WHAT IS THE NUMERICALLY CONVERGED AMPLITUDE OF MAGNETOHYDRODYNAMICS TURBULENCE IN STRATIFIED SHEARING BOXES?
We study the properties of the turbulence driven by the magnetorotational instability (MRI) in a stratified shearing box with outflow boundary conditions and an equation of state determined by self-consistent dissipation and radiation losses. A series of simulations with increasing resolution are performed within a fixed computational box. We achieve numerical convergence with respect to radial and azimuthal resolution. As vertical resolution is improved, the ratio of stress to pressure
... s only slowly, but the absolute levels of both the stress and the pressure increase noticeably. The vertical correlation length of the magnetic field within the core of the disk is highly time-dependent, but averaged over time, it is ≃ 3 times larger than found in previous unstratifed simulations. This correlation length decreases slowly as vertical resolution increases. We suggest that this greater degree of correlation in stratified than unstratified disks, even near the midplane, is due to field buoyancy. We further show that the undulatory Parker instability drives the magnetic field upwelling at several scaleheights from the midplane that is characteristic of stratified MHD-turbulent disks.