Wenda Cao

Context. High-resolution observations of the Sun reveal a multitude of small-scale striations throughout the photosphere. While these features are well observed in broad-band intensity images, spectropolarimetric observations remain rare. Aims. In this study, we characterize small dark striations at the pore-granulation boundary and bright grains moving along them. We seek to describe their magneto-convective nature. Methods. We analyzed restored context images and many-line Stokes inversions of a restored spectropolarimetric scan from GST/FISS-SP with a spatial resolution of 0.068″. In the inversion, we used 85 solar absorption lines within a 33 Å wide spectral window in the 5250 Å region. We compare the observations with a MURaM simulation to discern the magneto-convective nature of striations and grains. Results. We find multiple dark striations in the vicinity of pores or active region intergranular lanes with a typical width of 0.09″ and moving bright grains that migrate along some of those striations toward the adjacent pore. Grains forming in a high-resolution MURaM simulation of a pore show similar lifetimes of about 70 s. A comparison of the atmospheric configurations of simulated and observed grains reveals good qualitative agreement in structure, dynamics, thermal, and magnetic stratification. The simulation shows that the dark striations form at the top of a convective plume confined by the surrounding field, and that their dark appearance is caused by plasma trapped in the field cusp at optical depth unity. The moving bright grains are composed of hot plasma pulled upwards by turbulent flows at the tip of the striation. Conclusions. By combining high resolution spectro-polarimetry, many-line inversions, and MURaM simulations, we present the first analysis of the 3D fine structure of small-scale striations and moving bright grains in the vicinity of a pore and describe their magneto-convective nature.

Our study of H α line profiles of rapid blue- and redshifted excursions measured with Goode Solar Telescope revealed an asymmetrical three-peak structure present in the blue wing of the H α line, suggesting complex internal velocity fields that may include bidirectional flows and transverse and torsional motions. Blueshifted plasma predominates across the field of view (FOV), correlating with higher temperatures (>10 ⁴ K) and extending to greater heights. Conversely, redshifts are less spread over the FOV, are localized near network magnetic fields, and diminish rapidly with altitude. The frequency distribution associated with blueshifted events displays a bimodal structure with peaks centered at 7200 and 8800 K. The redshifted events display a much weaker and wide peak centered at about 9000 K. No significant difference in temperature distributions for blue- and redshifted events is observed in the range above 10,000 K. Variations of H α line profiles with height seem to indicate that the line-of-sight integration effects may be most significant within the 0–6 Mm layer above the photosphere, and it decreases with height.

Context. For the analysis of highly resolved solar spectra the simultaneous observation and interpretation (inversion) of only a few (often only one) spectral lines is still the norm. With modern instruments spatially highly resolved spectropolarimetric data covering many lines are available. Aims. For the first time we combine the information from 85 simultaneously observed absorption lines in spatially highly resolved data to test a proposed solar many-line inversion strategy. Methods. We inverted full Stokes spectra recorded with the FISS spectro-polarimeter (FISS-SP) at the 1.6-m Goode Solar Telescope in California, using the SPINOR code. We contrasted two different setups: one following the traditional approach of using a line doublet, and a new method inverting many-lines simultaneously. Results. Compared to results from an inversion using two lines of a line doublet, we discovered more fine-structure and better constrained values using the many-line technique. An average quiet Sun spectrum was successfully reproduced using a model atmosphere, but when inverting spatially resolved data, uncertainties in line parameters and blend configurations did not average out. Thus, a deliberate selection process of lines and line blends was required, in order to make the many-line case converge to a physically expected and coherent atmosphere. We successfully developed and tested such a selection method. Conclusions. Our results highlight that the many-line inversions method delivers more coherent results with superior line of sight (LOS) resolution of the atmospheric structure. Moreover, it effectively detects and utilizes even weak polarimetric signals in noisy data and thereby partly circumvents low noise requirements. It reveals uncertainties in atomic parameters of individual spectral lines and models, as the degree of freedom to compensate for these uncertainties by compromising the inferred atmospheric parameters is considerably reduced. It is thereby pointing to a need for improved atomic data, including log( gf ) values, of many lines in the solar spectrum. The many-line method presents significant potential for solar physics and may become the preferred option for future observations with upcoming spectrographs.

