Context.It is now possible to distinguish between two main models describing the mechanisms responsible for eruptive flares : the standard model, which assumes that most of the energy is released, by magnetic reconnection, in the region hosting the core of a sheared magnetic field, and the breakout model, which assumes reconnection occurs at first in a magnetic arcade overlaying the eruptive features. Aims. We analyze the phenomena observed in NOAA 10486 before and during an X17.2 flare that occurred on 2003 October 28, to study the relationship between the pre-flare and flare phases and determine which model is the most suitable for interpreting this event.Methods. We performed an analysis of multiwavelength data set available for the event using radio data (0.8-4.5 GHz), images in the visible range (WL and H α ), EUV images (1600 and 195 Å), and X-ray data, as well as MDI longitudinal magnetograms. We determined the temporal sequence of events occurring before and during the X17.2 flare and the magnetic field configuration in the linear force-free field approximation. Results. The active region was characterized by a multiple arcade configuration and the X17.2 flare was preceded, by ∼2 h, by the partial eruption of one filament. This eruption caused reconnection at null points located in the low atmosphere and a decrease in magnetic tension in the coronal field lines overlaying other filaments present in the active region. As a consequence, these filaments were destabilized and the X17.2 flare occurred. Conclusions. The phenomena observed in NOAA 10486 before and during the X17.2 flare cannot be explained by a simple scenario such as the standard or breakout model, but instead in terms of a so-called domino effect, involving a sequence of destabilizing processes that triggered the flare.

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Abstract. Using data acquired during an observational campaign carried out at the THEMIS telescope in IPM mode, coordinated with other ground-and space-based instruments (IOACT, TRACE, EIT/SOHO, MDI/SOHO), we have analyzed the first evolutionary phases of a recurrent active region (NOAA 10050), in order to study the morphology and dynamics of its magnetic structures during their emergence and early development. The main result obtained from this analysis concerns the dynamic evolution of the arch filament system (AFS) crossing the polarity inversion line: the line of sight velocities determined from Doppler measurements confirm that the loops forming the AFS show an upward motion at their tops and a downward motion at their extremities, but also indicate that the upward motion decreases while the active region develops. Moreover, it has been found that, within the limits of the temporal cadence and spatial resolution of the instruments used, the first evidence of the active region formation is initially observed in the transition region and lower corona, and later on (i.e. after about 6 h) in the inner layers (chromosphere and photosphere). Another interesting result concerns the analysis of the magnetograms, indicating that the initial increase in the magnetic flux seems to be synchronous with the appearance od the active region appearance in the transition region and lower corona, and that the rate of increase of the magnetic flux during the formation of the active region is not constant, but is steeper at the beginning (i.e. during the first 150 h) than in the following period. All these results may indicate the presence of some mechanism that decelerates the magnetic flux emergence as more and more flux tubes rise towards higher atmospheric layers. Finally, we would like to stress the observed asymmetries between the preceding and the following sides of NOAA 10050: the p-side is more extented than the f-side, the p-side moves forward from the initial outbreak position much faster than the f-side recedes; the AFS f-side exhibits higher downflows than the p-side.

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• ) 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.

Abstract. In this paper we present the results obtained from a statistical analysis carried out by correlating sunspot-group data collected at the INAF-Catania Astrophysical Observatory and in the NOAA reports with data on M and X flares obtained by the GOES-8 satellite in the soft X-ray range during the period January 1996-June 2003. These results allow us to provide a quantitative estimate of the parameters typical for an active region with very energetic flares. Moreover, the analysis of the flare productivity as a function of the group evolutionary stage indicates that the flaring probability of sunspots slightly increases with the spot age during the first passage across the solar disk, and that flaring groups are characterized by longer lifetimes than non-flaring ones.