NUMERICAL SIMULATIONS OF MAGNETIC ACCRETION DISKS

The global evolution of magnetic accretion disks is studied by means of time-dependent numerical simulations. Ideal magnetohydrodynamics is used to compute the evolution; the simulations assume the disk is adiabatic, axisymmetric, and initially threaded by a purely axial magnetic field. These restrictions permit a large number of exploratory simulations to be performed. The evolution of both Keplerian and sub-Keplerian disks with a variety of magnetic field strengths is investigated. In the case of an initially sub-Keplerian disk, collapse of the disk is halted at the centrifugal barrier, and rapid wind-up of the magnetic field results in the production of a strongly collimated, magnetic-pressure-driven wind. Although the evolution of the disk in this case is relatively independent of the magnetic field, the detailed properties of the wind are not. Surprisingly, in the case of an initially Keplerian disk, we also find collapse of the disk occurs on orbital timescales regardless of the initial field strength. In the case of an initially strong field the collapse is driven by external torques (magnetic braking), while in the case of an initially weak magnetic field, the collapse is driven by internal torques (the Balbus-Hawley instability). These simulations indicate that magnetized accretion disks and any magnetically driven winds that are associated with them may be intrinsically unsteady.