LOCAL 3-DIMENSIONAL MAGNETOHYDRODYNAMIC SIMULATIONS OF ACCRETION DISKS

We have performed three-dimensional magnetohydrodynamic numerical simu lations of an accretion disk to study the nonlinear development of the magnetorotational instability. We use a disk model that is local in t he sense that it incorporates tidal and Coriolis forces but neglects b ackground gradients in pressure and density. For simplicity we omit th e vertical component of gravity and employ periodic boundary condition s in the vertical and azimuthal directions, and shearing-periodic boun dary conditions in the radial direction. Our numerical method is an im plementation of the ''method of characteristics-constrained transport' ' algorithm. Most of the simulations begin with either a purely vertic al or purely azimuthal magnetic field. Our major result is that turbul ence is initiated and sustained by the magnetic instability. We provid e a detailed characterization of the saturated turbulent state. The tu rbulence is anisotropic in a sense that implies an outward flux of ang ular momentum. The turbulent energy and angular momentum flux is domin ated by magnetic stress rather than Reynolds stress. Most of the energ y and angular momentum flux is concentrated at the largest scales. We find that the magnetic energy density in the saturated state is propor tional to the product of the size of the simulation box and the initia l field strength and is independent of the sound speed.