The role of magnetic bald patches in surges and arch filament systems

The short-lived active region (AR) NOAA 7968 was thoroughly observed all along its disk transit (June 3 to 10, 1996) from space and from the ground. During the early stage of its evolution, flux emerged in between the two main polarities and arch filament systems (AFS) were observed to be linked to this emergence. New bipoles and a related surge were observed on June 9. We have modeled the magnetic configuration of AR 7968 using a magnetohydrostatic approach and we have analyzed its topology on June 6 and June 9 in detail. We have found that some of the AFS and the surge were associated with field lines having dips tangent to the photosphere (the so called ``bald patches'', BPs). Two interacting BP separatrices, defining a separator, have been identified in the configuration where these very different events occurred. The observed evolution of the AFS and the surge is consistent with the expected results of magnetic reconnection occuring in this magnetic topology, which is specific to 3D configurations. Previously BPs have been found to be related to filament feet, small flares and transition region brightenings. Our results are evidence of the importance of BPs in a much wider range of phenomena, and show that current layers can be formed and efficiently dissipated in the chromosphere. Fil: Mandrini, Cristina Hemilse. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Démoulin, Pascal. Centre National de la Recherche Scientifique. Observatoire de Paris; Francia