SOLARNET Conference: The Many Scales of the Magnetic Sun

A new method is developed for construction of coronal force-free fields. This method employs a poloidal-toroidal representation of magnetic field, not only to ensure the divergence-free condition, but to straightforwardly implement the boundary conditions Bn and Jn at the coronal base. The performance of the computational code has been compared with that of other existing codes and is found to excel others, especially in reproducing peculiar geometrical features of the magnetic field such as bold patches, quasi-separatrix layers (QSL) and hyperbolic flux tubes. We have used this code to identify the change of magnetic field connectivity responsible for each of the solar eruptions (two flares and a CME) that took place in AR 11974. The two flares are found to be caused by QSL reconnections, but the subsequent reconnections at nearby null-points played an important role in the overall evolution of the active region. A numerical solution is also constructed for a small local region containing a sunspot and reveals the field structure in the vicinity of the light bridge lying in the umbra.