Solar active region 11283 produced an X2.1 flare associated with a solar eruption on 2011 September 6. Observations revealed a preflare sigmoidal structure and a circular flare ribbon surrounding the typical two-ribbon structure, along with remote brightenings located at a considerable distance from the main flare site. To interpret these observations in terms of the dynamics of the three-dimensional coronal magnetic field, we conducted data-constrained magnetohydrodynamic simulations. Using a nonlinear force-free field as the initial condition, we reconstructed a realistic preflare magnetic environment, capturing a sheared sigmoid above the polarity inversion line surmounted by a fan–spine structure. Our simulations revealed that reconnection between the sigmoidal field, the adjacent fan–dome field lines, and the neighboring large loops facilitated the transfer of magnetic twist and led to the formation of a large magnetic flux rope (MFR). This transfer and propagation of twist are clearly visible throughout the MFR. As reconnection progresses, the entire fan–spine structure expands along with the evolving MFR. A notable outcome of the simulation is that the footpoints of the newly formed MFR align closely with the observed circular flare ribbon and the remote brightening region. Our findings suggest that a large MFR formed during the X2.1 flare, providing a coherent explanation for the observed phenomena.

Context. Small-scale events measuring only tens of kilometres can have significant implications for the overall energy state of various layers of the solar atmosphere. Current spectro-polarimetric observations lack either spatial or spectral resolution for a comprehensive study of these small-scale events. Aims. The slit-scanning spectro-polarimetric instrument described here is designed for spectral image reconstruction and, in combination with the excellent optical performance of the 1.6 metre Goode Solar Telescope, yields spectral hypercubes of the highest spatial and spectral resolution. Additionally, the instrument offers a huge spectral window of more than 30 Å, allowing many solar absorption lines to be observed simultaneously. Methods. We extended the existing Fast Imaging Solar Spectrograph (FISS) instrument with polarimetric capabilities, new customized cameras, and a context imager. We applied numerical methods to measure and correct for field-dependent instrumental and atmospheric degradations, to obtain diffraction-limited spectro-polarimetric scans. Results. In this work we present the instrument design, the data reduction workflow, and the first-light results. Compared to a typical HINODE/SP dataset, we find a higher signal-to-noise ratio in our data within the resolution limits of the respective telescopes when utilizing the signal of all simultaneously observed spectral lines. Conclusions. We have obtained the first diffraction-limited full Stokes spectro-polarimetric datasets recorded with a slit-scanning spectrograph on a telescope with an aperture exceeding 1.5 metres.

Solar active region 11283 produced an X2.1 flare associated with a solar eruption on September 6, 2011. Observations revealed a preflare sigmoidal structure and a circular flare ribbon surrounding the typical two ribbon structure, along with remote brightenings located at a considerable distance from the main flare site. To interpret these observations in terms of the three dimensional (3D) coronal magnetic field dynamics, we conducted data constrained magnetohydrodynamic (MHD) simulations. Using a non linear force free field (NLFFF) as the initial condition, we reconstructed a realistic pre flare magnetic environment, capturing a sheared sigmoid above the polarity inversion line (PIL) surmounted by a fan spine structure. Our simulations revealed that reconnection between the sigmoidal field, the adjacent fan dome field lines, and the neighboring large loops facilitated the transfer of magnetic twist and led to the formation of a large magnetic flux rope (MFR). This transfer and propagation of twist are clearly visible throughout the MFR. As reconnection progresses, the entire fan spine structure expands along with the evolving MFR. A notable outcome of the simulation is that the footpoints of the newly formed MFR align closely with the observed circular flare ribbon and the remote brightening region. Our findings suggest that a large MFR formed during the X2.1 flare, providing a coherent explanation for the observed phenomena.

