  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.
