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3U CubeSat stabilization towards the Sun is analyzed. The satellite is equipped with solar telescope of the vacuum ultraviolet spectrum. Stabilization of the optical axis of the telescope is achieved through the use of either reaction wheels or magnetorquers. The desired level of stabilization accuracy is set at two angular minutes per second. Performance of both control systems is compared on a Sun synchronous orbit. The study formulates the necessary operational conditions for the satellite's successful functioning and establishes the key parameters of its control systems. [ABSTRACT FROM AUTHOR]

The results of the spectral observation analysis in the Hα line of a site of active region NOAA 11 024, which has been in the main phase of development given its sharply increased activity, are discussed. The studied site (its length is 10 Mm) has been located in the region of a new serpentine magnetic flow emergence. An arch filament system (AFS) has been formed on it, under which an Ellerman bomb (EB) emerged and developed, and a pore formed at a distance of approximately 7.2 Mm from the EB. The evolution of the AFS is studied, and the formation and development of all Hα-ejections that formed in its magnetic loops during the observations are investigated. Spectral data with high spatial (approximately 1″) and temporal (approximately 3 s) resolution were obtained with the THEMIS French–Italian solar telescope (Tenerife, Spain) on July 4, 2009. The observation time is 20 min (to 9:52–10:11 UT). We use the spectral region that contains the central part of the Hα chromospheric line. In all spectra, Hα ejections (surges) are visible both in the long wavelength and short wavelength wings of the absorption line. The changes in the Stokes I profiles shape are studied, which are very diverse and appreciably different from the profile for the undisturbed chromosphere. Depending on whether the ejection moved to the upward direction or to the downward one, the profile component corresponding to it is projected onto the blue or red line wing. Substantially broadened and dual lobed profiles appear close to the end of the observations, which indicates that both downward and upward plasma flows exist nearby. It is found that surges can be comprised of several jets that are formed during successive and periodic magnetic reconnections. Doppler shifts of the profile components are used to calculate the line-of-sight velocities (Vlos) of chromospheric matter in surges. The changes in the Vlos along the cross section of the surge jets at the place of their maximum intensity are analyzed. The Vlos of jets are different and probably depend on the magnetic field structure in the surge and the surrounding environment. The direction of jet movement is also different, since it depends on the phase of surge development. Most of the curves of Vlos changes consist of several segments. This indicates that the large jets are composed of several smaller jets, i.e., they had a fibrous structure. The flows of ascending and descending surges often occur simultaneously and coincide in time with the increase of the EB brightness. A vortex motion of the plasma is observed in one of the surges for approximately 3 min, as evidenced by the inclined dark streaks in the spectra. At the instant of the greatest brightness of the EB, there are seven surges in the studied site of AR, and the plasma moves downward with Vlos up to 77 km/s in three of them and upward with a much lower Vlos up to –35 km/s in five of them. During our observations, the maximum upward velocities of the chromospheric matter in the surges reach –110 km/s, and the downward velocities reach 90 km/s. In the upper part of the magnetic loops, the plasma velocities vary between –25 km/s and 22 km/s. The Vlos values in the site without active formations do not exceed ±2 km/s. It is also analyzed whether the processes of AFS evolution and EB development phases are related. The study is based on a detailed analysis of observational data obtained with high spatial and temporal resolutions, which allowed the authors to better understand the dynamics of the evolution of the arch filament system under which an Ellerman bomb has emerged and developed, and to reveal the features of the formation and development of surges that form in its magnetic loops. They are probably the result of successive and periodic magnetic reconnections, which are associated with the emergence of a new serpentine magnetic flux and occur when its loops interact with the preexisting magnetic field surrounding the active region or between the magnetic loops of the flux itself. [ABSTRACT FROM AUTHOR]

The Spectro-Polarimeter (SP) is a new instrument installed at the upgraded Andrei B. Severny Solar Tower Telescope (STT) at the Crimean Astrophysical Observatory. The instrument is a traditional echelle slit dual-beam spectropolarimeter with temporal modulation of the polarization. STT-SP provides simultaneous spectropolarimetric observations of the Sun within three 15 Å wide spectral ranges around photospheric Fe I 5250 Å, Fe I 5324 Å, and chromospheric Mg I b2 5172 Å spectral lines. The spectral resolution of the instrument reaches 70,000 with the seeing-constrained slit width of 1 arcsec. The field-of-view of STT-SP is 200 arcsec allowing one to map a moderate size active region within a single raster scan. The instrument will provide new opportunities in the analysis of magnetic fields and thermodynamics of the lower atmosphere of the Sun. In this paper we describe the optical design of STT-SP and present the preliminary results acquired during the commissioning of the instrument. [ABSTRACT FROM AUTHOR]

