MRI turbulence in accretion discs at large magnetic Prandtl numbers

Abstract

The effect of large magnetic Prandtl number $\text{Pm}$ (the ratio of viscosity to resistivity) on the turbulent transport and energetics of the magnetorotational instability (MRI) is poorly understood, despite the realization of this regime in astrophysical environments as disparate as discs from binary neutron star mergers, the inner regions of low mass X-ray binaries and active galactic nuclei, and the interiors of protoneutron stars. We investigate the MRI dynamo and associated turbulence in the regime $\text{Pm}>1$ by carrying out fully compressible, 3D MHD shearing box simulations using the finite-volume code \textsc{PLUTO}, focusing mostly on the case of Keplerian shear relevant to accretion discs. We find that when the magnetic Reynolds number is kept fixed, the turbulent transport (as parameterized by $α$, the ratio of stress to thermal pressure) scales with the magnetic Prandtl number as $α\sim \text{Pm}^δ$, with $δ\sim 0.5-0.7$ up to $\text{Pm} \sim 128$. However, this scaling weakens as the magnetic Reynolds number is increased. Importantly, compared to previous studies, we find a new effect at very large $\text{Pm}$ -- the turbulent energy and stress begin to plateau, no longer depending on ${\rm Pm}$. To understand these results we have carried out a detailed analysis of the turbulent dynamics in Fourier space, focusing on the effect of increasing $\text{Pm}$ on the transverse cascade -- a key non-linear process induced by the disc shear flow that is responsible for the sustenance of MRI turbulence. Finally, we find that $α$-$\text{Pm}$ scaling is sensitive to the box vertical-to-radial aspect ratio, as well as to the background shear.