Resistivity-driven State Changes in Vertically Stratified Accretion Disks

TL;DR

This study examines how shear viscosity and Ohmic resistivity influence MRI-driven turbulence in stratified accretion disks, revealing magnetic field dynamics, dynamo activity, and variability relevant to observed accretion states.

Contribution

It provides new insights into the effects of resistivity and viscosity on MRI turbulence and magnetic field evolution in stratified disks through detailed simulations.

Findings

MRI turbulence persists within |z| < 2H for low resistivity

Magnetic polarity switches approximately every 10 orbits

Resistive decay and dynamo-driven regrowth cause variability

Abstract

We investigate the effect of shear viscosity and Ohmic resistivity on the magnetorotational instability (MRI) in vertically stratified accretion disks through a series of local simulations with the Athena code. First, we use a series of unstratified simulations to calibrate physical dissipation as a function of resolution and background field strength; the effect of the magnetic Prandtl number, Pm = viscosity/resistivity, on the turbulence is captured by ~32 grid zones per disk scale height, H. In agreement with previous results, our stratified disk calculations are characterized by a subthermal, predominately toroidal magnetic field that produces MRI-driven turbulence for |z| < 2 H. Above |z| = 2 H, magnetic pressure dominates and the field is buoyantly unstable. Large scale radial and toroidal fields are also generated near the mid-plane and subsequently rise through the disk. The polarity of this mean field switches on a roughly 10 orbit period in a process that is well-modeled by an alpha-omega dynamo. Turbulent stress increases with Pm but with a shallower dependence compared to unstratified simulations. For sufficiently large resistivity, on the order of cs H/1000, where cs is the sound speed, MRI turbulence within 2 H of the mid-plane undergoes periods of resistive decay followed by regrowth. This regrowth is caused by amplification of toroidal field via the dynamo. This process results in large amplitude variability in the stress on 10 to 100 orbital timescales, which may have relevance for partially ionized disks that are observed to have high and low accretion states.