On 2024 July 25, while observing the solar active region NOAA 13762 with the high-resolution 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory, we witnessed two mysterious phenomena: the partial detachment of filament strands from its main body in the chromosphere and the sudden disappearance of a sunspot penumbra in the photosphere, the former accompanied by small flares. Our analysis reveals a spatiotemporal correlation between the filament peeling process and the penumbral disappearance. To understand the above observations physically, we performed a magnetohydrodynamic simulation that successfully replicated the disappearance of the penumbra as a consequence of weakened horizontal magnetic field. The simulations demonstrate that both the filament peeling and the penumbral decay are driven by the same underlying process: the upward expansion of the magnetic flux rope induced by null point magnetic reconnection. These results suggest a novel mechanism by which the Sun sheds magnetic flux to interplanetary space in the form of filament peeling and penumbral disappearance.

The increasing demand for high-resolution solar observations has driven the development of advanced data processing and enhancement techniques for ground-based solar telescopes. This study focuses on developing a python-based package (GT-scopy) for data processing and enhancing for giant solar telescopes, with application to the 1.6 m Goode Solar Telescope (GST) at Big Bear Solar Observatory. The objective is to develop a modern data processing software for refining existing data acquisition, processing, and enhancement methodologies to achieve atmospheric effect removal and accurate alignment at the sub-pixel level, particularly within the processing levels 1.0–1.5. In this research, we implemented an integrated and comprehensive data processing procedure that includes image de-rotation, zone-of-interest selection, coarse alignment, correction for atmospheric distortions, and fine alignment at the sub-pixel level with an advanced algorithm. The results demonstrate a significant improvement in image quality, with enhanced visibility of fine solar structures both in sunspots and quiet-Sun regions. The enhanced data processing package developed in this study significantly improves the utility of data obtained from the GST, paving the way for more precise solar research and contributing to a better understanding of solar dynamics. This package can be adapted for other ground-based solar telescopes, such as the Daniel K. Inouye Solar Telescope (DKIST), the European Solar Telescope (EST), and the 8 m Chinese Giant Solar Telescope, potentially benefiting the broader solar physics community.

We present a unique observation of the X6.4-class flare SOL2024-02-22T22:34 using the Mid-InfraRed Imager (MIRI) at the Goode Solar Telescope. Three ribbon-like flare sources and one unidentified source were detected in MIRI’s two mid-infrared (mid-IR) bands at 5.2 and 8.2 μm. The two stronger ribbons displayed maximum mid-IR enhancements of 21% and 18% above quiet-Sun levels and 10% in Helioseismic and Magnetic Imager (HMI) continuum intensity (I _c ). The weak ribbon and the unidentified source had maximum mid-IR enhancements of 7% but showed HMI/I _c dimmings, instead of excess emissions. Our result suggests that mid-IR emission forms in a higher layer during the flare and is more sensitive to flare heating than HMI/I _c emission. The MIRI observations have high temporal resolution (2.6 s cadence in these observations) and show apparent source motions. One flare ribbon extends along weak vertical magnetic-field channels in the sunspot umbra, light bridge, and penumbra, with an approximately 30 s delay between HMI/I _c and 8.2 μm emissions. Meanwhile, the unidentified source moved at an apparent speed of 130 km s⁻¹ from a mixed-polarity area to one flare ribbon with a strong HMI/I _c enhancement. We studied available hard X-ray/microwave imaging spectroscopy and used nonlinear force-free field extrapolation modeling to identify flare structures. The observational evidence strongly favors the chromospheric origin of the unidentified mid-IR source. Comparison with the X1.0 flare SOL2022-10-02T20:25 indicates that the total amount of high-energy electron (>60 keV) flux density is a key factor in determining the total brightening area and the maximum intensity enhancement in HMI/I _c emissions.