The Ly α Solar Telescope (LST) is the first instrument to achieve imaging of the full solar disk and the coronal region in both white light (WL) and ultraviolet (UV) H i Ly α , extending up to 2.5 solar radii (Rs), contributing to solar physics research and space weather forecasting. Since its launch on 9 October 2022, LST has captured various significant solar activity phenomena, including flares, filaments, prominences, and coronal mass ejections (CMEs). On-orbit observation and test results show that LST covers a continuous spatial range and the wavelengths of 121.6, 360, and 700 nm. The Ly α Solar Disk Imager (SDI) has a field of view (FOV) of 38.4′ and a spatial resolution of around 9.5″, while the White-Light Solar Telescope (WST) has an FOV of 38.43′ and a spatial resolution of around 3.0″. The FOV of the Ly α Solar Corona Imager (SCI) reaches 81.1′ and its spatial resolution is 4.3″. The stray-light level in the 700 nm waveband is about 7.8 × 10−6 MSB at 1.1 Rs and 7.6 × 10−7 MSB at 2.5 Rs, and in Ly α waveband it is around 4.3 × 10−3 MSB at 1.1 Rs and 4.1 × 10−4 MSB at 2.5 Rs (MSB: mean solar brightness). This article will detail the results from on-orbit tests and calibrations. [ABSTRACT FROM AUTHOR]

High spectral resolution imaging spectroscopy plays a crucial role in solar observation, regularly serving as a backend instrument for solar telescopes. These instruments find direct application in deriving Doppler velocity from hyperspectral images, offering insights into the dynamic motion of matter on the solar surface. In this study, we present the development of a Fabry–Pérot interferometer (FPI) based imaging spectrometer operating at the Fe I (617.3 nm) wavelength for precise Doppler velocity measurements. The spectrometer features a moderate spectral resolution of λ / Δ λ ≈ 60 , 000 , aiming to balance the imaging signal-to-noise ratio (SNR). The instrument underwent successful observational experiments on the 65-cm Educational Adaptive-Optics Solar Telescope (EAST) at the Shanghai Astronomy Museum. Obtained Doppler velocities were compared with data from the Helioseismic and Magnetic Imager (HMI), the maximum column and row correlation coefficients are 0.9261 and 0.9603, respectively. The estimated cut-off normalized frequency of the power spectral density (PSD) curve for velocity map is approximately 0.4/0.21 times higher than that observed in the HMI data, with potentially higher spatial resolution achievable under better seeing conditions. Based on the estimated imaging SNR levels, the accuracy of velocity measurements is approximately 50 m s−1. [ABSTRACT FROM AUTHOR]

Minifilaments are widespread small-scale structures in the solar atmosphere. To better understand their formation and eruption mechanisms, we investigate the entire life of a sigmoidal minifilament located below a large quiescent filament observed by Big Bear Solar Observatory/Goode Solar Telescope on 2015 August 3. The H α structure initially appears as a group of arched threads, then transforms into two J-shaped arcades, and finally forms a sigmoidal shape. Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly observations in 171 Å show that two coronal jets occur around the southern footpoint of the minifilament before the minifilament eruption. The minifilament eruption starts from the southern footpoint, then interacts with the overlying filament and fails. The aforementioned observational changes correspond to three episodes of flux cancellations observed by SDO/Helioseismic and Magnetic Imager. Unlike previous studies, the flux cancellation occurs between the polarity where the southern footpoint of the minifilament is rooted and an external polarity. We construct two magnetic field models before the eruption using the flux rope insertion method and find a hyperbolic flux tube above the flux cancellation site. The observation and modeling results suggest that the eruption is triggered by the external magnetic reconnection between the core field of the minifilament and the external fields due to flux cancellations. This study reveals a new triggering mechanism for minifilament eruptions and a new relationship between minifilament eruptions and coronal jets. [ABSTRACT FROM AUTHOR]

