Abstract
Abstract Following the success of the first mission, the High-Resolution Coronal Imager (Hi-C) was launched for a third time (Hi-C 2.1) on 2018 May 29 from the White Sands Missile Range, NM, USA. On this occasion, 329 s of 17.2 nm data of target active region AR 12712 were captured with a cadence of ≈4 s, and a plate scale of 0.″129 pixel−1. Using data captured by Hi-C 2.1 and co-aligned observations from SDO/AIA 17.1 nm, we investigate the widths of 49 coronal strands. We search for evidence of substructure within the strands that is not detected by AIA, and further consider whether these strands are fully resolved by Hi-C 2.1. With the aid of multi-scale Gaussian normalization, strands from a region of low emission that can only be visualized against the contrast of the darker, underlying moss are studied. A comparison is made between these low-emission strands and those from regions of higher emission within the target active region. It is found that Hi-C 2.1 can resolve individual strands as small as ≈202 km, though the more typical strand widths seen are ≈513 km. For coronal strands within the region of low emission, the most likely width is significantly narrower than the high-emission strands at ≈388 km. This places the low-emission coronal strands beneath the resolving capabilities of SDO/AIA, highlighting the need for a permanent solar observatory with the resolving power of Hi-C.
Highlights
The NASA sounding rocket, High-Resolution Coronal Imager (Hi-C), was first launched on 2012 July 11 and it captured high-resolution (≈0 3–0 4), highcadence (≈5 s) images of active region 11520 (Kobayashi et al 2014) in a narrowband 19.3 nm channel
The low-emission loops are shown in more detail in Figure 4, while the high-emission regions include a selection of loops: large loops (Figure 5), open fan loop regions (Figures 6 and 7), and some small loops close to the center of the active region (Figure 8)
Continuing on from the success of Hi-C, Hi-C 2.1 reveals structures throughout and around its targeted active region (AR 12712) in the 17.2 nm line that cannot be resolved by Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA)
Summary
The NASA sounding rocket, Hi-C, was first launched on 2012 July 11 and it captured high-resolution (≈0 3–0 4), highcadence (≈5 s) images of active region 11520 (Kobayashi et al 2014) in a narrowband 19.3 nm channel. Further work on this by Brooks et al (2016) combines IRIS observations and HYDRAD modeling (Bradshaw & Cargill 2013) to investigate 108 transition region loops whose FWHM ranges between 266 and 386 km They argue that at these spatial scales the structures appear to be composed of monolithic stands rather than multi-stranded bundles. The advancements made in high-resolution measurements of coronal loops, by Brooks et al (2016) and Aschwanden & Peter (2017), appear to highlight evidence that current instrumentation is at a stage of resolving individual plasma strands within the corona and provides some possible constraint on the heat input required for these features. Renormalizing the background-subtracted slices has the effect of focusing in on the structures themselves, which means that ΔI will appear larger
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