Magnetorotationally driven wind cycles in local disc models

TL;DR

This study uses shearing-box simulations to reveal that magnetorotational instability induces cyclic magneto-centrifugal winds in accretion discs, with activity periods of 10-20 orbital times, potentially explaining observed variability in disk-jet systems.

Contribution

It demonstrates the cyclic nature of MRI-driven winds in stratified disc models and links the activity to specific magnetic and wind launching phases, advancing understanding of disc-jet dynamics.

Findings

Winds exhibit 10-20 Omega^{-1} cycle periods.

Cycle activity increases with stronger vertical magnetic fields.

Solutions are robust across parameter variations and numerical setups.

Abstract

Jets, from the protostellar to the AGN context, have been extensively studied but their connection to the turbulent dynamics of the underlying accretion disc is poorly understood. Following a similar approach to Lesur et al. (2013), we examine the role of the magnetorotational instability (MRI) in the production and acceleration of outflows from discs. Via a suite of one-dimensional shearing-box simulations of stratified discs we show that magneto-centrifugal winds exhibit cyclic activity with a period of 10-20 Omega^{-1}, a few times the orbital period. The cycle seems to be more vigorous for strong vertical field; it is robust to the variation of relevant parameters and independent of numerical details. The convergence of these solutions (in particular the mass loss rate) with vertical box size is also studied. By considering a sequence of magnetohydrostatic equilibria and their stability, the periodic activity may be understood as the succession of the following phases: (a) a dominant MRI channel mode, (b) strong magnetic field generation, (c) consequent wind launching, and ultimately (d) vertical expulsion of the excess magnetic field by the expanding and accelerating gas associated with the wind. We discuss potential connections between this behaviour and observed time-variability in disk-jet systems.