Turbulence Can Persist in the Inner Regions of Weakly-Ionized Planet Forming Disks

TL;DR

This study investigates how non-ideal MHD effects like Ohmic diffusion, ambipolar diffusion, and the Hall effect influence turbulence in weakly-ionized protoplanetary disks, revealing the potential for turbulence persistence via the ambipolar diffusion shear instability.

Contribution

First demonstration of the ambipolar diffusion shear instability operating non-linearly in protoplanetary disks, highlighting its role in sustaining turbulence despite low ionization.

Findings

Hall effect increases magnetic field saturation without driving turbulence.

Ambipolar diffusion can lead to the ambipolar diffusion shear instability (ADSI).

MRI may still operate in regions with weak magnetic fields and flux concentrations.

Abstract

Identifying the mechanisms responsible for angular momentum transport in protoplanetary disks, and the extent to which those mechanisms produce turbulence, is a crucial problem in understanding planet formation. The bulk of the gas in protoplanetary disks is weakly ionized, which leads to the emergence of three non-ideal effects, Ohmic diffusion, ambipolar diffusion, and the Hall effect. These low-ionization processes can in some cases suppress turbulence driven by the magnetorotational instability (MRI). However, it has recently been shown that these non-ideal terms can also affect the dynamics of the gas in fundamentally different ways than simple diffusion. In order to further study the role of low-ionization on disk gas dynamics, we carry out a 3D local shearing box simulation with both Ohmic diffusion and ambipolar diffusion and an additional simulation with the Hall effect included. The strength of each non-ideal term, when present, is representative of gas at a radius of 5 AU in a realistic protoplanetary disk. We find the Hall effect increases the saturation strength of the magnetic field, but does not necessarily drive turbulence, consistent with previous work. However, interactions between ambipolar diffusion and the Keplerian shear lead to the ambipolar diffusion shear instability (ADSI), which can drive the initial growth, not damping, of magnetic perturbations. To our knowledge, this is the first work that explicitly demonstrates the viability of the ADSI in the non-linear regime within protoplanetary disks. At later times in the disk, the MRI (reduced in strength by ambipolar-diffusion), may also be present in regions of weak magnetic field between strong concentrations of vertical magnetic flux and sustain turbulence locally in protoplanetary disks.