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

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5. Discussion

This work studies coronal loops by taking advantage of the complementary information provided by NIXT and Yohkoh/SXT, which observed the solar corona simultaneously. NIXT works in a softer and narrower spectral band, and its spatial resolution (1.2" pixel size) is four times higher than the other instrument, while Yohkoh/SXT allows direct temperature diagnostics by using different filters. In this work, for the first time, we have applied a diagnostic method (Paper I) which allows us to evaluate the plasma pressure inside loops observed with NIXT.

The analysis described above is based on the assumption that hydrostatic loop models provide a good description of loop plasma conditions, a well-established fact for most loops (e.g. Rosner et al. 1978). The presence of moderate subsonic siphon flows driven by pressure differences between the loop footpoints should not much affect our results, because the overall physical characteristics would not change much (Orlando et al. 1995). Shocking siphon flows instead should make loops appear highly asymmetric, an effect that is not observed on our selected loops. Finally although the exact values are, to some extent, subject to improvement, their orders of magnitude are unequivocal and represent a significant result.

Since the two instruments are sensitive to plasma at different coronal temperatures, NIXT mostly around 1 MK, SXT mostly higher than 2 MK, in general we do not expect an exact correspondence between the morphology of loops detected by the two instruments. In fact, although there is a good correspondence between the location of all the selected structures observed by the two telescopes, their morphology appears clearly different. A close inspection of loop structure A  shows that the path chosen along the loop in the NIXT image corresponds to a relatively "dark" structure related to the surrounding loops in the SXT images. A more detailed analysis, which includes the pressure and filling factor evaluations in both instrument bands, seems to indicate that the two instruments are looking at distinct loop structures with different lengths, falling in the same field of view. In other words the low pressure derived from NIXT data would be confined in a loop not visible by the SXT, which, instead, detects the emission from co-existing higher pressure and more compact structures.

The pressure values obtained from SXT data are systematically higher than those obtained from NIXT. This result can probably be ascribed to the different spectral bands: the SXT is sensitive to hotter plasma, which is in general also at higher pressure for the same loop length. This indicates that plasmas at different temperature and pressure may coexist in the same loop structures, or, at most, in neighboring loops. Notice that our analysis shows that density values inferred from the emission measure as in Eq. (2) can be wrong and can lead to unrealistic values of pressure and filling factor.

The same detailed comparison cannot be made for the smaller loops B , C  and E  because of the limited resolution of the SXT. The morphology of such more compact regions appears quite similar in the SXT and NIXT images. The brightest parts detected by SXT are located close to the center of the NIXT-imaged structures and seem to coincide with the apex of the loops. For the small loops, it appears as if the combination of SXT and NIXT images would show the entire loop structure, NIXT being more sensitive to the footpoints and SXT to the apex. However, the relatively low resolution of the SXT does not allow us to put a tighter constraint on this aspect, and we cannot exclude, for instance, that the two instruments observe two adjacent loops at different pressure (SXT observing a loop at higher pressure).

We could not identify a clear counterpart of the structure D  on the SXT images, probably because it is a relatively low pressure loop located in a region with other complex and relatively bright structures.

These results allow us to derive a more complete scenario than previously obtained, for instance, by Yoshida et al. (1995). In summary, smaller and higher pressure structure are those showing the better morphological agreement between NIXT and SXT. In contrast, large structures (A  and D ) appear different to the two instruments both in morphology and in plasma parameters. In particular, the large loop A , clearly visible in the NIXT image, is not visible in the SXT band.

The filling factor in region A , as observed by NIXT, is large and close to 1; for the smaller structures of regions B , C  and E , for which a comparison between NIXT and SXT results is sound, we find a large filling factor in Yohkoh-imaged loops and a small one in NIXT-imaged ones.

All these results may be interpreted in the light of the following scenario, known since Skylab observations: coronal loops evolve from small, hot and compact structures to progressively large, cool and extended structures. Our results add the information that smaller loops are also completely filled with high temperature plasma, detectable almost only by SXT. As they get cooler, more and more plasma filaments become visible by NIXT, although coexisting with many other hot and dense filaments. Later in the cooling, most of the loop would contain relatively cooler plasma, i.e. visible with NIXT only, and with a high filling factor. The fact that this occurs just for the large loop A  is consistent with the general trend of the gradual expansion of small loops to large loops.

In this scenario, the large loop observed by NIXT in region A  would be a relatively evolved and cool structure, invisible to the SXT (which instead detects under/over-lying hotter structures); the more compact loops in B , C  and E , would instead be hotter and with a small number of filaments visible by NIXT; region D  would be in an intermediate stage between the two extremes.

We note that the low filling factors obtained for the compact loops (1/100-1/1000) strongly suggest that there is a very fine filamentation of magnetized loops within loops and provide a quantitative estimate of its value. This work therefore contributes quantitatively to the comprehension of the filamentary structure of coronal magnetic loops. Also our findings suggest that the plasma does not cool down uniformly in the loop, and that a progressively higher number of filaments get cooler, and thus become gradually visible by NIXT. This hypothesis needs further checks and verification which will surely come from forth-coming high resolution observation.

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999