Investigation of force-freeness of emerging magnetic fields via application of virial theorem to MHD simulations
Force-freeness of emerging magnetic fields is a key to reconstructing invisible coronal
magnetic structure of solar active regions where flares and coronal mass ejections occur. We used virial theorem to investigate the force-freeness in emerging flux regions with different magnetic field configurations, which were obtained by performing a series of magnetohydrodynamic simulations with different degrees of field-line twist applied to an emerging flux tube. We examined
applicability of the theorem to find that the theorem may only be applied to a late phase when an axis of the emerging flux tube exists well above a solar surface (photosphere). We calculated a force-free range of emerging magnetic field in
each of these emerging flux regions to show how the range depends on the degree of field-line twist. The upper limit of the force-free
range tended to increase continuously with time regardless of the degree of field-line twist, whereas the temporal development of the lower limit seemed saturated, especially settling into an order
of photospheric pressure scale height above the photosphere in the emerging flux region produced by the most strongly twisted flux tube. The saturated value of the lower limit tended to decrease (close to the photosphere) as the field-line twist became strong.
Reference
Kang, J. H. & Magara, T.
Publication of Astronomical Society of Japan, 66, 121
