The weakly nonlinear magnetorotational instability in a local geometry

TL;DR

This paper develops a weakly nonlinear theory of the magnetorotational instability (MRI) in a local geometry, revealing how the instability saturates and forms patterns, especially considering non-ideal effects like ambipolar diffusion.

Contribution

It extends the understanding of MRI saturation by deriving a Ginzburg-Landau equation in a local setting with non-ideal effects, supported by simulations and force balance analysis.

Findings

The perturbation amplitude follows a Ginzburg-Landau equation.

Pattern formation predicted by the model is observed in simulations.

Saturation involves reducing shear and modifying the vertical magnetic field.

Abstract

The magnetorotational instability (MRI) is a fundamental process of accretion disk physics, but its saturation mechanism remains poorly understood despite considerable theoretical and computational effort. We present a multiple scales analysis of the non-ideal MRI in the weakly nonlinear regime -- that is, when the most unstable MRI mode has a growth rate asymptotically approaching zero from above. Here, we develop our theory in a local, Cartesian channel. Our results confirm the finding by Umurhan et al. (2007) that the perturbation amplitude follows a Ginzburg-Landau equation. We further find that the Ginzburg-Landau equation will arise for the local MRI system with shear-periodic boundary conditions when the effects of ambipolar diffusion are considered. A detailed force balance for the saturated azimuthal velocity and vertical magnetic field demonstrates that even when diffusive effects are important, the bulk flow saturates via the combined processes of reducing the background shear and rearranging and strengthening the background vertical magnetic field. We directly simulate the Ginzburg-Landau amplitude evolution for our system and demonstrate the pattern formation our model predicts on long length and time scales. We compare the weakly nonlinear theory results to a direct numerical simulation of the MRI in a thin-gap Taylor Couette flow.