Magnetorotational instability and dynamo action in gravitoturbulent astrophysical discs

TL;DR

This study explores the interaction between magnetorotational and gravitational instabilities in astrophysical discs through 3D simulations, revealing how gravito-turbulence influences magnetic field generation and MRI activity.

Contribution

It provides the first detailed numerical analysis of combined MRI and GI in stratified discs, demonstrating how gravito-turbulence affects magnetic dynamo processes and MRI suppression.

Findings

Gravito-turbulence weakens the zero-net-flux MRI.

Efficient cooling suppresses MRI but sustains strong magnetic fields.

Strong net-vertical-flux can revive MRI and lead to magnetically dominated states.

Abstract

Though usually treated in isolation, the magnetorotational and gravitational instabilities (MRI and GI) may coincide at certain radii and evolutionary stages of protoplanetary discs and active galactic nuclei. Their mutual interactions could profoundly influence several important processes, such as accretion variability and outbursts, fragmentation and disc truncation, or large-scale magnetic field production. Direct numerical simulations of both instabilities are computationally challenging and remain relatively unexplored. In this paper, we aim to redress this neglect via a set of 3D vertically stratified shearing-box simulations, combining self-gravity and magnetic fields. We show that gravito-turbulence greatly weakens the zero-net-flux MRI. In the limit of efficient cooling (and thus enhanced GI), the MRI is completely suppressed, and yet strong magnetic fields are sustained by the gravitoturbulence. This turbulent `spiral wave' dynamo may have widespread application, especially in galactic discs. Finally, we present preliminary work showing that a strong net-vertical-flux revives the MRI and supports a magnetically dominated state, in which the GI is secondary.