A Comparison Between Nonlinear Force-Free Field and Potential Field   Models Using Full-Disk SDO/HMI Magnetogram
  Measurements of magnetic fields and electric currents in the pre-eruptive corona are crucial to study solar eruptive phenomena, like flare and coronal mass ejections(CMEs). However, spectro-polarimetric measurements of certain photospheric lines permit a determination of the vector magnetic field at the photosphere. Thus, substantial collection of magnetograms relate to the photospheric surface field only. Numerical modeling is carried out by applying state-of-the-art nonlinear force-free field (NLFFF) reconstruction. Cartesian nonlinear force-free field (NLFFF) codes are not well suited for larger domains, since the spherical nature of the solar surface cannot be neglected when the field of view is large. One of the most significant results of Solar Dynamic Observatory (SDO) mission to date has been repeated observations of large, almost global scale events in which large scale connection between active regions may play fundamental role. Therefore, it appears prudent to implement a NLFFF procedure in spherical geometry for use when large scale boundary data are available, such as from the Helioseismic and Magnetic Imager (HMI) on board SDO. In this work, we model the coronal magnetic field above multiple active regions with the help of a potential field and a NLFFF extrapolation codes in a full-disk using HMI data as a boundary conditions. We compare projections of the resulting magnetic field lines solutions with full-disk coronal images from the Atmospheric Imaging Assembly (SDO/AIA) for both models. This study has found that the NLFFF model reconstructs the magnetic configuration better than the potential field model. We have concluded that much of trans-equatorial loops connecting the two solar hemispheres are current-free. 
Tilaye Tadesse, T. Wiegelmann, B.Inhester, K.Olson, P. J. MacNeice, A.   Pevtsov
astro-ph.SR