamo effect, are still found to decrease with increasing Reynolds number -- just like in nonshearing dynamos. However, in contrast to nonshearing dynamos, where the generated mean magnetic field declines with increasing magnetic Reynolds number, it is now found to remain independent of it. This suggests that catastrophic dynamo quenching is alleviated by the shear-induced hemispheric small-scale magnetic helicity fluxes that can even overcompensate the fluxes between large and small scales and thereby cause resistive contributions.