Turbulence and Angular Momentum Transport in a Global Accretion Disk Simulation

The global development of magnetohydrodynamic turbulence in an accretion disk is studied within a simplified disk model that omits vertical stratification. Starting with a weak vertical seed field, a saturated state is obtained after a few tens of orbits in which the energy in the predominantly toroidal magnetic field is still subthermal. The efficiency of angular momentum transport, parameterized by the Shakura-Sunyaev α parameter, is of the order of 10. The dominant contribution to α comes from magnetic stresses, which are enhanced by the presence of weak net vertical fields. The power spectra of the magnetic fields are flat or decline only slowly towards the largest scales accessible in the calculation, suggesting that the viscosity arising from MHD turbulence may not be a locally determined quantity. I discuss how these results compare with observationally inferred values of α, and possible implications for models of jet formation. Subject headings: accretion, accretion disks — hydrodynamics — instabilities — magnetic fields — MHD — turbulence For visualization of simulations see http://www.cita.utoronto.ca/∼armitage/global abs.html