MAGNETIC-FIELD TOPOLOGY OF AN ACTIVE REGION

The global 3-dimensional topology of the magnetic field of the weakly flaring active region AR 5420 in the early phase has been estimated using a series of photospheric longitudinal and vector magnetograms together with H-alpha-filtergrams and white-light images. As a physical model for this task, we have used the assumption of a force-free magnetic field character. An arcade-like structure of the magnetic field has been identified in the preflare phase with an apparent sheared configuration already present in the current-free field case. This system might have been an important source of generation of a field-aligned current system for the eruptions that followed. That was, in particular, suggested by a significant extension of the active region area in the observation period from June 22 to 25, 1984, that resulted in a stronger shear of the detected arcade-like system. The atmospheric electric currents generated in such a way and the transition triggered by a large-amplitude perturbation generated by new flux emergences on June 23 at this place together might be the reason of the sequence of flares observed. Despite the changes in the H-alpha-structure, the global force-free character of the field did not change significantly over the time of observations. The invariance (in the first order) of the force-free (nearly current-free) character of the magnetic field is consistent with the occurrence of relatively small flares only.