Connections Between Local and Global Turbulence in Accretion Disks

TL;DR

This study investigates whether local turbulence scaling laws in accretion disks hold true in global simulations, emphasizing the role of magnetic fields and the importance of resolving the corona for accurate modeling.

Contribution

It demonstrates the correlation between local magnetic fields and stress in global MHD simulations, highlighting the significance of magnetic linkages and corona resolution for turbulence modeling.

Findings

Local Maxwell stress correlates with local vertical magnetic field.

Vertical magnetic patches influence angular momentum transport.

Proper corona resolution is essential for simulation convergence.

Abstract

We analyze a suite of global magnetohydrodynamic (MHD) accretion disk simulations in order to determine whether scaling laws for turbulence driven by the magnetorotational instability, discovered via local shearing box studies, are globally robust. The simulations model geometrically-thin disks with zero net magnetic flux and no explicit resistivity or viscosity. We show that the local Maxwell stress is correlated with the self-generated local vertical magnetic field in a manner that is similar to that found in local simulations. Moreover, local patches of vertical field are strong enough to stimulate and control the strength of angular momentum transport across much of the disk. We demonstrate the importance of magnetic linkages (through the low-density corona) between different regions of the disk in determining the local field, and suggest a new convergence requirement for global simulations -- the vertical extent of the corona must be fully captured and resolved. Finally, we examine the temporal convergence of the average stress, and show that an initial long-term secular drift in the local flux-stress relation dies away on a time scale that is consistent with turbulent mixing of the initial magnetic field.