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Appendix A: Chromospheric observations of mutual- and self-helicity in the two loops

To relate the transition region plasma to emission from chromospheric origin, we analyzed the Mg II k (2796 Å) line of these two loops observed by IRIS in the same way as for the Si IV data shown in the main part of this study.

Figure A.1 displays the loops during the second scan in the mutual-helicity phase. Figure A.1b shows the intensity of the Mg II line. Two fainter loops are detected (roughly) co-spatially with the Si IV loops, as displayed in Fig. A.1a (see also Fig. 4a). This applies especially to the region surrounding the middle part of these two loops, which is marked by two red arrows.

The yellows crosses in the two panels of Fig. A.1 are at exactly the same positions. The alignment between the Si IV and Mg II maps is easily achieved using the fiducial marks on the slit that are visible in the spectro-heliograms as horizontal black lines. The yellow crosses mark two positions along the Si IV loops, and a comparison of panel a and b of Fig. A.1 shows that the loops of the Mg II k line are located ~1′′ to the south of the Si IV loops. Thus the chromospheric and transition region plasma is not at exactly the same position, but they most likely populate different strands of the same loop system.

Figure A.2 displays the profiles of Mg II k at 2796 Å in the mutual-helicity phase. The positions of the spectra are the same as those of the Si IV profiles shown in Fig. 5. The red and green marks here indicate the positions of the Si IV loops as displayed in Fig. 5. By comparing Fig. A.2 with Fig. 5, we can find some similar indications of the Doppler motions of the loops, even though the signals are weaker in the Mg II k line.

Figure A.3 shows the loops during the third scan in the self-helicity phase. Here, again, the Mg II line shows roughly the same loop patterns. Just as in the earlier mutual-helicity phase, here the loops seen in Mg II seem to be offset by ~1′′ to the south of the Si IV loops (because the FOV is different in this raster-scan map, the fiducial mark is not visible in Fig. A.3).

In Fig. A.4 we show the spectra of Mg II now in the self-helicity phase, again at the same positions as those of Si IV in the main text (cf. Fig. 7). For the north loop, loop 1, some spectral tilt is detected (see Figs. A.4e−h). These spectral tilts are similar to those displayed in Fig. 7 for Si IV, but with weaker Doppler shifts.

	Fig. A.1 Intensity maps of IRIS Si IV (1394 Å) a) and Mg II k (2796 Å) b) during the second scan in the mutual-helicity phase. The yellow pluses mark the positions of the Si IV loops. The FOV is the same as in Fig. 4a.
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	Fig. A.2 Similar as Fig. 5, but for the Mg II k (2796 Å) line.
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From these considerations we conclude that the spectro-heliograms in Mg II show loop structures similar to those in Si IV, albeit offset by about 1′′ perpendicular to the loop spine. Most importantly, even the spectral profiles of Mg II and Si IV share similar properties. This implies that the source region of the chromospheric Mg II k line and the transition region line of Si IV share the same volume defined by the loop system. Thus the loops are indeed multithermal structures, probably with the chromospheric and transition region plasma found on different strands within the loop system. However, because the signals in the Mg II k line are much weaker than in the Si IV line, the results we discussed here need to be considered carefully.

	Fig. A.3 Same as Fig. A.1, but for the self-helicity phase, now showing the Si IV (1403 Å) a) and Mg II k (2796 Å) line b).
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	Fig. A.4 Same as Fig. A.2, but for the self-helicity phase.
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© ESO, 2014