MRI channel flows and their parasites

Local simulations of the magnetorotational instability (MRI) in accretion discs 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 clean isolated channels, which appear in systems near criticality or which emerge from low amplitude initial conditions. These structures can achieve large amplitudes and are only then destroyed, giving rise to eruptive behaviour.