On Self-Sustained Dynamo Cycles in Accretion Discs

TL;DR

This paper investigates how magnetohydrodynamic turbulence persists in zero-net-flux shearing boxes, revealing a nonlinear dynamo mechanism that sustains large-scale magnetic fields and links to the magnetorotational instability.

Contribution

It identifies a nonlinear dynamo process responsible for sustaining turbulence in zero-net-flux accretion disk models, which was previously unexplained by linear theory.

Findings

Large-scale azimuthal magnetic field persists for several orbits.

Turbulence is maintained through a dynamo process coupled with MRI.

The mechanism explains turbulence survival without net magnetic flux.

Abstract

(abridged) MHD turbulence is known to exist in shearing boxes with either zero or nonzero net magnetic flux. However, the way turbulence survives in the zero-net-flux case is not explained by linear theory and appears as a purely numerical result. Aims: We look for a nonlinear mechanism able to explain the persistence of MHD turbulence in shearing boxes with zero net magnetic flux, and potentially leading to large-scale dynamo action. Method: Spectral nonlinear simulations of the magnetorotational instability are shown to exhibit a large-scale axisymmetric magnetic field, maintained for a few orbits. The generation process of this field is investigated using the results of the simulations and an inhomogeneous linear approach. Results: The mechanism by which turbulence is sustained in zero-net-flux shearing boxes is shown to be related to the existence of a large-scale azimuthal field, surviving for several orbits. In particular, it is shown that MHD turbulence in shearing boxes can be seen as a dynamo process coupled to a magnetorotational-type instability.