CONICET | Buscador de Institutos y Recursos Humanos

Through the determination of the magnetic field topology, we focus this study on the flaring activity occurring in a nest of five bipoles. These bipoles belonged to a ``great complex of activity" (Mc Math 15314). We are interested in the largest and best observed flare on May 28, 1978, a class 1B/X1, occurring in a delta spot. The usual spreading action during the growth of the bipoles lead to the formation of a delta-configuration: the preceding and following spots of the two inner regions overlapped into a single penumbra. In this case, the spots approached continuously during five days. We first compute the coronal magnetic field using subphotospheric sources to model the longitudinal magnetograms and derive the location of separatrices, for May 27 and 28. Quasi-separatrix layers are a generalization of separatrices for any magnetic field configuration, these are thin volumes where the connectivity of field lines changes drastically. We calculate them using a model of the field obtained by extrapolation of the observations based on the fast Fourier transform method. With both approaches, we show that the plage brightenings, on the 27, and the flare kernels, on the 28, are located at the intersection of separatrices with the photosphere. This confirms the importance of the magnetic field topology for solar flares. Taking into account Halpha observations and the magnetic field modelling, we conclude that energy is released in the solar corona, between the preceding and following spots, in the region of the separator. Bright and dark post-flare loops follow the location of separatrices and quasi-separatrix layers. This is consistent with magnetic reconnection models. Our results confirm the importance of the field topology for the comprehension of the active phenomena and allow us to characterize the energy release mechanism that is at their origin.

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