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

Investigation of Force-Freeness of Solar Emerging Magnetic Field via Application of the Virial Theorem to MHD Simulations