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Abstract Resolving fine structures in the Sun’s corona may provide key insights into rapid eruptions and the heating of the corona. Adaptive optics systems have been used for over two decades to reach the diffraction limit of large telescopes, thereby compensating for atmospheric image blur. Current systems, however, are still limited to observations of the solar disk and fail with coronal objects, leaving fundamental coronal dynamics hidden in that blur. Here we present observations with coronal adaptive optics reaching the diffraction limit of a 1.6-m telescope to reveal very fine coronal details. Furthermore, we discovered a short-lived, fast-moving, finely twisted feature occurring during the decay phase of a flare that quickly destabilized. Coronal adaptive optics increased the spatial resolution by an order of magnitude at visible wavelengths. We report here a large portion of off-limb coronal rain material with observed scales below 100 km. This new adaptive optics scheme opens opportunities for observational discoveries at small scales beyond the solar limb in the highly dynamic corona by exploiting the diffraction limit of large ground-based telescopes. 

Abstract Recent high-resolution solar observations have unveiled the presence of small-scale loop-like structures in the lower solar atmosphere, often referred to as unresolved fine structures, low-lying loops, and miniature hot loops. These structures undergo rapid changes within minutes, and their formation mechanism has remained elusive. In this study, we conducted a comprehensive analysis of two small loops utilizing data from the Interface Region Imaging Spectrograph (IRIS), the Goode Solar Telescope (GST) at Big Bear Solar Observatory, and the Atmospheric Imaging Assembly and the Helioseismic Magnetic Imager on board the Solar Dynamics Observatory, aiming to elucidate the underlying process behind their formation. The GST observations revealed that these loops, with lengths of ∼3.5 Mm and heights of ∼1 Mm, manifest as bright emission structures in Hαwing images, particularly prominent in the red wing. IRIS observations showcased these loops in 1330 Å slit-jaw images, with transition region (TR) and chromospheric line spectra exhibiting significant enhancement and broadening above the loops, indicative of plasmoid-mediated reconnection during their formation. Additionally, we observed upward-erupting jets above these loops across various passbands. Furthermore, differential emission measurement analysis reveals an enhanced emission measure at the location of these loops, suggesting the presence of plasma exceeding 1 MK. Based on our observations, we propose that these loops and associated jets align with the minifilament eruption model. Our findings suggest a unified mechanism governing the formation of small-scale loops and jets akin to larger-scale X-ray jets. 

Abstract The dynamic structures of solar filaments prior to solar flares provide important physical clues about the onset of solar eruptions. Observations of those structures under subarcsecond resolution with high cadence are rare. We present high-resolution observations covering preeruptive and eruptive phases of two C-class solar flares, C5.1 (SOL2022-11-14T17:29) and C5.1 (SOL2022-11-14T19:29), obtained by the Goode Solar Telescope at Big Bear Solar Observatory. Both flares are ejective, i.e., accompanied by coronal mass ejections (CMEs). High-resolution Hαobservations reveal details of the flares and some striking features, such as a filament peeling process: individual strands of thin flux tubes are separated from the main filament, followed shortly thereafter by a flare. The estimated flux of rising strands is in the order of 10¹⁷Mx, versus the 10¹⁹Mx of the entire filament. Our new finding may explain why photospheric magnetic fields and overall active region and filament structures as a whole do not have obvious changes after a flare, and why some CMEs have been traced back to the solar active regions with only nonerupting filaments, as the magnetic reconnection may only involve a very small amount of flux in the active region, requiring no significant filament eruptions. We suggest internal reconnection between filament threads, instead of reconnection to external loops, as the process responsible for triggering this peeling of threads that results in the two flares and their subsequent CMEs. 

Abstract We analyze high-resolution observations of an X-1.0 white-light flare, triggered by a filament eruption, on 2022 October 2. The full process of filament formation and subsequent eruption was captured in the Hαpassband by the Visible Imaging Spectrograph (VIS) on board the Goode Solar Telescope (GST) within its center field of view. White-light emissions appear in flare ribbons following the filament eruption and Hαribbon brightening. GST Broadband Filter Imager data show that the continuum intensity, as compared to the nearby quiet-Sun area, has increased by up to 20% in the photospheric TiO band around 7057 Å. The Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory reported 10% contrast enhancement in the continuum near Fei6173 Å line. The separation motion of two white-light kernels is recorded by the high-cadence GST/TiO images and is well accompanied by the motion of the VIS Hαflare ribbon leading edge. One kernel, located in a 150 Gauss field within a granulation area, exhibited an average apparent motion speed of 55 km s⁻¹, which is the highest average speed ever reported. The other kernel drifted at 9 km s⁻¹in an 800 Gauss magnetic field area. Hard X-ray (HXR) emissions reaching up to 300 keV have been observed for this flare. The simultaneous occurrence of high-cadence HXR, microwave, and white-light emissions strongly suggests that the energetic particles from the flare directly contribute to the heating. The inverted HXR energy flux density corresponding to 10% TiO brightening is 2.07 ± 0.23 × 10¹¹erg cm⁻²s⁻¹during the flare peak.