The Nature of Turbulence in the Outer Regions of Protoplanetary Disks

TL;DR

This study uses local simulations to analyze turbulence and angular momentum transport in the outer regions of protoplanetary disks, highlighting the impact of FUV ionization on MRI activity and magnetic flux transport.

Contribution

It provides new insights into how FUV ionization influences MRI suppression and magnetic flux dynamics in the outer disk regions.

Findings

MRI can be sustained at the mid-plane with weak magnetic fields without FUV.

FUV layers transport magnetic flux that suppresses MRI at the mid-plane.

Angular momentum transport is dominated by laminar magnetic fields in FUV-ionized regions.

Abstract

We carry out a series of local, shearing box simulations of the outer regions of protoplanetary disks, where ambipolar diffusion is important due to low ionization levels, to better characterize the nature of turbulence and angular momentum transport in these disks. These simulations are divided into two groups, one with far ultraviolet (FUV) ionization included, and one without FUV. In both cases, we explore a large range in diffusivity values. We find that in the simulations without FUV, the properties of the turbulence are similar to the unstratified simulations of Bai & Stone (2011); for a given diffusivity, the MRI can still be present so long as the magnetic field is sufficiently weak. Furthermore, the dynamics of the mid-plane in these simulations are primarily controlled by the MRI. In the FUV simulations on the other hand, the MRI-active FUV layers transport strong toroidal magnetic flux to the mid-plane, which shuts off the MRI. Instead, angular momentum transport at the mid-plane is dominated by laminar magnetic fields, resulting in lower levels of turbulent Maxwell stress compared to the no-FUV simulations. Finally, we perform a temporal correlation analysis on the FUV simulations, confirming our result that the dynamics in the mid-plane region is strongly controlled by the FUV-ionized layers.