Shear-flow-driven and reconnection-controlled dynamo*

Turbulence tangles magnetic fields, generating strong small-scale currents. Yet large-scale magnetic fields are observed in astrophysics. To understand such and to model transport of energy and momentum, MHD Kelvin-Helmholtz-instability (KHI)-driven, quasi-stationary two- and three-dimensional (3D) turbulence is studied here with a mean flow forced toward its initial profile. The resulting turbulence is analyzed using the large-scale stable and unstable modes of the system.

In 2D, the magnetic field is rapidly folded by the KHI-driven turbulence when stable modes are removed; but when retained, large-scale structures emerge. While in both cases kinetic and magnetic energies cascade to small scales, the cascading energy fluxes are reduced by orders of magnitude due to the stable modes.^1,2,3

In 3D, large-scale magnetic fields are amplified by a KHI-driven dynamo, with the aid of the stable modes. The fields generated have field line reversals. The accompanying current layers then undergo disruption but are regenerated by turbulence. The field-reversal scale is compared against an analytical theory by varying the (magnetic) Reynolds number, which suggests that the KHI-driven dynamo can trigger tearing instability, an effect seen also in reconnection-driven turbulence.