Unresolved Fine-Scale Structure in Solar Coronal Loop-Tops

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 sub-structures within coronal loop crosssections, in order to understand the nature of the solar corona. Here, we push this lower limit further to search for the finest coronal loop sub-structures, through taking advantage of the resolving power of the Swedish 1m Solar Telescope (SST) / CRisp Imaging Spectro-Polarimeter (CRISP), together with co-observations from the Solar Dynamics Observatory (SDO) / 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 datasets, large-scale catastrophic cooling in coronal loop-tops and the existence of multi-thermal, multi-stranded sub-structures. Many cool strands even extend fully-intact from loop-top to foot-point. We discover that coronal loop fine-scale strands can appear bunched with as many as 8 parallel strands, within an AIA coronal loop cross-section. The strand number density vs 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 sub-structure strands are statistically unresolved in AIA coronal loops.

Dark Post-Flare Loops Observed by the Solar Dynamics Observatory

“…The results confirmed that the DPFLs originate from the cooling process due to the thermal instability in PFLs. The blob-like features in DPFLs behaved like coronal rain in non-flare AR loops (Foukal 1976;Kamio et al 2011;Antolin et al 2012;Scullion et al 2014;Antolin et al 2015). These blob-like features are most likely produced by the same mechanism that creates coronal rain, but with a different amount of mass from the chromospheric evaporation.…”

“…Although DPFLs have been observed for years, little attention has been paid to their fine structures in EUV wavelengths and their relationship to coronal rain (Scullion et al 2014;Antolin et al 2015). Now, the unprecedented observations from the Atmospheric Imaging Assembly (AIA; Lemen et al 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al 2012) allow us to investigate the fine structures of DPFLs in multiple EUV wavelengths.…”

Polarization Characteristics of Zebra Patterns in Type IV Solar Radio Bursts

“…The formation of such a boundary is closely related to the microscopic structure of the corona. Due to the recent development of imaging instruments with high resolutions in the EUV wavelength range, such as SDO/AIA and HINODE/EIS, the fine structure of the corona has been revealed (e.g., Del Zanna 2008; Van Doorsseleaere et al 2008;Warren et al 2008;Brooks et al 2012;Peter et al 2013;Scullion et al 2014). …”

Coronal rain in magnetic bipolar weak fields

“…The electron density of rain clumps was found to be about 1.8-7.1×10 10 cm −3 , through estimation based on absorption in multiple extreme ultraviolet (EUV) channels. Besides this quiescent coronal rain, which occurs in nonflaring coronal loops with relatively weak variation of energy and mass, flare-driven coronal rain, which appears in postflare loops as a result of catastrophic cooling, often emerges as a bunch of parallel strands extending from loop top to footpoint (Scullion et al 2014(Scullion et al , 2016.…”