On the appearance of non-local MRI in Keplerian accretion discs

Abstract

We revisit the modal analysis of small perturbations in Keplerian ideal gas flows leading to magneto-rotational instability (MRI) using the non-local approach. We consider the case of constant vertical background magnetic field, as well as the case of radially dependent background Alfv\'en velocity. In the case of constant Alfv\'en velocity, MRI modes are described by a Schr\"odinger-like differential equation with some effective potential including 'repulsive' ($1/r^2$) and 'attractive' ($-1/r^3$) terms. Taking into account the radial dependence of the background Alfv\'en speed leads to a qualitative change in the shape of the effective potential. It is shown that there are no stationary energy levels corresponding to unstable modes $\omega^2 < 0$ in ``shallow'' potentials. In thin accretion disks, the wavelength of the disturbance $\lambda=2\pi/k_z$ is smaller than the half-thickness $h$ of the disk only in ``deep'' potentials. The limiting value of the background Alfv\'en speed $(c_A)_\mathrm{cr}$, above which the magnetorotational instability does not occur, is found. In thin accretion disks with low background Alfv\'en speed $c_A\ll (c_A)_\mathrm{cr}$, the increment of the magnetorotational instability $\omega\approx -\sqrt{3}\mathrm{i}c_Ak_z$ is suppressed compared to the value obtained in the local perturbation analysis.