SUSTAINED MAGNETOROTATIONAL TURBULENCE IN LOCAL SIMULATIONS OF STRATIFIED DISKS WITH ZERO NET MAGNETIC FLUX

Abstract

We examine the effects of density stratification on magnetohydrodynamic\nturbulence driven by the magnetorotational instability in local simulations\nthat adopt the shearing box approximation. Our primary result is that, even in\nthe absence of explicit dissipation, the addition of vertical gravity leads to\nconvergence in the turbulent energy densities and stresses as the resolution\nincreases, contrary to results for zero net flux, unstratified boxes. The ratio\nof total stress to midplane pressure has a mean of ~0.01, although there can be\nsignificant fluctuations on long (>~50 orbit) timescales. We find that the time\naveraged stresses are largely insensitive to both the radial or vertical aspect\nratio of our simulation domain. For simulations with explicit dissipation, we\nfind that stratification extends the range of Reynolds and magnetic Prandtl\nnumbers for which turbulence is sustained. Confirming the results of previous\nstudies, we find oscillations in the large scale toroidal field with periods of\n~10 orbits and describe the dynamo process that underlies these cycles.\n