Magnetorotational and Magnetic Buoyancy Instabilities in Differentially Rotating Disks

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1.By means of three-dimensional local magnetohydrodynamic (MHD) simulations, we studied the nonlinear evolution of magnetorotational instabilities and magnetic buoyancy instabilities in gravitationa11y stratified, differentially rotating disks. We found that magnetic accretion disks have two states ; a gas pressure dominated quasi-steady state and a magnetic pressure dominated state. In the latter state, buoyancy driven magnetic field amplification occurs.2.We carried out global MHD simulations of differentially rotating torus threaded by large scale vertical magnetic fields and studied the dependence of jet speed and mass outflow rates on the magnetic field strength. By 3D MHD simulations, we showed that the growth of non-axisymmetric instabilities in the torus also create non-axisymmetric structures inside the jet.3.We studied the time evolution of a torus threaded by weak toroidal magnetic fields. The amplification of magnetic fields inside the torus saturates after 10 rotation period when magnetic turbulence develops. The ratio of the gas pressure to magnetic pressure in the torus is about 10 in this state. Because of the effcient angular momentum transport, the angular momentum distribution approaches to the Keplerian rotation. The effective value of the angular momentum transport rate, alpha, is between 0.01 and 0.1. We also found that filamentary magnetic pressure dominated regions appear and that magnetic loops rise buoyantly.4.We carried out axisymmetric 2D simulations of the interaction between the dipole magnetic field of the central star and surrounding accretion disks. The magnetic helicity injected from the disk drives magnetic reconnection and ejection of hot plasmoids. We applied this mechanism to X-ray flares and optical jets observed in protostars.5.We wrote parallelized MHD codes for various parallel computers and confirmed that the efficiency of the code is almost proportional to the number of processing elements.