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Unresolved Fine-Scale Structure in Solar Coronal Loop-Tops

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Unresolved Fine-Scale Structure in Solar Coronal Loop-Tops The Astrophysical Journal, 797:36 (10pp), 2014 December 10 doi:10.1088/0004-637X/797/1/36 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. UNRESOLVED FINE-SCALE STRUCTURE IN SOLAR CORONAL LOOP-TOPS E. Scullion1,2, L. Rouppe van der Voort1, S. Wedemeyer1, and P. Antolin3 1 Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, NO-0315 Oslo, Norway 2 Trinity College Dublin, College Green, Dublin 2, Ireland; [email protected] 3 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Received 2014 August 7; accepted 2014 October 1; published 2014 November 24 ABSTRACT New and advanced space-based observing facilities continue to lower the resolution limit and detect solar coronal loops in greater detail. We continue to discover even finer substructures within coronal loop cross-sections, in order to understand the nature of the solar corona. Here, we push this lower limit further to search for the finest coronal loop substructures, through taking advantage of the resolving power of the Swedish 1 m Solar Telescope/CRisp Imaging Spectro-Polarimeter (CRISP), together with co-observations from the Solar Dynamics Observatory/Atmospheric Image Assembly (AIA). High-resolution imaging of the chromospheric Hα 656.28 nm spectral line core and wings can, under certain circumstances, allow one to deduce the topology of the local magnetic environment of the solar atmosphere where its observed. Here, we study post-flare coronal loops, which become filled with evaporated chromosphere that rapidly condenses into chromospheric clumps of plasma (detectable in Hα) known as a coronal rain, to investigate their fine-scale structure. We identify, through analysis of three data sets, large-scale catastrophic cooling in coronal loop-tops and the existence of multi-thermal, multi-stranded substructures. Many cool strands even extend fully intact from loop-top to footpoint. We discover that coronal loop fine-scale strands can appear bunched with as many as eight parallel strands within an AIA coronal loop cross-section. The strand number density versus cross-sectional width distribution, as detected by CRISP within AIA-defined coronal loops, most likely peaks at well below 100 km, and currently, 69% of the substructure strands are statistically unresolved in AIA coronal loops. Key words: methods: data analysis – methods: observational – techniques: image processing – techniques: spectroscopic – telescopes Online-only material: color figures ∼ ∼ 1. INTRODUCTION from EUV to higher energies) of 1000 km ( 1.5: 1 arcsec () ≈720 km), it is likely that most observations represent It has long been claimed (for, e.g., Gomez et al. 1993, and superpositions of hundreds of unresolved strands that can earlier) that coronal loops consist of bundles of thin strands, exist at various stages of heating and cooling (Klimchuk to scales below the current instrumental resolution. Today, that 2006). Other studies based both on models and on analysis statement continues to remain as prevalent as ever. Coronal of observations independently suggest that elementary loop loops were first detected in coronagraphic observations in components should be even finer, with typical cross-sections the 1940s (Bray et al. 1991). These loops are observed to of the strands on the order of 10–100 km (Beveridge et al. 2003; extend into the low plasma-β environment of the solar corona, Cargill & Klimchuk 2004; DeForest 2007). The space-based arching over active regions, and are filled with relatively dense (Hinode; Kosugi et al. 2007) EUV Imaging Spectrometer (EIS; plasma (in the range of ∼108–1010 cm−3) and confined by Culhane et al. 2007) has been used together with the Solar a dipole-like magnetic field (Aschwanden 2004; Reale 2010, Dynamics Observatory (SDO)/Atmospheric Image Assembly and references therein). Coronal loops are observed to have a (AIA; Lemen et al. 2012) to investigate the fundamental spatial broad range of temperatures, and they are observed in extreme scales of coronal loops; the results suggest that most coronal ultraviolet (EUV) from ∼105 K (cool loops) to a few 106 K loops remain unresolved (Ugarte-Urra et al. 2012)giventhe (warm loops) and up to a few 107 K (flaring loops). In coronal 1.2(∼860 km) and 2(∼1440 km) resolution of SDO/AIA and loops, neighboring magnetic field lines are considered to be Hinode/EIS, respectively. thermally isolated; hence, each field line can be considered However, contrary to this, Brooks et al. (2012) presented independently, which we call a “strand” herein. results of multi-stranded loop models calculated at a high reso- To explain the nature of coronal loops is to understand the lution. They show that only five strands with a maximum radius origin of solar coronal heating. One fundamental issue is that of 280 km are required in order to reproduce the observed coro- we do not know what the