Dynamo effects in magnetorotational turbulence with finite thermal diffusivity

TL;DR

This study explores how thermal diffusivity influences magnetorotational turbulence and dynamo action in accretion disks, highlighting the importance of boundary conditions and heat transport modes in turbulence saturation.

Contribution

It provides new insights into the impact of thermal diffusivity and boundary conditions on turbulence saturation and dynamo processes in non-isothermal accretion disk models.

Findings

Transition from conductive to convective heat transport at critical diffusivity

Boundary conditions significantly influence heat transport and magnetic activity

Heating from turbulence affects disk thermodynamics and dynamo properties

Abstract

We investigate the saturation level of hydromagnetic turbulence driven by the magnetorotational instability in the case of vanishing net flux. Motivated by a recent paper of Bodo, Cattaneo, Mignone, & Rossi, we here focus on the case of a non-isothermal equation of state with constant thermal diffusivity. The central aim of the paper is to complement the previous result with closure parameters for mean-field dynamo models, and to test the hypothesis that the dynamo is affected by the mode of heat transport. We perform computer simulations of local shearing-box models of stratified accretion disks with approximate treatment of radiative heat transport, which is modeled via thermal conduction. We study the effect of varying the (constant) thermal diffusivity, and apply different vertical boundary conditions. In the case of impenetrable vertical boundaries, we confirm the transition from mainly conductive to mainly convective vertical heat transport below a critical thermal diffusivity. This transition is however much less dramatic when more natural outflow boundary conditions are applied. Similarly, the enhancement of magnetic activity in this case is less pronounced. Nevertheless, heating via turbulent dissipation determines the thermodynamic structure of accretion disks, and clearly affects the properties of the related dynamo. This effect may however have been overestimated in previous work, and a careful study of the role played by boundaries will be required.