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We propose a model of hard X-ray flares in protostars observed by ASCAsatellite. Assuming that the dipole magnetic field of the protostar threads theprotostellar disk, we carried out 2.5-dimensional magnetohydrodynamic (MHD)simulations of the disk-star interaction. The closed magnetic loops connectingthe central star and the disk are twisted by the rotation of the disk. As thetwist accumulates, magnetic loops expand and finally approach to the open fieldconfiguration. A current sheet is formed inside the expanding loops. In thepresence of resistivity, magnetic reconnection takes place in the currentsheet. Outgoing magnetic island and post flare loops are formed as a result ofthe reconnection. The time scale of this `flare' is the order of the rotationperiod of the disk. The released magnetic energy partly goes into the thermalenergy and heats up the flaring plasma up to $10^8$ K. The length of theflaring loop is several times of the radius of the central star, consistentwith 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 1M$_\odot$ protostar. The hot plasma outflow can explain the speed and mass flowrate of optical jets. Dense, cold, magnetically accelerated wind ($v \sim150-250$ km s$^{-1}$) emanates from the surface of the disk along the partiallyopen magnetic field lines threading the disk. This dense, cold wind maycorrespond to high velocity neutral winds.

X-Ray Flares and Mass Outflows Driven by Magnetic Interaction between a Protostar and Its Surrounding Disk