Accretion Disks in Large-scale Magnetic Fields
  This paper presents a survey of a set of simulations of accretion disks orbiting a rapidly rotating Kerr black hole and embedded in a large-scale initial magnetic field. Each simulation uses a common state for the initial torus, including an MRI seed-field consisting of poloidal loops along isodensity contours, and differ only in the strength of the initial large-scale magnetic field in relation to these poloidal loops. Simulations with a weak large-scale initial field differ little from simulations that evolve from only the poloidal field. Simulations where the large-scale initial field distorts or completely overwhelms the poloidal loops show more extensive regions of turbulence, due to the action of the MRI on the large-scale field. However, the overall structure of the late-time state of the simulations remains qualitatively unchanged, that is each simulation sees the emergence of a turbulent accretion disk, axial jets, a funnel-wall outflow and an extensive corona. 
Jean-Pierre De Villiers