MRI channel flows and their parasites

TL;DR

This paper investigates the formation, stability, and destruction of channel flows in MRI turbulence, revealing that most channels are destroyed by turbulence rather than parasitic instabilities, with implications for accretion disk dynamics.

Contribution

It clarifies the roles of parasitic modes and turbulence in channel flow evolution, linking classical plasma instabilities to astrophysical MRI phenomena.

Findings

Channels often emerge from turbulence and are destroyed by turbulent mixing.

Isolated channels near criticality can grow large and be destroyed eruptively.

Parasitic instabilities are not the primary destruction mechanism in most cases.

Abstract

Local simulations of the magnetorotational instability (MRI) in accretion disks can exhibit recurrent coherent structures called channel flows. The formation and destruction of these structures may play a role in the development and saturation of MRI-induced turbulence, and consequently help us understand the time-dependent accretion behaviour of certain astrophysical objects. Previous investigations have revealed that channel solutions are attacked by various parasitic modes, foremost of which is an analogue of the Kelvin-Helmholtz instability. We revisit these instabilities and show how they relate to the classical instabilities of plasma physics, the kink and pinch modes. However, we argue that in most cases channels emerge from developed turbulence and are eventually destroyed by turbulent mixing, not by the parasites. The exceptions are the clean isolated channels which appear in systems near criticality or which emerge from low amplitude initial conditions. These structures inevitably achieve large amplitudes and are only then destroyed, giving rise to eruptive behaviour.