Hypermagnetized Accretion Disks: A Global Stability Analysis

We examine the stability properties of hypermagnetized accretion disks in a global framework, with a particular focus on the evolution of the weak-field magnetorotational instability (MRI), which is believed to trigger angular momentum transport in accretion flows. Local magnetohydrodynamic (MHD) simulations have shown that in the presence of a sufficiently strong but subthermal (plasma-β ≫1) vertical magnetic flux, a large-scale, suprathermal (plasma-β < 1) toroidal field is generated, which becomes thedominant source of pressure support in the accretion disk. We term such magnetically dominated disks as hypermagnetized accretion disks, which have the potential to resolve several observational shortcomings of the standard, geometrically thin, accretion disk model. The suprathermal toroidal field makes the curvature effects due to the cylindrical geometry of the disk non-negligible, which are otherwise ignored in weak-field studies. Furthermore, these intrinsically global effects may not be accurately captured in a local model. Hence, in order to self-consistently account for the curvature effects, we perform a global eigenvalue analysis of the linearized MHD equations for a compressible, differentially rotating flow in cylindrical geometry. We find that MRI gets highly suppressed at a critical suprathermal toroidal field and two new instabilities appear beyond this limit. These results are additionally verified using numerical simulations. Our analysis partly confirms the predictions of a local model but reveals important differences, which highlight the necessity of a global treatment to accurately model hypermagnetized accretion disks.