Dissipative structures in magnetorotational turbulence

TL;DR

This study investigates the small-scale dissipative structures in magnetorotational turbulence, revealing elongated magnetic reconnection ribbons that influence large-scale dynamics and energy dissipation in accretion disks.

Contribution

It provides the first detailed numerical analysis of the morphology and evolution of dissipative structures in MRI turbulence, highlighting their meso-scale nature and role in turbulence dynamics.

Findings

Dissipation occurs in elongated magnetic reconnection ribbons.

These structures are meso-scale, extending up to a scale height.

Couplings between density waves and small-scale structures affect turbulence behavior.

Abstract

Via the process of accretion, magnetorotational turbulence removes energy from a disk's orbital motion and transforms it into heat. Turbulent heating is far from uniform and is usually concentrated in small regions of intense dissipation, characterised by abrupt magnetic reconnection and higher temperatures. These regions are of interest because they might generate non-thermal emission, in the form of flares and energetic particles, or thermally process solids in protoplanetary disks. Moreover, the nature of the dissipation bears on the fundamental dynamics of the magnetorotational instability (MRI) itself: local simulations indicate that the large-scale properties of the turbulence (e.g. saturation levels, the stress-pressure relationship) depend on the short dissipative scales. In this paper we undertake a numerical study of how the MRI dissipates and the small-scale dissipative structures it employs to do so. We use the Godunov code RAMSES and unstratified compressible shearing boxes. Our simulations reveal that dissipation is concentrated in ribbons of strong magnetic reconnection that are significantly elongated in azimuth, up to a scale height. Dissipative structures are hence meso-scale objects, and potentially provide a route by which large scales and small scales interact. We go on to show how these ribbons evolve over time --- forming, merging, breaking apart, and disappearing. Finally, we reveal important couplings between the large-scale density waves generated by the MRI and the small-scale structures, which may illuminate the stress-pressure relationship in MRI turbulence.