A non-local magneto-curvature instability in a differentially rotating disk

TL;DR

This paper identifies a new global non-axisymmetric instability in magnetized, differentially rotating disks that dominates over the standard MRI at strong magnetic fields, driven by magnetic curvature and global geometry.

Contribution

It introduces the magneto-curvature instability, a global mode driven by spatial curvature, extending understanding of disk instabilities beyond local MRI models.

Findings

Global non-axisymmetric modes dominate at strong magnetic fields.

Transition from MRI turbulence to global mode dominance observed in nonlinear simulations.

Instability potentially explains nonlinear transport in high-field accretion disks.

Abstract

A global mode is shown to be unstable to non-axisymmetric perturbations in a differentially rotating Keplerian disk containing either vertical or azimuthal magnetic fields. In an unstratified cylindrical disk model, using both global eigenvalue stability analysis and linear global initial-value simulations, it is demonstrated that this instability dominates at strong magnetic field where local standard MRI becomes stable. Unlike the standard MRI mode, which is concentrated in the high flow shear region, these distinct global modes (with low azimuthal mode numbers) are extended in the global domain and are Alfv\'en continuum driven unstable modes. As its mode structure and relative dominance over MRI is inherently determined by the global spatial curvature as well as the flow shear in the presence of magnetic field, we call it the magneto-curvature (magneto-spatial-curvature) instability. Consistent with the linear analysis, as the field strength is increased in the nonlinear simulations, a transition from a MRI-driven turbulence to a state dominated by global non-axisymmetric modes is obtained. This global instability could therefore be a source of nonlinear transport in accretion disks at higher magnetic field than predicted by local models.