Structure formation through magnetohydrodynamical instabilities in protoplanetary and accretion disks

Structure formation in various astronomical systems through magnetohydrodynamical (MHD) instabilities has been investigated. The effect of magnetic field enhancement in sheared flows is studied as the eigenmode problem in a non-self-adjoint system, and new mathematical and physical aspects of the instability are shown. The mechanism for the faster structure formation in a protoplanetary disk with MHD instabilities is suggested with linear analysis and simulation. Experiments to simulate the plasmas of protoplanetary and active galactic nuclei are also suggested. The coupling of magnetic field enhancement and magnetic buoyancy has been studied and mode coupling of two instabilities are shown.; The stability of nonaxisymmetric perturbations in a differentially rotating astrophysical accretion disk is analyzed by fully incorporating the properties of shear flows. The magnetorotational instability has been investigated as the most promising instability to explain anomalous angular momentum transfer in accretion disks.; The apparent narrow window of the age difference of the Sun and the Earth is one of the most difficult problems in the planetary formation theory. The magnetorotational, Parker, and kinematic dynamo instabilities under the presence of magnetic fields in the protoplanetary disk can greatly facilitate the formation of density structures that may provide seeds of protoplanetesimals prior to the onset of the gravitational Jeans instability.; The unstable eigenmodes with complex and pure imaginary eigenvalues without any artificial disk edge boundaries is shown to be discrete. The nonlocal behavior of eigenmodes in the vicinity of Alfvén singularities at ω _D = ±ω_A is investigated mathematically and physically, where ω_D is the Doppler-shifted wave frequency and ω_A = k_∥v_A is the Alfvén frequency.; The effects of the Parker, magnetorotational (Balbus-Hawley), and kinematic dynamo instabilities are evaluated by comparing the properties of these instabilities in protoplanetary disks. The mass spectra of aggregated density structures are calculated by the above mechanism in the radial direction for axisymmetric MHD torus equilibrium and power-law density profile models.; The coupling of the magnetorotational and the Parker instabilities is studied here by three-dimensional MHD simulations. The coupling of these instabilities is analyzed linearly, and preliminary results are shown. (Abstract shortened by UMI.)...