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M. Machida (Chiba Univ.), M.R. Hayashi (NAOJ), K.E. Nakamura (JST), R. Matsumoto (Chiba Univ.)
We present the results of three-dimensional global magnetohydrodynamic (MHD) simulations of differentially rotating disks. An equilibrium model of an MHD torus (Okada et al. 1989) is adopted as an initial condition. The differentially rotating torus subjects to the magnetorotational instability and the Parker instability.
When \beta=Pgas/Pmag \gg 1 at the initial state, magnetic energy is amplified exponentially due to the dynamo action driven by the instabilities. In the nonlinear stage, the system approaches to a quasi-steady state with \beta ~10. Turbulent motions inside the torus tangle magnetic field lines but in large scale magnetic fields show low azimuthal nunber spiral structure. Due to the efficient angular momentum transport by Maxwell stress, the torus evolves to a nealy Keprerian accretion disk.
The \alpha-value defined by \alpha \equiv \langle Br B\phi/ (4 \pi P0 )\rangle is \alpha ~0.01-0.1. Large amplitude fluctuations of mass accretion rate is observed. Inside the disk, magnetic fields show intermittent structures; althogh \beta ~10 in average, low-\beta (\beta < 1) filaments are formed. The volume filling factor of the region where \beta < 0.3 is 3-5%. Magnetic reconnection takeing place in low-\beta region heats up the disk and creates X-ray emitting hot plasmas. Numerical results also show that magnetic flux escapes from the disk by buoyancy. Magnetic loops emerging from the disk create magnetically structured corona similar to the solar corona. Differential rotation can drive expansion of the loop, flares and outflows.