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Astron. Astrophys. 342, 563-574 (1999)

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2. The loop structures

Figs. 1 and 2 show the full-sun NIXT and SXT images, respectively, taken virtually simultaneously on April 12, 1993. The NIXT data were taken between 17:17 and 17:23 UT, the SXT data between 17:11 and 17:28 UT. The NIXT image is made of 2000[FORMULA]2000 pixels of 1.2"[FORMULA]1.2", the SXT image of 512[FORMULA]512 pixels of 4.91"[FORMULA]4.91". The SXT image results from a combination of two single exposures of 78 and 2668 ms in the Al.1 filter (Tsuneta et al. 1991, see Sect. 4 for more details). The images have been aligned by using a fitting procedure applied to the disk limbs.

[FIGURE] Fig. 1. NIXT solar image of April 12, 1993: the insets expand the regions where loop structures have been selected for analysis. The brightness color scales, indicated at the bottom for all panels (full disk and subregions), are logarithmic (cgs units).

[FIGURE] Fig. 2. Yohkoh/SXT solar image of April 12, 1993: the insets expand the same regions as the corresponding insets in Fig. 1. As in Fig. 1, the color scales are logarithmic (DN/s).

As described by Yoshida et al. (1995), many loop structures are clearly visible in the SXT image, both inside and outside the large active regions close to the disk center. There are two other active regions, one on the south-west and the other, very large, on the east limb of the disk. The loop structures are less evident in the NIXT image, although some of them are clearly visible inside and close to the active region at the center of the disk.

The loops to which we apply our pressure diagnostics have been selected from the NIXT full disk image (Fig. 1). The identification of loop structures from X-ray images is, in general, non-trivial; since the Skylab era (e.g. Vaiana 1976) it has become clear the need to identify loops with the aid of magnetograms. Therefore we have taken advantage of magnetograms to identify the loop footpoints. The magnetograms were taken by P. Hartmann of Kitt Peak Solar Observatory between 15:11 and 16:06 UT, about one hour before the X-ray images. We have corrected for the shift due to solar rotation and aligned the magnetograms to the images. We have then selected rectangular regions including single loop structures with the following two basic requirements:

  1. the candidate loop structures in the NIXT image have to be isolated from other structures;

  2. the extremes of the loop structures in the X-ray image must correspond to opposite polarity spots in the magnetograms.

The need for an unequivocal identification of the loop structures may impose a bias to select bright, and therefore preferentially high pressure, loops but we will show that our small sample includes pressure values spanning more than a decade, from values typical of quiet regions to those of active regions. We have selected five regions, which we will identify in the following with A , B , C , D , E  in decreasing order of solar latitude, as shown Fig. 1 and, in more detail, in Fig. 3. Their characteristics, as derived from the analysis of the NIXT image, are reported in Table 1, together with relevant physical parameters derived according to the procedures illustrated in Sect. 3.

[FIGURE] Fig. 3. Analyzed loop structures: the first and second columns show the insets of Figs. 1 (NIXT image) and 2 (SXT image) respectively, the third column the corresponding magnetograms (courtesy of NSO/Kitt Peak, from a collaboration of NSF/NOAO, NASA/GSFC and NOAA/SEL). In the NIXT frames the solid strips bound the inferred isolated loops, divided into rectangular sectors for the analysis. The corresponding strips and the appropriate division into sectors are marked also in the SXT insets. In the NIXT frames, the arrows indicate the positive direction of the NIXT brightness profiles in Fig. 5. Contour plots of NIXT brightness are shown in the magnetograms. The color scales of the magnetograms are linear (blue negative, green neutral, red positive) and range between -146 G and 146 G for regions A and C and between -73 G and 73 G for regions B, D, and E.


Table 1. Analysis of loop structures imaged with NIXT. Notes: [FORMULA] Coordinates of the center of the selected regions; [FORMULA] Distance of the footpoints; [FORMULA] Loop half-length; [FORMULA] Width, length and total number, respectively, of the sectors in which each loop has been divided for the analysis; [FORMULA] Average luminosity contrast between the sectors at the footpoints and the apex of the loops; [FORMULA] pressure, maximum temperature (obtained from scaling laws) and volume filling factor, respectively, of the loop plasma emitting in the NIXT band.

Region A , slightly to the east of the central active region (Fig. 1), shows a large loop structure (the distance between the footpoints is [FORMULA] cm) with evident footpoints, which correspond to regions of opposite polarity in magnetograms. The loop appears to lie in a plane almost perpendicular to the line of sight, and it does not overlap with other structures, in spite of its large extension, except for a bright structure not far from the apex.

Regions B , C  and E  contain candidate isolated loops of relatively small extension ([FORMULA] cm). The loops connect opposite polarities, thus suggesting that they are seen from above.

The loop structure in region D  is very faint and not well defined. The results concerning this region will therefore be taken with proper care. Along this arch there are some small bright structures, which mix significantly with the loop.

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999