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Astron. Astrophys. 363, 779-788 (2000)

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5. Conclusion

The aim of present study is to further quantify the energy release processes involved in flares. We focus here on the understanding of the link between the observed flare emissions (in H[FORMULA] and soft X-rays) and the magnetic configuration (photospheric magnetograms and coronal computed field) in two interacting active regions (ARs 7031 and 7038), observed on January 1992. Previous studies of several flares (see references in the Introduction) have shown that energy release occurs at the location of quasi-separatrice layers (QSLs) in different kinds of magnetic configurations. In this work we confirm these results with additional examples. Within the spatial resolution of the observations and the limit of the magnetic field model we have found that flare H[FORMULA] kernels and soft X-ray brightenings lie in the close vicinity of the computed QSLs (see Fig. 5). For all flares, as well as for a soft X-ray arc linking the two ARs, we are able to identify the two magnetic bipoles which interact and the magnetic connectivies before and after reconnection (see Fig. 6). Only reconnected loops are expected to be seen in the observations since pre-reconnected loops are not dense enough. During reconnection, the energy released is enough to evaporate locally the top of the chromosphere and so, the emission of the reconnected loops is strongly enhanced. We observe such pairs of X-ray loops in the three flare events. We then extend our conclusions to the plausible origin of flares and of the interconnecting arc as summarised below.

The computation of QSLs in the extrapolated field from a given magnetogram allows us to find the possible places of reconnection in the magnetic configuration as we have shown for the flares in AR 7031 (Fig. 5a-d). However, the computation of the magnetic topology at a given time cannot precise which portions of the QSLs are, or will be, involved in reconnection. These locations are indeed determined by the evolution of the magnetic configuration; that is to say, the evolution determines basically where the stronger current layers are formed and where the free magnetic energy is located. We have so far found two different kinds of magnetic field evolutions in flaring regions: emergence of new field (e.g. Mandrini et al. 1996) and photospheric horizontal displacements of magnetic polarities (e.g. Gaizauskas et al. 1998). Both cases are present in AR 7031: flux emergence for flares A and A' and photospheric flux displacements for flare B , respectively. The presence of a long-duration faint soft X-ray arc between AR 7031 and AR 7038 belongs to the second kind: reconnection between magnetic loops belonging to the two ARs is driven mainly by the long term rotation (of at least a week) of AR 7038. This region emerged in a North-South direction, and rotated in such a way to adopt the Hale orientation corresponding to the southern hemisphere for cycle 22 (see SGD 571 Part I).

A third possibility to drive magnetic reconnection is an evolution of the magnetic configuration as a result of another nearby magnetic reconnection process. Indeed, three "sympathetic" flaring processes were observed associated to flare B . First, the initial energy release in flare B is associated to a set of QSLs (Fig. 5c,d), then it stimulates energy release in nearby QSLs: the eastern elongated ribbon extends North and the western ribbon shifts North to the nearby QSL (Fig. 5e,f). Second, flare B stimulated energy release at the QSLs of the morning flares (A , A' ). Finally, flare B , which is located in the magnetic configuration of AR 7031, stimulated magnetic reconnection between the two ARs (7031 and 7038), forming a brighter soft X-ray interconnecting arc.

This study indicates also that a probable origin of observed soft X-ray interconnecting arcs between ARs can be a long-term magnetic reconnection process. AR 7038 emerged near AR 7031 in such a way that the positive polarities of both ARs were close by. As AR 7038 turned towards the Hale orientation, the magnetic connectivity between the ARs became more favourable since this implies a lower energetic field; then, the rotation of AR 7038 leads to a progressive reconnection between the magnetic configurations of the two ARs. This is the origin of the weak soft X-ray arc observed since January 24, 1992. When flare B occured, it stimulated reconnection forming a brighter soft X-rays arc (see above). In the case of the arc, we were unable to find in X-rays the other reconnected loop, both because the event is weak and because such a loop is expected to be very extended; these two reasons contribute to a low plasma density, so to a low X-ray enhancement.

Then, the present analysis confirms previous results which conclude that QSLs are the places where magnetic reconnection can occur. Which of the many QSLs that are present in a configuration will be the site of magnetic reconnection depends strongly on the magnetic field evolution. In the analysed regions, three drivers are present at different locations: flux emergence, photospheric motions and sympathetic flaring. We find that the same process, magnetic reconnection, is at the origin of flaring inside one AR and of the formation of the interconnecting arc between the two ARs.

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© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000