Evidence for and Analysis of Multiple Hidden Coronal Strands in Cross-sectional Emission Profiles: Further Results from NASA’s High-resolution Solar Coronal Imager

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.

Multi-Stranded Coronal Loops: Quantifying Strand Number and Heating Frequency from Simulated Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) Observations

“…Subsequently, the superior resolution images from NASA's sounding rocket High-resolution Coronal imager (Hi-C: Kobayashi et al, 2014, 0.1 pixel) has led to observational assertions of the detection of magnetic braiding (Cirtain et al, 2013), nanoflares in inter-moss flares (Winebarger et al, 2013), and the determination of plasma loop parameters (Brooks et al, 2013;Peter et al, 2013;Scullion et al, 2014;Aschwanden and Peter, 2017;Williams et al, 2020b). It is still not fully settled whether the loop structures we observe with our current capabilities are actually spatially resolved or if they may be comprised of several individually isolated sub-resolution strands (Peter et al, 2013;Xie et al, 2017;Williams et al, 2020a).…”

The Cross-sectional Shape and Height Expansion of Coronal Loops: High-resolution Coronal Imager (Hi-C) Analysis of AR 12712

“…3 Each cross section normal to each strand is taken to be 3 pixels deep and the background emission is then subtracted. As outlined in Figure 2 of Williams et al (2020b), this background subtraction is performed by first finding all the local minima of a slice, and interpolating through these values using a cubic spline (Yi et al 2015) to obtain a global trend (blue dashed line). The global trend is then subtracted from the intensity profile along the slice, leaving behind the background subtracted coronal strands (similar to Aschwanden & Schrijver 2011;Williams et al 2020a).…”

“…The observed Hi-C 2.1 intensity profile of a cross-sectional slice is reproduced by simultaneously fitting Gaussian profiles, the number of which is determined by the Akaike information criterion (AIC; Akaike 1974) along with a corrective term (AICc) for small sample sizes. This is fully described in the appendix of Williams et al (2020b). Subsequently, the FWHM of the Gaussian profile is measured to provide an estimate of the possible width of the substructures likely present within the Hi-C 2.1 data.…”

“…As is discussed in Klimchuk (2000), the lack of cross-sectional expansion in loop observations have important implications for coronal heating models. The first is that energy may be deposited in an axial symmetric manner at the same spatial scales as the diameter of monolithic coronal loops/strands (FWHM 200 km; Williams et al 2020aWilliams et al , 2020b. Alternatively, the energy deposited within strands and loops occur at spatial scales much smaller than the diameters of these structures-perhaps on spatial scales as small as 15 km (Peter et al 2013)-and is then transported orthogonal to the loop abscissa in an axisymmetric manner.…”

“…In light of recent Hi-C findings indicating that monolithic loops may have approximately circular cross sections that exhibit no expansion along their observable length (Klimchuk & DeForest 2020), this paper employs the methods used in previous studies (Section 2) to examine the spatial scales along the loop envelope (Williams et al 2020a) and individual loop sub-element strands (Williams et al 2020b) of eight distinct coronal structures. In Section 3, the relationship between loop width and their peak intensities are explored to classify whether their cross sections may be approximately circular (Klimchuk & DeForest 2020) or oval such as a twisted ribbon (Malanushenko & Schrijver 2013;McCarthy et al 2021).…”