Is the High-Resolution Coronal Imager Resolving Coronal Strands? Results from AR 12712

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 29 th May 2018 from the White Sands Missile Range, NM, USA. On this occasion, 329 seconds of 17.2 nm data of target active region AR 12712 was captured with a cadence of ≈ 4 s, and a plate scale of 0.129 pixel. 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 (MGN), 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 with 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 more typical strands 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 of a permanent solar observatory with the resolving power of Hi-C.

Evolution of downflows in the transition region above a sunspot over short time-scales

“…White arrows indicate the fingers in each panel. The widths of each finger in the Si iv 1394 Å channel was less than 1 (similar to the inferred widths of coronal loops in the highest resolution coronal imaging currently available; Williams et al 2020) and the lengths were between 3-5 . The lifetimes of these fingers ranged from a few minutes to more than half an hour.…”

Oscillation and Evolution of Coronal Loops in a Dynamical Solar Corona

“…Due to LOS integration of optically thin EUV emission it is difficult to interpret fine structure in transverse intensity profiles, particularly when using a single channel as may be the case for coronal loops which are often identifiable only in the 171 Å channel of SDO/AIA. Observations using higher resolution instruments such as CRisp Imaging Spectro-Polarimeter (CRISP; Antolin et al, 2012;Scullion et al, 2014) and the High-Resolution Coronal Imager (Hi-C; Aschwanden and Peter, 2017;Williams et al, 2020) reveal that loop structuring indeed exists below the resolution of SDO/AIA. Our results demonstrate that the non-linear evolution of the loop profile by KHI produces a time-dependent (increasing) damping rate for kink oscillations that would not be expected for structuring due to loops being multi-stranded.…”

Case study on the identification and classification of small-scale flow patterns in flaring active region

“…2 Philishvili et al: Flow patterns However, the observations of smaller scale flows are rather puzzling as most of these flows are of the order of a few times the spatial resolution limit, provided by the instruments on the abovementioned space missions. Recently, Williams et al (2020) reported the discovery of a large population of so-called coronal strands in active regions made with the High-Resolution Coronal Imager (Hi-C 2.1, with 0 .129 pixel −1 resolution, see Rachmeler et al (2019)), which is the advanced version of the original Hi-C NASA sounding rocket experiment performed in 2012 (Kobayashi et al 2014). The existence of these strands reveals that large-scale magnetic objects like active regions have fine small-scale structures.…”

“…As to the aperiodic motions of the observed intensity blobs in the same active region (AR11429), these are the main objective of the present study. In particular, we aim to formulate a mathematically rigorous identification and statistical classification method using SDO/AIA 171Å for flows in the small-scale strands of similar spatial scales as those observed with the higher resolution instruments (Williams et al 2020). Our proposed method provides a set of identified aperiodic flow trajectories together with their estimated velocity bounds (taking into account corrections in pixel distances due to the projection deformation of the solar spherical surface in flat CCD images) and classification as either subsonic, sonic, or supersonic.…”