  We perform two-dimensional MHD simulations on the solar flux emergence. We
set the initial magnetic flux sheet at z=-20,000 km in the convection zone. The
flux sheet rises through the convective layer due to the Parker instability,
however, decelerates beneath the photosphere because the plasma on the flux
sheet piles up owing to the convectively stable photosphere above. Meanwhile,
the flux sheet becomes locally unstable to the Parker instability within the
photosphere, and the further evolution to the corona occurs (two-step emergence
model). We carry out a parameter survey to investigate the condition for this
two-step model. We find that magnetic fluxes which form active regions are
likely to have undergone the two-step emergence. The condition for the two-step
emergence is 10^21 - 10^22 Mx with 10^4 G at z=-20,000 km in the convection
zone.
