Abstract
Abstract Previous work utilizing NASA’s High-resolution Coronal Imager (Hi-C 2.1) 172 Å observations revealed that, even at the increased spatial scales available in the dataset, there may be evidence for coronal structures that are still not fully resolved. In this follow-up study, cross-section slices of coronal strands are taken across the Hi-C 2.1 field of view. Following previous loop-width studies, the background emission is removed to isolate the coronal strands. The resulting intensity variations are reproduced by simultaneously fitting multiple Gaussian profiles using a nonlinear least-squares curve-fitting method. In total, 183 Gaussian profiles are examined for possible structures that are hinted at in the data. The full width at half maximum is determined for each Gaussian, which are then collated and analyzed. The most frequent structural widths are ≈450–575 km with 47% of the strand widths beneath NASA’s Solar Dynamics Observatory Atmospheric Imaging Assembly (AIA) resolving scale (600–1000 km). Only 17% reside beneath an AIA pixel width (435 km) with just 6% of the strands at the Hi-C 2.1 resolving scale (≈220–340 km). These results suggest that non-Gaussian shaped cross-sectional emission profiles observed by Hi-C 2.1 are the result of multiple strands along the integrated line of sight that can be resolved, rather than being the result of even finer sub-resolution elements.
Highlights
Observational investigations of coronal loop structure have been undertaken since the 1940s (Bray et al 1991); due to insufficient spatial resolution of current and previous instrumentation, the definitive resolved widths of these fundamental structures have not been fully realized
Aschwanden & Peter (2017) sampled 105 loop-width measurements from the Highresolution Coronal imager (Hi-C) field of view (FOV) with their analysis finding the most-likely width ≈550 km, arguing the possibility that Hi-C fully resolved the 193 Å loops/strands. This agrees with previous work (Peter et al 2013) where it is proposed that at least some of the wider loops with diameters ≈1 Mm observed by NASA’s Solar Dynamic Observatory Atmospheric Imaging Assembly (AIA; Lemen et al 2012) do not appear to show what they consider to be obvious signs of substructure when compared to the coincident Hi-C dataset
Employing the nonlinear least-squares curve-fitting method discussed in the previous section, a total of 183 Gaussian profiles are fitted to 24 Hi-C 2.1 cross-sectional slices
Summary
Observational investigations of coronal loop structure have been undertaken since the 1940s (Bray et al 1991); due to insufficient spatial resolution of current and previous instrumentation, the definitive resolved widths of these fundamental structures have not been fully realized. Aschwanden & Peter (2017) sampled 105 loop-width measurements from the Hi-C field of view (FOV) with their analysis finding the most-likely width ≈550 km, arguing the possibility that Hi-C fully resolved the 193 Å loops/strands. This agrees with previous work (Peter et al 2013) where it is proposed that at least some of the wider loops with diameters ≈1 Mm observed by NASA’s Solar Dynamic Observatory Atmospheric Imaging Assembly (AIA; Lemen et al 2012) do not appear to show what they consider to be obvious signs of substructure when compared to the coincident Hi-C dataset. Combining IRIS data with hydrodynamic simulations, Brooks et al (2016) find transition region temperature loops with widths between 266 and 386 km, and showcase that these structures appear to be composed of singular magnetic threads
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