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 could 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 and found 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 then calculated a force-free range of emerging magnetic field for
each of these emerging flux regions to see how the range depends on the degree of field-line twist. The upper limit of the force-free
range tends to increase continuously with time regardless of the degree of field-line twist, whereas the temporal development of the lower limit is rather saturated, and the saturation value tends to decrease (close to the photosphere) as the field-line twist becomes strong. In the emerging flux region produced by the most strongly twisted flux tube, the value settled down into an order
of photospheric pressure scale height above the photosphere.
Reference
Kang, J. H. & Magara, T.
Publication of Astronomical Society of Japan, 66, 121
