Local outflows from turbulent accretion disks

TL;DR

This study uses numerical simulations to explore how weak vertical magnetic fields influence turbulence and outflows in accretion disks, revealing the complex interplay and robustness of flow morphology, with implications for disk mass-loss estimates.

Contribution

It demonstrates for the first time that accretion disks can exhibit both MRI-driven turbulence and magneto-centrifugal outflows simultaneously, highlighting the impact of box size on outflow mass-loss rates.

Findings

Outflows are driven by MHD turbulence in the disk.

Outflow mass-loss rates are highly sensitive to simulation box size.

Flow morphology shows invariants similar to Blandford & Payne mechanism.

Abstract

The aim of this paper is to investigate the properties of accretion disks threaded by a weak vertical magnetic field, with a particular focus on the interplay between MHD turbulence driven by the magnetorotational instability (MRI) and outflows that might be launched from the disk. For that purpose, we use a set of numerical simulations performed with the MHD code RAMSES in the framework of the shearing box model. We concentrate on the case of a rather weak vertical magnetic field such that the initial ratio beta0 of the thermal and magnetic pressures in the disk midplane equals 10^4. As reported recently, we find that MHD turbulence drives an efficient outflow out of the computational box. We demonstrate a strong sensitivity of that result to the box size: enlargements in the radial and vertical directions lead to a reduction of up to an order of magnitude in the mass-loss rate. Such a dependence prevents any realistic estimates of disk mass-loss rates being derived using shearing-box simulations. We find however that the flow morphology is robust and independent of the numerical details of the simulations. Its properties display some features and approximate invariants that are reminiscent of the Blandford & Payne launching mechanism, but differences exist. For the magnetic field strength considered in this paper, we also find that angular momentum transport is most likely dominated by MHD turbulence, the saturation of which scales with the magnetic Prandtl number, the ratio of viscosity and resistivity, in a way that is in good agreement with expectations based on unstratified simulations. This paper thus demonstrates for the first time that accretion disks can simultaneously exhibit MRI-driven MHD turbulence along with magneto-centrifugally accelerated outflows.