Global three-dimensional magnetohydrodynamic simulations of accretion disks and the surrounding magnetosphere

We present the results of global three-dimensional numerical simulations of magnetized accretion disks which have been performed in order to study the evolution of an accretion disk and its surrounding corona within the frame work of ideal magnetohydrodynamics. A self-consistent study of this coupled system is a necessary step toward gaining an understanding of both the pro duction and structure of outflows and the angular momentum transport in acc retion disks. We study model disks that are initially threaded by a uniform , homogenous magnetic held parallel to the rotational axis. The evolution o f these disks is examined under the assumption of an adiabatic equation of state. When the disk is threaded with a weak initial magnetic field, we observe ne w, intrinsically three-dimensional nonaxisymmetric phenomena that do not oc cur in the two-dimensional axisymmetric approximation. Our three-dimensiona l simulations show that in the weak field case MHD waves are generated at t he surface of the disk and subsequently propagate into the corona, possibly contributing to coronal heating. To investigate the nature and the origin of these waves, we have performed a series of exploratory simulations with different rotation profiles of the corona and different density relations b etween disk and corona, as well as simulations with a purely toroidal initi al magnetic field. When the simulations are computed with a strong initial field, we find that there is very little difference between two-dimensional and three-dimensio nal evolution. In both the weak field and strong field cases, in agreement with previous two-dimensional simulations, the disk collapses on orbital ti mescales. Disk collapse arises from angular momentum transport by the Balbu s-Hawley instability in the weak field case and by the external torque of a n outflow in the strong field case. We observe the generation of winds in a ll our simulations. In the weak field case, these winds are unsteady, and t hey do not collimate within the time frame of the simulation.