MHD Simulations of Global Accretion Disks with Vertical Magnetic Fields

Abstract: (Abridged) We report results of three dimensional MHD simulations of global accretion disks threaded with weak vertical magnetic fields. We perform the simulations in the spherical coordinates with different temperature profiles and accordingly different rotation profiles. In the cases with a spatially constant temperature, because the rotation frequency is vertically constant in the equilibrium condition, general properties of the turbulence are quantitatively similar to those obtained in local shearing box simulations. On the other hand, in the cases with a radially variable temperature profile, the vertical differential rotation, which is inevitable in the equilibrium condition, winds up the magnetic field lines, in addition to the usual radial differential rotation. As a result, the coherent wound magnetic fields contribute to the Maxwell stress in the surface regions. We obtain nondimensional density and velocity fluctuations ~0.1-0.2 at the midplane. The azimuthal power spectra of the magnetic fields show shallow slopes, ~m^0- m^{-1}, than those of velocity and density. We observe intermittent and structured disk winds driven by the Poynting flux associated with the MHD turbulence. The Poynting flux originating from magnetic tension is injected from the regions above a scale height towards both the midplane and the surfaces. Related to this, sound waves are directed to the midplane from the surface regions. The mass accretion mainly occurs near the surfaces and the gas near the midplane slowly moves outward, which causes large-scale meridional circulations. The vertical magnetic fields are also dragged inward in the surface regions, while they stochastically move outward and inward around the midplane. We also discuss an observational implication of induced spiral structure in the simulated turbulent disks to protoplanetary disks.