Abstract
Abstract Coronal loop observations have existed for many decades yet the precise shape of these fundamental coronal structures is still widely debated since the discovery that they appear to undergo negligible expansion between their footpoints and apex. In this work a selection of eight EUV loops and their 22 sub-element strands are studied from the second successful flight of NASA’s High-resolution Coronal Imager (Hi-C 2.1). Four of the loops correspond to open fan structures with the other four considered to be magnetically closed loops. Width analysis is performed on the loops and their sub-resolution strands using our method of fitting multiple Gaussian profiles to cross-sectional intensity slices. It is found that while the magnetically closed loops and their sub-element strands do not expand along their observable length, open fan structures may expand an additional 150% of their initial width. Following recent work, the Pearson correlation coefficient between peak intensity and loop/strand width are found to be predominantly positively correlated for the loops (≈88%) and their sub-element strands (≈80%). These results align with the hypothesis of Klimchuk & DeForest that loops and—for the first time—their sub-element strands have approximately circular cross-sectional profiles.
Highlights
It has long been shown that coronal loops may have uniform cross sectional widths along their observable length (Klimchuk et al 1992; Klimchuk 2000; Watko & Klimchuk 2000) where the cross-sectional profile is roughly circular
A total of eight loops are selected from the High Resolution Coronal Imager (Hi-C) 2.1 field of view (FOV) (Figure 1) that are relatively free from emission of surrounding coronal structures
From the selection of Hi-C 2.1 loops chosen for this study, loops A, B, C, and E are from open fan regions5, 4 MPFIT is freely available at: http://purl.com/net/mpfit
Summary
It has long been shown that coronal loops may have uniform cross sectional widths along their observable length (Klimchuk et al 1992; Klimchuk 2000; Watko & Klimchuk 2000) where the cross-sectional profile is roughly circular This behavior is contrary to what one would expect; that is, flux tubes, and coronal loops are expected to expand with height to maintain the pressure balance between the internal and external loop plasma due to gravitational stratification and decreasing magnetic field strength. This perceived circular cross section may be explained by locally twisted flux tubes (Klimchuk et al 2000; López Fuentes & Klimchuk 2006), which have been demonstrated to undergo less expansion than untwisted structures (McClymont & Mikic 1994).
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