Evolution of the magnetorotational instability on initially tangled magnetic fields

TL;DR

This study investigates whether the magnetorotational instability (MRI) can develop from turbulent, small-scale magnetic fields typical of astrophysical plasmas, finding that MRI growth depends on initial field strength and coherence.

Contribution

It demonstrates that MRI can grow from turbulent initial conditions if the fields are sufficiently strong and coherent, extending understanding of MRI development in realistic astrophysical environments.

Findings

MRI growth depends on initial field strength and coherence.

Weak or incoherent fields decay faster than MRI can amplify.

Saturated states resemble those with large-scale initial fields.

Abstract

The initial magnetic field of previous magnetorotational instability (MRI) simulations has always included a significant system-scale component, even if stochastic. However, it is of conceptual and practical interest to assess whether the MRI can grow when the initial field is turbulent. The ubiquitous presence of turbulent or random flows in astrophysical plasmas generically leads to a small-scale dynamo (SSD), which would provide initial seed turbulent velocity and magnetic fields in the plasma that becomes an accretion disc. Can the MRI grow from these more realistic initial conditions? To address this we supply a standard shearing box with isotropically forced SSD generated magnetic and velocity fields as initial conditions, and remove the forcing. We find that if the initially supplied fields are too weak or too incoherent, they decay from the initial turbulent cascade faster than they can grow via the MRI. When the initially supplied fields are sufficient to allow MRI growth and sustenance, the saturated stresses, large-scale fields, and power spectra match those of the standard zero net flux MRI simulation with an initial large scale vertical field.