Three-dimensional magnetohydrodynamic simulations of an accretion disk with star-disk boundary layer

We present global three-dimensional MHD simulations of geometrically thin but unstratified accretion disks in which a near-Keplerian disk rotates between two bounding regions with initial rotation profiles that are stable to the magnetorotational instability (MRI). The inner region models the boundary layer between the disk and an assumed more slowly rotating central, nonmagnetic star. We investigate the dynamical evolution of this system in response to initial vertical and toroidal fields imposed in a variety of domains contained within the near-Keplerian disk. Cases with both nonzero and zero net magnetic flux are considered, and sustained dynamo activity is found in runs for up to 50 orbital periods at the outer boundary of the near-Keplerian disk. We find a progression of behavior regarding the turbulence resulting from the MRI and the evolving structure of the disk and boundary layer according to the initial field configuration. Simulations starting from fields with small radial scale and with zero net flux lead to the lowest levels of turbulence and smoothest variation of disk mean state variables. As found in local simulations, the final outcome is shown to be independent of the form of the imposed field. For our computational setup, average values of the Shakura Sunyaev alpha-parameter in the Keplerian disk are typically 0.004 +/- 0.002. The magnetic field eventually always diffuses into the boundary layer, resulting in the buildup of toroidal field, inward angular momentum transport, and the accretion of disk material. The mean radial velocity, while exhibiting large temporal fluctuations, is always subsonic. Simulations starting with net toroidal flux may yield an average alpha similar to 0.04. While being characterized by 1 order of magnitude larger average alpha, simulations starting from vertical fields with large radial scale and net flux may lead to the formation of persistent nonhomogeneous, nonaxisymmetric magnetically dominated regions of very low density. In these gaps, angular momentum transport occurs through magnetic torques acting between regions on either side of the gap. Local turbulent transport occurs where the magnetic field is not dominant. These simulations are indicative of the behavior of the disk when threaded by magnetic flux originating from an external source. However, the influence of such presumed sources in determining the boundary conditions that should be applied to the disk remains to be investigated.