# Shearing-box simulations of MRI-driven turbulence in weakly collisional accretion discs

## TL;DR

This study investigates MRI-driven turbulence in weakly collisional accretion disks using shearing-box simulations with anisotropic viscosity, revealing that turbulence levels are similar to MHD results but flow structures and heating are significantly affected.

## Contribution

It is the first systematic shearing-box study including anisotropic (Braginskii) viscosity in MRI turbulence, exploring a broad parameter space and its effects on turbulence and flow structure.

## Key findings

- Turbulence levels are similar to ideal MHD despite large anisotropic viscosities.
- Anisotropic viscosity alters flow structure and redistributes turbulence.
- At high Reynolds numbers, anisotropic transport becomes negligible, but dominates plasma heating.

## Abstract

We present a systematic shearing-box investigation of MRI-driven turbulence in a weakly collisional plasma by including the effects of an anisotropic pressure stress, i.e. anisotropic (Braginskii) viscosity. We constrain the pressure anisotropy ($\Delta p$) to lie within the stability bounds that would be otherwise imposed by kinetic microinstabilities. We explore a broad region of parameter space by considering different Reynolds numbers and magnetic-field configurations, including net vertical flux, net toroidal-vertical flux and zero net flux. Remarkably, we find that the level of turbulence and angular-momentum transport are not greatly affected by large anisotropic viscosities: the Maxwell and Reynolds stresses do not differ much from the MHD result. Angular-momentum transport in Braginskii MHD still depends strongly on isotropic dissipation, e.g., the isotropic magnetic Prandtl number, even when the anisotropic viscosity is orders of magnitude larger than the isotropic diffusivities. Braginskii viscosity nevertheless changes the flow structure, rearranging the turbulence to largely counter the parallel rate of strain from the background shear. We also show that the volume-averaged pressure anisotropy and anisotropic viscous transport decrease with increasing isotropic Reynolds number (${\rm Re}$); e.g., in simulations with net vertical field, the ratio of anisotropic to Maxwell stress ($\alpha_{\rm A} / \alpha_{\rm M}$) decreases from $\sim 0.5$ to $\sim 0.1$ as we move from ${\rm Re} \sim 10^3$ to ${\rm Re} \sim 10^4$. Anisotropic transport may thus become negligible at high ${\rm Re}$. Anisotropic viscosity nevertheless becomes the dominant source of heating at large ${\rm Re}$, accounting for $\gtrsim 50 \%$ of the plasma heating. We conclude by briefly discussing the implications of our results for RIAFs onto black holes.