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Astron. Astrophys. 336, 359-366 (1998)

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3. Evolution of magnetic configuration

From Oct. 26 to Oct. 27, considerable changes were taking place in the magnetic configuration, which was characterized by the emerging flux and the motions of spots. Two vector magnetograms on Oct. 26 and 27 are compared in Fig. 1 ; the latter was taken just before the onset of the 01:44UT flare. Seen in the figure, the major spots N and S were present and made the main poles in the active region. East to the N spot, a group of satellite spots made up the S2 polarity. The inversion line of the line-of-sight field is drawn. For convenience, we name the segment of the inversion line that separates N from S as M1 and the segment separating N from S2 as M2.

[FIGURE] Fig. 1. The photospheric vector magnetograms (contours) taken on a  Oct. 26, 03:34UT and b  Oct. 27, 01:44UT superposed on the white light images showing the spots. The white contours in both figures demonstrate the intensity of the longitudinal field (levels = [FORMULA]500, 1500, 2500, and 3500 Gauss); solid lines show positive field and dashed lines show the negative field. The short arrows indicate the transverse field; the length of each arrow gives the magnitude of the field and the arrow gives the direction. The longitudinal inversion line is plotted in thick dark lines and the grey, thick curves indicate the magnetic connection patterns and the white straight segment drawn by hand indicate L3 (see the text). The field of view (FOV) is 170[FORMULA]170".

On Oct. 26 (Fig. 1 a), due to the emerging flux along the inversion line, the non-potential feature was already strong in the active region, as manifested by the transverse field which was almost aligned with the inversion line. The shear angle thus defined by Hagyard et al. (1984) was near 90o along both M1 and M2. The alignment of transverse field lines also defines the magnetic connectivity in this region, which was roughly divided into two patterns by M1. The first pattern, labeled as L1 in Fig. 1 a, connected the minor spots of N1 and S1 and the second pattern L2 connected N[FORMULA] with S2. It is distinctive that emerging flux along the inversion line made up the connectivity patterns in this region. The connectivity patterns shown in Fig. 1 are plotted by tracing the vector of transverse field lines. They can be supposed as magnetic loops projected on the photosphere (Liu et al. 1998; Wang et al. 1997), which, in Sect. 5, will be further confirmed by the comparison with the flare configuration in space.

The emerging flux system was fully developed on Oct. 27 (Fig. 1 b). Compared with the magnetogram taken one day before, several new spots, N3, S3, and S4 appeared, and N1 were enhanced evidently. The emerging magnetic structures brought in new connectivity patterns as well. Seen in Fig. 1 b, the alignment of the transverse field along M1 was changed, perhaps due to the appearance of a new pattern L3 (drawn by hand in thick white lines in the figure), which connected N3 with S3. With the emergence of L3, the shape of M1 was different from one day before and the spots N and N1 were pushed southwest. As a result, L1 was twisted and the magnetic structure of the spot N rotated clockwisely by an angle of about 20o. For the pattern L2, not much change was taking place and the magnetic shear here remained high, but the spots of S2 group showed a running-away motion. Besides, between N1 and S4, yet another pattern L4 was formed, which was not seen one day before.

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998