  We have performed 3-D numerical simulations to investigate the effect of
partial ionization on the process of magnetic flux emergence. In our study, we
have modified the single-fluid MHD equations to include the presence of
neutrals and have performed two basic experiments: one that assumes a fully
ionized plasma (FI case) and one that assumes a partially ionized plasma (PI
case). We find that the PI case brings less dense plasma to and above the solar
surface. Furthermore, we find that partial ionization alters the emerging
magnetic field structure, leading to a different shape of the polarities in the
emerged bipolar regions compared to the FI case. The amount of emerging flux
into the solar atmosphere is larger in the PI case, which has the same initial
plasma beta as the FI case, but a larger initial magnetic field strength. The
expansion of the field above the photosphere occurs relatively earlier in the
PI case, and we confirm that the inclusion of partial ionization reduces
cooling due to adiabatic expansion. However, it does not appear to work as a
heating mechanism for the atmospheric plasma. The performance of these
experiments in three dimensions shows that PI does not prevent the formation of
unstable magnetic structures, which erupt into the outer solar atmosphere.
