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

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5. Spatial correlation between the flare and magnetic configuration

5.1. Flare identity of the magnetic loops

The 1N/M1.1 flare occurred at 01:44UT on October 27. A comprehensive study of the coordinated observations of the event was given in Paper I. From the observations, the configuration of the flare from the low chromosphere up to the corona can be established, which may serve as a means to investigate the magnetic configuration in the space. Fig. 3 gives the coaligned observations of the flare in SXR and [FORMULA] images, superposed on the photospheric magnetogram taken on Oct. 27. The magnetic connection patterns are drawn in Fig. 4 and are found well identified with the flare regions.

[FIGURE] Fig. 3. The magnetic longitudinal field (contours) on Oct. 27, 01:44UT superposed on the SXR images (grey map) on a  Oct. 27, 01:44:33UT and b  Oct. 27, 01:45:31UT taken by Yohkoh SXT from the filter Al.1. To illustrate clearly the structures at different times, the two SXR images are not shown in the same grey scale. The contour levels are the same as in Fig. 1. The thick dark line indicates the longitudinal field inversion line. In b , the [FORMULA] flare kernels (01:55:26UT on Oct. 27) are plotted in thick grey contours. The field of view (FOV) is 160[FORMULA]160".

[FIGURE] Fig. 4. Coalignment of SXR images on a  Oct. 27, 01:44:33UT, b  Oct. 27, 01:45:31UT taken by Yohkoh SXT from the filter Al.1 with the vertical current distribution (contour) on Oct. 27, 01:44UT and with the magnetic connection patterns (thick grey curves). The white straight segment drawn by hand indicates L3 (see the text). The solid (dashed) lines represent the current flowing out of (into) the photosphere. The contour levels of both figures are [FORMULA]0.6, 1.2, 1.8, 2.4 [FORMULA]10-2A/m2. In both figures,the longitudinal inversion line is drawn in thick dark line and the current reverse line in thick grey line. In b , the [FORMULA] flare is overlapped in thick grey contours. The FOV is the same as that in Fig. 3.

A compact flaring loop (loop AB in Paper I) is prominently shown in the SXR images. Co-ordinated observations from other wavelengths show that A and B, the two ends of the loop, give compact HXR (up to 53keV) and [FORMULA] emissions, suggesting that they make the foot points of the SXR loop. The loop is well coaligned with the pattern L3, which emerged on this day along the magnetic inversion line M1. At the onset of the flare (Fig. 3 a), three bright loci in SXR emission along this pattern seem to indicate the two emitting feet (A and B) and the bright top (D) of the L3 loop. Similarly, the magnetic connection pattern L2 finds its coronal counterpart in well coaligned SXR emission streak, which runs across the inversion line M2 and heads into an extended foot area (region C in Paper I) in S2 region. The co-alignment seen for this pattern also suggests that possibly L2 and L3 share a common foot which is foot A. For the pattern L1, the alignment of SXR emission even portrays the feature of the twist along this loop, which is rooted in E and F and seems to interact with the shorter loop L3 at the site of D. This point will be discussed in more details in Sect. 6. The coalignment in L4 just gives the last example in this comparison.

For all patterns, [FORMULA] emission is located at the ends of SXR streaks, or the footpoints of the loops in the chromosphere (seen in Fig. 3 b). The above co-ordination is a comparison of the projections of the magnetic loop structure in the photosphere, chromosphere and corona, which offers an indirect way to view vividly the spatial configuration of the magnetic field, specifically, the magnetic structures associated with the energy storing, releasing or depositing sites where flares are produced.

5.2. Vertical current system and flares

In Fig. 4, we further demonstrate the spatial relationship between the flare regions and the vertical current distribution on October 27 when the flare occurred. It is seen that the feet of the flaring loop L3 in SXR are located immediately on the two sides of the current reverse line, which is virtually coaligned with the magnetic inversion line here (M1)  (seen in Fig. 4 a). One foot (A) is found just at the edge of the strong current concentration [FORMULA], while the other (B) is almost above the reverse line of a current pair. However, the emergence of this bipole is continuously accompanied by the input of magnetic shear from below the photosphere, and the existence of horizontal current inside the loop L3 is probable.

The flaring loop L1 travels from the positive current concentration to the negative one, connecting the current pair of [FORMULA]-[FORMULA], and the two feet E and F are well coaligned with it; one foot (E) is exactly located in the vertical current maximum of [FORMULA], while the other one (F) is in between the maxima of two current concentrations [FORMULA] and [FORMULA] (seen in Fig. 4 b). Since the magnetic field in the chromosphere and corona is generally regarded free from force (Metcalf et al. 1995), the pattern L1, given its long expanse, can be well extending to the corona and carrying the field-aligned current whose footpoints are perched in the photosphere. The pattern L2 also straddles the vertical current reverse line around M2 from [FORMULA] to [FORMULA], but the site of C is not a region of strong current concentration.

The above comparison offers the information that the flare may originate from the instability at the interface between two current carrying loops, one being the pre-existing loop L1, and the other being the newly emerging and strongly sheared loop L3.

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998