spatial scale of the mechanisms that nal loops, and a maximum of only eight strands where needed heat the solar corona is (Reale 2010). It has been considered to reproduce all of the detected loops. More recently (2012 July that in order to form stable overdense, warm coronal loops, 11), the High-resolution Coronal Imager (Hi-C; Cirtain et al. it may be required to assume that coronal loops consist of 2013) took images of the 1.5 MK corona at an unprecedented unresolved magnetic strands, each heated impulsively, non- resolution of 0.3–0.4(∼220—290 km; Winebarger et al. 2014), uniformly, and sequentially (Gomez et al. 1993; Cargill 1994; which is unique for direct imaging of coronal loops in this pass- Klimchuk 2000; Klimchuk & Cargill 2001; Reale et al. 2005; band. As a follow-up, Brooks et al. (2013) measured the Gaus- Klimchuk 2006; Klimchuk et al. 2008). At a typical spatial sian widths of 91 Hi-C loops observed in the solar corona, resolution (of most current space-based instruments observing and the resulting distribution had a peak width of 270 km. 1 The Astrophysical Journal, 797:36 (10pp), 2014 December 10 Scullion et al. In other words, the finest-scale substructures of coronal loops the chromosphere. Fundamentally, the magnetic substructuring are already observable. Other studies concerning the variations of the coronal loops near loop-tops should remain the same or of intensity across a variety of hot loops, co-observed by AIA similar for all coronal loops (flaring and non-flaring). However, and Hi-C, have continued to speculate on whether or not strand the possibility of probing the fine-scale magnetic structure of substructures could potentially exist well below what Hi-C or coronal loops during the post-flare phase with Hα via high- AIA can resolve (Peter et al. 2013). Most recently, Winebarger resolution imaging can present itself. et al. (2014) performed a statistical analysis on how the pixel In this paper, we report on the distribution of threaded intensity scales from AIA resolution to Hi-C resolution. They substructures within a coronal loop from loop-top toward claim that 70% of the Hi-C pixels show no evidence for sub- footpoint via direct imaging of coronal loop cross-sections structuring, except in the moss regions within the field of view through a coordinated SST and SDO analysis, which comprises (FOV) and in regions of sheared magnetic field. three data sets. There is strong evidence to suggest that coronal loops are, in- deed, so finely structured when we consider loop legs from coor- 2. OBSERVATIONS dinated observations involving high-resolution spectral imaging with ground-based instruments. Antolin & Rouppe van der Voort CRISP, installed at the SST, is an imaging spectropolarimeter (2012) performed a detailed and systematic study of coronal rain that includes a dual Fabry–Perot´ interferometer, as described by (Kawaguchi 1970; Schrijver 2001; De Groof et al. 2004) via the Scharmer (2006). The resulting FOV is about 55 ×55 . CRISP Swedish 1 m Solar Telescope (SST; Scharmer et al. 2003a)/ allows for fast wavelength tuning (∼50 ms) within a spectral (CRISP; Scharmer et al. 2008) instrument at very high spatial range and is ideally suited for spectroscopic imaging of the and spectral resolutions (0.0597 image scale). They detected chromosphere. For Hα 656.3 nm, the transmission FWHM of narrow clumps of coronal rain in Hα down to the diffraction CRISP is 6.6 pm and the prefilter is 0.49 nm. The image quality limit (130 km) in the cross-sectional area, with average lengths of the time series data is greatly benefited from the correction between ∼310 km and ∼710 km and widths approaching the of atmospheric distortions by the SST adaptive optics system diffraction limit of the instrument. These measurements where (Scharmer et al. 2003b) in combination with the image restora- repeated for on-disk coronal rain by (Antolin et al. 2012). Coro- tion technique Multi-Object Multi-Frame Blind Deconvolution nal rain is considered to be a consequence of a loop-top thermal (MOMFBD; van Noort et al. 2005). Although the observations instability driving catastrophic cooling of dense plasma (see suffered from seeing effects, every image is close to the theo- Antolin et al. 2010; Antolin & Rouppe van der Voort 2012, retical diffraction limit for the SST. We refer to van Noort & and references therein). Radiation cooling of dense evaporated Rouppe van der Voort (2008) and Sekse et al. (2013)formore plasma (filling coronal loops) leads to the onset of plasma de- details on the MOMFBD processing strategies applied to the pletion from the loops, slowly at first and then progressively CRISP data. We followed the standard procedures in the reduc- faster. tion pipeline for CRISP data (de la Cruz Rodr´ıguez et al. 2014), From an observational standpoint, the next step is to reveal which includes the post-MOMFBD correction for differential evidence for substructuring along the full length of the coronal stretching suggested by Henriques (2012).
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