  Protostellar flares are rapid magnetic energy release events associated with
formation of hot plasma in protostars. In the previous models of protostellar
flares, the interaction between a protostellar magnetosphere with the
surrounding disk plays crucial roles in building-up and releasing the magnetic
energy. However, it remains unclear if protostars indeed have magnetospheres
because vigorous disk accretion and strong disk magnetic fields in the
protostellar phase may destroy the magnetosphere. Considering this possibility,
we investigate the energy accumulation and release processes in the absence of
a magnetosphere using a three-dimensional magnetohydrodynamic simulation. Our
simulation reveals that protostellar flares are repeatedly produced even in
such a case. Unlike in the magnetospheric models, the protostar accumulates
magnetic energy by acquiring large-scale magnetic fields from the disk by
accretion. Protostellar flares occur when a portion of the large-scale magnetic
fields are removed from the protostar as a result of magnetic reconnection.
Protostellar flares in the simulation are consistent with observations; the
released magnetic energy (up to $\sim 3\times 10^{38}$ erg) is large enough to
drive observed flares, and the flares produce hot ejecta. The expelled magnetic
fields enhance accretion, and the energy build-up and release processes are
repeated as a result. The magnetic flux removal via reconnection leads to
redistribution of magnetic fields in the inner disk. We therefore consider that
protostellar flares will play an important role in the evolution of the disk
magnetic fields in the vicinity of protostars.
