3 Description of the observed features

3.1 H$\alpha$ structures

From the beginning of the observational period several features of different types were visible in the analyzed active region: (i) a big spot (S) located a little above the centre of the field of view (see Fig. 2); (ii) filaments: a curved filament or a strong fibril (P1) situated to the north-east of the spot, a long filament (P2) running from the eastern periphery of the spot to the south up to a brightening (B3, described below) and a very southern filament (P3) running from B3 to the south; (iii) brightenings: the most important brightenings in the region were located to the east of the spot. The first brightening (B1) was located in an elongated region at the eastern base of anchorage of fibrils F1. The second one (B2) lays to the west of brightening B1, just between the northern part of filament P2 and the spot S. Brightening B2 was situated in the area of the eastern group of the superpenumbral fine filaments. There were also two significant brightenings B3 and B4 situated to the south of the spot S; (iv) three systems of fibrils, the first one (F1) was situated more or less to the north-east of the spot, the second one (F2) consisting of a group of spot's superpenumbral fibrils was located to the west of the spot. The third system of fibrils (F3) was situated to the south of the spot. The eastern legs of fibrils F3 seem to be anchored in the area close to brightening B3 while the western ones at the other brightening B4.

In some high resolution pictures the brightening B3 displayed well visible fibril structure oriented perpendicularly to the P2 and P3 filaments axes. The brightening B3, located between filaments P2 and P3, seems to be an important region connecting these two filaments.

At 10:29 UT a B3.2 class flare started in the NOAA 7912 active region, to the south-east of the spot S. MSDP images taken in the red and blue wings of the H$\alpha$ line at $\pm 0.78$ Å reveal a highly dynamical response of the chromosphere to the flare. The line of sight velocity maps Fig. 3) show several regions with velocities toward the observer (blueshifts) or from the observer (redshifts), well related to certain structures of the active region. Detailed description of the evolution of the H$\alpha$ structures is given in Sect. 4.1.

3.2 Magnetic fields

The active region NOAA 7912 consisted of a round, leading spot of negative polarity and a dispersed following area of positive polarity (Fig. 4a). A few small parasitic polarity spots disturbed the bipolar structure of the region [14, (van Driel-Gesztelyi et al. 1997;] [12, Schmieder et al. 1998;] [, van Driel-Gesztelyi et al. 1998).] During the 22nd solar activity cycle, leading spots of the south hemisphere had positive polarity. Thus, NOAA 7912 located at S12, did not follow Hale's law, but had inversed polarity. The magnetic field was highly sheared as indicated by the clockwise vortex fibril pattern (see Fig. 2, fibrils systems F1 and F2) around the leading spot at the chromospheric level, as well as by the shape and connectivity of coronal loops, which out of two sets of loops formed a "forward S" shape - both pointing to the presence of positive helicity [9, (Pevtsov et al. 1997).]

  

Figure 4: NOAA 7912 H$\alpha$ structures at 10:58 UT and magnetic fields at 14:59 UT rotated in accordance with the 10:58 UT H$\alpha$ image orientation. a) Magnetic fields and absorbing features seen in the H$\alpha$ line wing (+0.78 Å); b) magnetic fields and bright features seen in the H$\alpha$ line center

  

Figure 5: X-ray loops and H$\alpha$ structures: a) Yohkoh/SXT full resolution image taken at 10:27 UT; b) H$\alpha$ line centre structures at 10:58 UT and SXR contours at 10:27 UT

3.3 X-ray structures

The soft X-ray loops had a highly sheared appearance, which is not surprising in case of an inversed polarity active region. One set of loops connected the south and south-western parts of the penumbra of the leader spot S with the south-east part of the region, where the strongest positive polarity was located (cf. Figs. 1 and 4). Another set of loops connected the north-western penumbra with the weak positive polarity concentrations in the north-east part of the AR (Figs. 4 and 5). The brightest stationary X-ray loop visible to the north of the leading spot S was rooted in the penumbra of the negative magnetic polarity spot and in the H$\alpha$ brightening region of positive magnetic polarity (B1 region in Fig. 2). This bright loop was above the fibrils F1 (Fig. 5b).

At 10:29 UT the GOES satellite recorded an onset of a B3.2 class flare, which occurred to the south-east of the spot S. The flare was accompanied by a spectacular jet event, which reached a length of $2.5 - 3 \ 10^5$ km toward the south [15, (van Driel-Gesztelyi et al. 1998a).] The X-ray jet had its footpoints in the vicinity of the filament P2, where small areas of various polarities created a mixed-polarity region (cf. Figs. 4 and 5).