  Force-freeness of a solar magnetic field is a key to reconstructing invisible
coronal magnetic structure of an emerging flux region on the Sun where active
phenomena such as flares and coronal mass ejections frequently occur. We have
performed magnetohydrodynamic (MHD) simulations which are adjusted to
investigate force-freeness of an emerging magnetic field by using the virial
theorem. Our focus is on how the force-free range of an emerging flux region
develops and how it depends on the twist of a pre-emerged magnetic field. As an
emerging flux region evolves, the upper limit of the force-free range
continuously increases while the lower limit is asymptotically reduced to the
order of a photospheric pressure scale height above the solar surface. As the
twist becomes small the lower limit increases and then seems to be saturated.
We also discuss the applicability of the virial theorem to an evolving magnetic
structure on the Sun.
