We propose a model of hard X-ray flares in protostars observed by ASCA satellite. Assuming that the dipole magnetic field of the protostar threads the protostellar disk, we carried out 2.5-dimensional magnetohydrodynamic (MHD) simulations of the disk-star interaction. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. As the twist accumulates, magnetic loops expand and finally approach to the open field configuration. A current sheet is formed inside the expanding loops. In the presence of resistivity, magnetic reconnection takes place in the current sheet. Outgoing magnetic island and post flare loops are formed as a result of the reconnection. The time scale of this `flare' is the order of the rotation period of the disk. The released magnetic energy partly goes into the thermal energy and heats up the flaring plasma up to $10^8$ K. The length of the flaring loop is several times of the radius of the central star, consistent with observations. The speed of the hot plasmoid ejected by the reconnection is $200-400 $ km s$^{-1}$ when the footpoint of the loop is at 0.03 AU from 1 M$_\odot$ protostar. The hot plasma outflow can explain the speed and mass flow rate of optical jets. Dense, cold, magnetically accelerated wind ($v \sim 150-250$ km s$^{-1}$) emanates from the surface of the disk along the partially open magnetic field lines threading the disk. This dense, cold wind may correspond to high velocity neutral winds.Comment: 14 pages, uses aasms4.sty,2 PostScript figures, tar'ed and gzip'ed.Full postscript text, figures (color) and mpeg simulations available at http://pleiades.c.chiba-u.ac.jp/~hayashi/lanlxxx.html Accepted for publication in 'ApJ Letters

We present the results of three-dimensional global magnetohydrodynamic simulations of the Parker-shearing instability in a differentially rotating torus initially threaded by toroidal magnetic fields. An equilibrium model of a magnetized torus is adopted as an initial condition. When beta0=Pgas&solm0;Pmag approximately 1 at the initial state, magnetic flux buoyantly escapes from the disk and creates looplike structures similar to those in the solar corona. Inside the torus, the growth of nonaxisymmetric magnetorotational (or Balbus & Hawley) instability generates magnetic turbulence. Magnetic field lines are tangled on a small scale, but on a large scale they show low azimuthal wavenumber spiral structure. After several rotation periods, the system oscillates around a state with beta approximately 5. We found that magnetic pressure-dominated (beta<1) filaments are created in the torus. The volume filling factor of the region in which beta