MRI channel flows in vertically-stratified models of accretion disks

TL;DR

This paper investigates the behavior of MRI channel flows in vertically-stratified accretion disk models, showing their energy dominance, vulnerability to instabilities, and implications for disk magnetic field strength and outburst phenomena.

Contribution

It extends the understanding of MRI channel flows from unstratified to stratified models, providing analytical and numerical insights into their dynamics and stability.

Findings

Channel flows rapidly capture magnetic and kinetic energy.

Maximum amplitudes before destruction are estimated.

Implications for magnetic field strengths and outburst behavior.

Abstract

Simulations of the magnetorotational instability (MRI) in 'unstratified' shearing boxes exhibit powerful coherent flows, whereby the fluid vertically splits into countermoving planar jets or `channels'. Channel flows correspond to certain axisymmetric linear MRI modes, and their preponderance follows from the remarkable fact that they are approximate nonlinear solutions of the MHD equations in the limit of weak magnetic fields. We show in this paper, analytically and with one-dimensional numerical simulations, that this property is also shared by certain axisymmetric MRI modes in vertically-stratified shearing boxes. These channel flows rapidly capture significant amounts of magnetic and kinetic energy, and thus are vulnerable to secondary shear instabilities. We examine these parasites in the vertically stratified context, and estimate the maximum amplitudes that channels attain before they are destroyed. These estimates suggest that a dominant channel flow will usually drive the disk's magnetic field to thermal strengths. The prominence of these flows and their destruction place enormous demands on simulations, but channels in their initial stages also offer a useful check on numerical codes. These benchmarks are especially valuable given the increasing interest in the saturation of the stratified MRI. Lastly we speculate on the potential connection between 'run-away' channel flows and outburst behaviour in protostellar and dwarf nova disks.