Magnetic helicity fluxes in an alpha-squared dynamo embedded in a halo

TL;DR

This study uses simulations to investigate magnetic helicity fluxes in an alpha-squared dynamo, revealing that small-scale magnetic helicity reaches a steady state and fluxes are turbulently diffusive, with boundary conditions affecting magnetic energy.

Contribution

It demonstrates that magnetic helicity fluxes in an alpha-squared dynamo are turbulently diffusive and explores boundary effects on magnetic field saturation.

Findings

Small-scale magnetic helicity reaches a steady state.

Magnetic helicity fluxes are turbulently diffusive.

Boundary conditions influence the saturated magnetic energy.

Abstract

We present the results of simulations of forced turbulence in a slab where the mean kinetic helicity has a maximum near the mid-plane, generating gradients of magnetic helicity of both large and small-scale fields. We also study systems that have poorly conducting buffer zones away from the midplane in order to assess the effects of boundaries. The dynamical alpha quenching phenomenology requires that the magnetic helicity in the small-scale fields approaches a nearly static, gauge independent state. To stress-test this steady state condition we choose a system with a uniform sign of kinetic helicity, so that the total magnetic helicity can reach a steady state value only through fluxes through the boundary, which are themselves suppressed by the velocity boundary conditions. Even with such a set up, the small-scale magnetic helicity is found to reach a steady state. In agreement with earlier work, the magnetic helicity fluxes of small-scale fields are found to be turbulently diffusive. By comparing results with and without halos, we show that artificial constraints on magnetic helicity at the boundary do not have a significant impact on the evolution of the magnetic helicity, except that "softer" (halo) boundary conditions give a lower energy of the saturated mean magnetic field.