Revisiting linear dynamics of non-axisymmetric perturbations in weakly magnetized accretion discs

TL;DR

This paper explores the linear behavior of non-axisymmetric perturbations in weakly magnetized accretion discs, revealing a new mode coupling process and showing that non-axisymmetric growth can dominate axisymmetric MRI, affecting disc stability.

Contribution

It introduces eigenvariables for coupled modes and uncovers a new linear coupling process, highlighting the significance of non-axisymmetric modes in disc dynamics.

Findings

Non-axisymmetric modes can outgrow axisymmetric MRI during dynamical times.

A new linear coupling process between inertia-gravity and magnetic modes is identified.

Maximum non-axisymmetric growth occurs near the disc's scale height, indicating the role of compressibility.

Abstract

We investigate linear dynamics of non-axisymmetric perturbations in incompressible, vertically stratified Keplerian discs with a weak vertical magnetic field in the shearing box approximation. Perturbations are decomposed into shearing waves whose evolution is followed via numerical integration of the linearized ideal MHD equations. There are two basic modes in the system -- inertia-gravity waves and magnetic mode, which displays the magnetorotational instability (MRI). As distinct from previous studies, we introduce `eigenvariables' characterizing each (counter-propagating) component of the inertia-gravity and magnetic modes, which are governed by a set of four first order coupled ordinary differential equations. This allowed us to identify a new process of linear coupling of the two above non-axisymmetric modes due to the disc's differential rotation. We did a comparative analysis of the dynamics of non-axisymmetric and axisymmetric magnetic mode perturbations. It is shown that the growth of optimal and close-to-optimal non-axisymmetric harmonics of this mode, having transient nature, can prevail over the exponential growth of axisymmetric ones (i.e., over the axisymmetric MRI) during dynamical time. A possible implication of this result for axisymmetric channel solutions is discussed. Specifically, the formation of the channel may be affected/impeded by non-axisymmetric modes already at the early linear stage leading to its untimely disruption -- the outcome strongly depends on the amplitude and spectrum of initial perturbation. So, this competition may result in an uncertainty in the magnetic mode's non-linear dynamics. It is also shown that a maximum non-axisymmetric growth is at vertical wavelengths close to the scale-height for which compressibility effects are important. This indirectly suggests that compressibility plays a role in the dynamics of the non-axisymmetric MRI.