Magnetic loops: A comparison of extrapolations from the photosphere with chromospheric measurements

Direct observations of chromospheric and coronal magnetic fields are difficult and usually one has to reconstruct the 3D magnetic field from photospheric measurements. The extrapolation method depends on assumptions regarding the electric current flow in the coronal plasma. Due to the low plasma beta it is justified to assume that the currents are parallel or antiparallel to the magnetic field, the so called force-free assumption. Simplifications of the in general non-linear force-free magnetic model are linear force-free and potential fields. We compare the different magnetic field models (potential, linear force-free, non-linear force-free) with the observationally inferred structure of magnetic loops in a newly developed active region. This is the first time that the reconstructed 3D-topology of the magnetic field is available to test the extrapolations. This comparison reveals that a potential field extrapolation is not suitable for a reconstruction of the magnetic field in this young, developing active region. The inclusion of field-line-parallel electric currents gives much better results. Furthermore, a non-linear force-free computation reproduces the observations better than the linear force-free approximation.