A C-level flare observed in an arch filament system: reconnection between pre-existing and emerging field lines?

Context. Observations show that solar flares are often caused by the emergence of new magnetic flux from subphotospheric layers and by the interaction of the rising field lines with the ambient magnetic field. In this framework, recent theoretical models of magnetic flux emergence have investigated the effectiveness of magnetic reconnection as a function of the old and new flux system relative orientations. Aims. We aim to compare phenomena that occurred in an active region, before and during a small flare, with the effects of magnetic reconnection between nearly parallel magnetic field lines, foreseen by these models. Methods. We analyzed high resolution photospheric and chromospheric data acquired during a coordinated observational campaign performed with the THEMIS telescope in IPM mode, as well as MDI magnetograms and TRACE 1600 and 171 Å images, to investigate the dynamics and the magnetic configuration of the active region hosting the flare. Results. An emerging arch filament system (AFS) was observed in the area between the two main sunspots: it showed typical upward motion at the arch tops and plasma downward motion at the footpoints. A C-level flare, characterized by a factor of 3 peak enhancement in the GOES X-ray emission with respect to the pre-event background, occurred in this zone, where the configuration of the emerging magnetic field lines showed a small (∼$12^{\circ}$) relative inclination with respect to the old flux system. Conclusions. In an active region (age ≥6 days) a new magnetic flux bundle emerged between the two main polarity spots. It gave rise to the formation of pores in the photosphere and to an AFS in the chromosphere. The interaction between the new and the pre-existing field lines, characterized by a small relative inclination, might have caused a weak reconnection process and given rise to the C-level flare. This result is in broad agreement with numerical simulations predicting very limited reconnection when the two flux systems have an almost parallel orientation