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K. Noguchi (T-CNLS, LANL), T. Tajima (IFS, Dept. of Physics, Univ. of Texas, Austin, LLNL), W. Horton (IFS, Dept. of Physics, Univ. of Texas)
The magnetorotational(MRI) and the Parker instabilities may be the key to resolve the early stage of evolution of accretion disks. MRI and the Parker instabitity can greatly facilitate the formation of density structures that serve as seeds prior to the onset of the Jeans instability. Due to the presence of magnetic field and flow shear, MRI is dominant near the equatorial plane of a disk, whereas the Parker instability may affect near the surface. We investigate the coupling of MRI and the Parker instabilities both with liniear eigenmode analysis and nonlinear MHD simulations. The linear analysis shows that nonaxisymmetric MRI is robust when magnetic field pressure is low(\beta=8\pi P/B2\gg1), whereas the Parker instability will compete with MRI when \beta\leq1. The three dimentional nonlinear simulations in local shearing box shows that the vortices exited by the eigenmodes will evolve to create blob- and strip-like dense structure in the vicinity of the equatorial plane of an accretion disk. These dense structures would produce the observed structures of protoplanetary disks, and become the seed of a rocky core at the center of massive planets.