Turbulent resistivity driven by the magnetorotational instability

TL;DR

This paper measures the turbulent resistivity and magnetic Prandtl number in MRI-driven turbulence using numerical simulations, finding that the Prandtl number is approximately one and robust across different conditions.

Contribution

The study provides the first detailed numerical measurement of turbulent resistivity and Prandtl number in MRI turbulence, confirming their approximate unity and robustness across simulation parameters.

Findings

Turbulent resistivity aligns with mean field theories.

Turbulent magnetic Prandtl number is around unity.

Prandtl number increases with imposed forcing.

Abstract

We measure the turbulent resistivity in the nonlinear regime of the MRI, and evaluate the turbulent magnetic Prandtl number. We perform a set of numerical simulations with the Eulerian finite volume codes Athena and Ramses in the framework of the shearing box model. We consider models including explicit dissipation coefficients and magnetic field topologies such that the net magnetic flux threading the box in both the vertical and azimuthal directions vanishes. We first demonstrate good agreement between the two codes by comparing the properties of the turbulent states in simulations having identical microscopic diffusion coefficients (viscosity and resistivity). We find the properties of the turbulence do not change when the box size is increased in the radial direction, provided it is elongated in the azimuthal direction. To measure the turbulent resistivity in the disk, we impose a fixed electromotive force on the flow and measure the amplitude of the saturated magnetic field that results. We obtain a turbulent resistivity that is in rough agreement with mean field theories like the Second Order Smoothing Approximation. The numerical value translates into a turbulent magnetic Prandtl number Pm_t of order unity. Pm_t appears to be an increasing function of the forcing we impose. It also becomes smaller as the box size is increased in the radial direction, in good agreement with previous results obtained in very large boxes. Our results are in general agreement with other recently published papers studying the same problem but using different methodology. Thus, our conclusion that Pm_t is of order unity appears robust.