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

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2. Magnetic field data

2.1. The magnetic evolution of AR 7031

AR 7031 traversed the solar disk during the last week of January and the first week of February, 1992. Mees Solar Observatory (MSO) obtained the magnetic data using the Mees Stokes Polarimeter (Mickey 1985). On January 30, the day in which the three most important flares happened, the AR was located at S07 W06 on the solar disk.

In Fig. 1 we present the magnetogram rasters corresponding to January 28, 29, 30 and 31, 1992. The time identification is the starting time of the raster, although the scan took up to 3 hours to complete a full magnetogram. The line measured in this opportunity was Fe I ([FORMULA] 6301). The spatial resolution was 5.7", covering a field of view (FOV) of (5'[FORMULA]7'). The highest spatial resolution magnetograms (2.8"), were obtained for single spots only, so we have used them to visualize some morphological details in smaller areas of the AR, and the direction of the transverse field.

[FIGURE] Fig. 1a-d. Longitudinal magnetograms corresponding to AR 7031 on a January 28 (22:17 UT); b January 29 (01:48 UT); c January 30 (18:06 UT) and d January 31 (18:54 UT), 1992. Full (dashed) lines represent positive (negative) values of [FORMULA] G. The axes are expressed in magnetogram pixels. In c we show the transverse magnetic field above 200 G, the length of the arrows is proportional to the logarithmic value of the field intensity. In this figure and in Fig. 2, terrestrial North is up and West is to the right.

AR 7031 is mainly bipolar (see Fig. 1), with a positive and very concentrated preceding spot and a diffuse and more extended following polarity. The strongest positive longitudinal region remained almost unchanged during these days, while some new positive flux was seen emerging in the weakest field region towards the East of the main spot. The negative region underwent several modifications. Negative magnetic flux emerged towards the South between January 28 and 29, and also towards the East between January 29 and 30. By January 31, the negative flux in the southeastern portion of the AR had started to decrease. On the other hand, a new bipole started to emerge at the North of the main positive spot on the 28 and is clearly seen on the 30 and 31. The transverse field (see Fig. 1c) looks in general potential. However, there are two localized zones of non-negligible shear related to the events studied in this paper; these are: a) the eastern portion of the longitudinal inversion line between the main positive and negative polarities and b) the longitudinal inversion line between the new bipole negative polarity and the main positive spot. The vertical current density, obtained from the observed transverse field, is above [FORMULA] only at the location named in b). At the place named in a), the tranverse field is too low to obtain reliable current density measurements; in this case the presence of shear in the field is mainly indicated by the direction of the H[FORMULA] fibrils.

2.2. The combined magnetic field of AR 7031 and 7038

We have found in the SXT largest FOV an interconnection arc linking AR 7031 with another region (AR 7038) (see Sect. 3.2.2). We then use the full-disk longitudinal magnetograms obtained with the Vacuum Telescope at Kitt Peak National Observatory (KPNO, Livingston et al. 1976), in which both ARs can be observed. However, the values of the field in regions where [FORMULA] G are uncertain in KPNO magnetic maps, probably due to a problem of stray light in the telescope (Harvey J., private communication); for that reason, we have used the MSO magnetograms in our study, although none of the magnetograms have both ARs in their field of view. MSO took a magnetogram of AR 7038 on January 30 at 21:54 UT. AR 7038 is a bipolar region smaller and with a lower intensity field than AR 7031. In this magnetogram we observe an important departure from potentiality but, as the field intensity is low, the transverse field measurements are hardly above the noise level.

As we intend to compare the location of the interconnection arc with the topological structures of the field (see Sect. 4), we have to combine both ARs in the same map. To do so we have to take into account: a) the relative location of the ARs (for this purpose the magnetogram of KPNO was used), b) the differential rotation of the Sun since both ARs were located at different latitudes, c) the time and the time interval during which the magnetograms were taken. This last point is the issue because, although we know the starting time of the observation, the magnetograph took more than 2 hours to scan each complete field. AR 7031 was located at S07W06 and AR 7038 at S12W17. The magnetograph scans the field following a zig-zag path, beginning with a pixel at the North East. Taking this into account, and considering the total duration of the scan, we can estimate the time when the southwestest pixel of the northern region (AR 7031) was taken. After that, we corrected by differential rotation and we found in this way the coordinates (latitude and longitude) of the nearest pixels of both regions. As in the full disk KPNO magnetogram the ARs appeared isolated and surrounded by very low field concentrations, we created a matrix where both ARs were placed at their "true" locations and we filled with 0 G the pixels without measurements (Fig. 4d).

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© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000