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

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3. The events in the complex AR 7031 - AR 7038

3.1. An overview

Concerning the flare activity, AR 7031 was clearly more important than AR 7038. AR 7031 produced 12 flares, classified as type C or higher in soft X-rays, between January 27 and 31, 1992, and several weakest flares and subflares (Solar Geophysical Data, SGD 575 Part II). SXT observed almost all the flares that occurred in this region, and the Mees CCD Imaging Spectrograph (MCCD, Penn et al. 1991) at MSO recorded their H[FORMULA] emission.

The loop that connects AR 7031 and AR 7038 was bright in soft X-rays at least since its appearance on the disk, on January 24, 1992. By that date no bright plage was seen in H[FORMULA] at the location of AR 7038, while at photospheric level a bipolar flux concentration of very low intensity was observed indicating the growing stage of AR 7038. AR 7038 was reported as a new active region just on January 27. From then on, the new AR started to flare weakly. Between January 28 and 30, 1992, AR 7038 produced 18 subflares and an H[FORMULA] 1F flare on January 29, 13:25 UT. Simultaneously, the emission of the interconnection arc increased. Two sympathetic subflares occurred in AR 7031 around that time, one before (at 13:18 UT) and one after (at 13:28 UT) the 1F flare in AR 7038.

3.2. The flares on January 30, 1992

The 3 most intense flares in the analyzed period were observed on January 30 in AR 7031, at 02:23 UT (classified 2N in H[FORMULA] and M 1.6 in X-rays), 09:35 UT (classified 2B and M 4.9) and at 19:35 UT (classified as 2B and M 2.4). The first two were homologous, but the other one did not happen in the same zone of the AR. We describe here the evolution of these flares using SXT data and H[FORMULA] data from MCCD.

To co-align the data in different wavelengths we have used the sunspots for corregistration. SXT provided, at that time, white-light images through the narrow-band filter, the MCCD images include both the line and the continuous flux, while the magnetograph data allow us to relate higher values of the longitudinal field with white-light spots. Taking a conservative approach, we consider that the overlays among SXT, MCCD and magnetic data are accurate to [FORMULA] [FORMULA] 6".

3.2.1. The data in H[FORMULA]

The H[FORMULA] data have been analyzed using mainly co-aligned movies. In Figs. 2 we can see H[FORMULA] images for the main flares on January 30, 1992. For comparison, the preceding spot (positive) can easily be superimposed with the main positive polarity in Fig. 1c. The emission for the flare at 02:23 UT, to which from now on we will call "A ", starts at the North of the preceding spot over the inversion line between the small negative and positive flux concentrations (see Fig. 1c). Almost all the flares in this AR started at this place. MCCD shows some very elongated H[FORMULA] dark fibrils extending from East to West in the northern portion of the AR prior to the flare onset. This orientation is coherent with the observed transverse field and indicates the presence of magnetic shear in the AR. As the flare evolved, the emission, first localized, expands towards the North and it goes out of the MCCD field of view (see e.g. Fig. 2a). We observe that material flows towards the North East starting probably at the same place where the flare erupted. The trajectory followed by this material agrees with the shape of an arcade observed in soft X-rays (see next section). During the main phase, dark elongated loops are seen at the North of two main polarities (pointed by arrows in Fig. 2b). The flare at 09:35 UT, to which from now on we will call "A' ", evolves in a similar way, although we don't observe the same expansion of the H[FORMULA] emission.

[FIGURE] Fig. 2a-d. Emission in H[FORMULA] for the flares A and B on January 30, 1992. a and b Flare A at 02:22 UT and at 02:37 UT, respectively. c and d Flare B at 19:44 UT and 20:44 UT, respectively. In a the arrow points to the place where we observe the H[FORMULA] emission expanding towards the North East, while in b they point to the long H[FORMULA] loops (which extend partially out of the observed frame).

The two-ribbon flare at 19:35 UT, that we will call "B " started emitting in a very elongated band, following the shape of the main eastern inversion line, and a shorter one, located almost over the positive spot (Fig. 2c). After the impulsive phase, the eastern band widened and extended; while the western one seemed to rotate counter clockwise and evolved into an elongated brightening very close to the inversion line at the North East of the main positive polarity (Fig. 2d).

3.2.2. The data in soft X-rays

We have used the SXT data to investigate the morphology of the hot flaring plasma. Images were taken in the four SXT filters: Al0.1, AlMg, Al12 and Be119, listed in increasing order of energies. The response of these filters lies in the range 1-2 keV. During the three flares, images were taken both in flare and quiet mode and in the three different available resolutions. Images shown in Figs. 3 were taken using the best resolution (2.5") provided by the instrument and the smaller FOV (64[FORMULA]64 pixels in size). In these figures the magnetogram obtained on January 30 has been superimposed.

[FIGURE] Fig. 3a-d. Soft X-ray (negative) images using the Al12 filter and the best resolution of SXT: a flare A' at 09:50 UT; b flare A' as a composition of two images recorded at 09:55 and 10:00 UT; c flare B at 19:47 UT; d flare B at 20:04 UT. The longitudinal magnetogram on January 30 (contours of [FORMULA]  G) has been superimposed. In these and the following figures heliographic North is up and West is to the right.

The soft X-ray emission from flares A and A' is concentrated in the same zone as the H[FORMULA] brightenings. It started on the same portion of the inversion line between the small negative and positive flux concentrations, at the North of the main positive polarity (Fig. 3a). As the flares evolved, the initial brightening splitted in two and we can also see a large arcade with the same orientation of the dark H[FORMULA] loops, clearly connecting this zone with the northeastern portion of the main negative polarity (Fig. 3b). The difference between flare A and A' is that in A we can see the arcade emitting faintly, while in A' it is as bright as the main flaring kernel.

Flare B starts with a single intense brightening at the South East of the positive spot (Fig. 3c); as the flare evolves, we observe two nuclei (Fig. 3d) probably corresponding to the footpoints of a set of loops. Fig. 4 shows a temporal sequence of SXT images for flare B in the Al0.1 filter at quarter resolution (9.8"). The total FOV is the largest provided by the SXT partial frame (covering four times the smallest FOV). We see that when the emission reaches its maximum another zone, where AR 7038 is located, emits in sympathy with AR 7031. The interconnection arc is clearly seen connecting both ARs. This arc was far from being quiet, we first identify a bright loop, as shown in Fig. 4 a at 19:46 UT. As the time proceeds, the emission seems to bend towards the South and afterwards we can identify several independent loops (Fig. 4b). The northwestern zone of AR 7031, where flares A and A' developed, was also seen emitting in X-rays. From this time, and until 19:56 UT, all SXT images are saturated. In Fig. 4c, the loop is again observed having the shape it had before the flare.

[FIGURE] Fig. 4a-d. Temporary sequence of SXT images (largest FOV in Al0.1) for flare B at: 19:46 UT (a ), 19:50 UT (b ) and 20:06 UT (c ). The flare appears saturated so the much fainter arc can be observed. d corresponds to the magnetic field model of the combined regions. Some field lines following the shape of the interconnecting arc have been added. These lines have their footpoints at the computed QSLs (see text), which are shown at photospheric level as thick continuous lines. The isocontour values of [FORMULA] are [FORMULA] G. The box axes are labelled in Mm.

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Online publication: December 11, 2000