Stretching, mixing, and tearing: Magnetic self-organization in high-resolution simulations of the turbulent dynamo in Pm>1 plasma

Turbulence in a conducting plasma can amplify seed magnetic fields in what is known as the turbulent, or small-scale, dynamo. The associated growth rate and emergent magnetic-field geometry depends sensitively on the material properties of the plasma, in particular the magnetic Prandtl number Pm. For Pm>1, the amplified magnetic field is gradually arranged into a folded structure, with direction reversals at the resistive scale and field lines curved at the larger scale of the flow. As the magnetic energy grows to come into approximate equipartition with the fluid motions, this folded structure is thought to persist. Using analytical theory and high-resolution MHD simulations with the Athena++ code, it is shown that these magnetic folds become unstable to tearing during the nonlinear stage of the dynamo for magnetic Reynolds numbers Rm≳10⁴. An Rm- and Pm-dependent tearing scale, at and below which fold disruption occurs, is theoretically predicted and found to match well the characteristic field-reversal scale measured in the simulations. This disruption increases the amount of viscous dissipation in this tearing-limited dynamo. In the saturated state, the spectral peak of the magnetic energy appears to be independent of the resistive scale, belying the customary “small-scale” moniker.