The investigation of plasma motions in the solar chromosphere is crucial for understanding the transport of mechanical energy from the interior of the Sun to the outer atmosphere and into interplanetary space. We report the finding of large-amplitude oscillatory transverse motions prevailing in the non-spicular Hα chromosphere of a small quiet region near the solar disk center. The observation was carried out on 2018 August 25 with the Microlensed Hyperspectral Imager (MiHI) installed as an extension to the spectrograph at the Swedish Solar Telescope (SST). MiHI produced high-resolution Stokes spectra of the Hα line over a two-dimensional array of points (sampled every 0.066″ on the image plane) every 1.33 s for about 17 min. We extracted the Doppler-shift-insensitive intensity data of the line core by applying a bisector fit to Stoke I line profiles. From our time–distance analysis of the intensity data, we find a variety of transverse motions with velocity amplitudes of up to 40 km s−1 in fan fibrils and tiny filaments. In particular, in the fan fibrils, large-amplitude transverse MHD waves were seen to occur with a mean velocity amplitude of 25 km s−1 and a mean period of 5.8 min, propagating at a speed of 40 km s−1. These waves are nonlinear and display group behavior. We estimate the wave energy flux in the upper chromosphere at 3 × 106 erg cm−2 s−1. Our results contribute to the advancement of our understanding of the properties of transverse MHD waves in the solar chromosphere. [ABSTRACT FROM AUTHOR]

Context. Flat-fielding spectro-polarimetric data with one spatial and one spectral dimension is inherently difficult as the imprint of the spectral lines needs to be separated from other wavelength-dependent instrumental effect (e.g., fringes or prefilter profiles) and wavelength-independent effects (e.g., dust and sensor response). Current approaches for spectrometers are often based on moving the grating or they depend on optical models and/or on lab calibration data. They are also limited to small spectral regions and are instrument-specific. Approaches that would be suitable for polarimeters have not been reported yet. Aims. We present an approach that allows for flat-field calibration data to be to obtained for diffraction-grating-based, long-slit spec-trographs combined with temporally modulated polarimetry from high-resolution solar telescopes. This approach is based on nominal flat-fielding procedures performed during the instrument's science operations. Methods. We performed a precise and field-dependent correction of the spectrographic distortion effect (resulting in curved spectral lines, typically denoted as a "smile" effect) to ensure the orthogonality of the spectral and spatial dimensions. We identified distortions by tracking the position of multiple spectral lines within the full spectral field of view. From the raw modulated flats, we then removed the solar line imprints and derived separate flat-fields for sensor and slit dust features. Optionally, wavelength calibration and continuum correction can be included in this process. Results. We have created generic Python libraries that can be plugged into existing Python-based data reduction pipelines or used as a standalone calibration tool. We show that for spectrographs covering many spectral lines, a correction of the smile distortion based on optical models alone is not sufficient. Our results demonstrate a suppression of fringes, sensor artifacts, and fixed-pattern imprints in demodulated data by one order of magnitude. For intensity images, the photon noise level can be closely attained after calibration. Our correction works across the full spectral range. The algorithm was tested for different wavelength regimes with emission (EUV range) or absorption (near-UV, VIS, IR range) spectra, on data acquired with ground-based (SST/TRIPPLE-SP, GREGOR/GRIS), balloon-borne (SUNRISE-III/SUSI), and space-based (SolO/SPICE) instruments. The data calibrated with our method offer robust and precise inversion results. Conclusions. We have extended existing spectroscopic flat-field techniques to modern instruments with large imaging sensors covering many spectral lines simultaneously, and with polarimetric capabilities, where methods described so far are not adequate. We believe that our method is applicable as a standard calibration approach for most modern high resolution large-FOV, long-slit spectrographs – both with and without polarimetric capabilities. [ABSTRACT FROM AUTHOR]

We introduce a new solar feed for the PEPSI nighttime spectrograph of the LBT. It enables spectroscopy of the Sun‐as‐a‐star in circular polarization (CP) and linear polarization (LP) with a spectral resolution of 250,000 (≈0.025Å or 600ms−1$$ 600\kern0.1em \mathrm{m}\kern0.1em {\mathrm{s}}^{-1} $$) for the wavelength range 383–907 nm. The polarimeter is a dual‐beam design with a modified Wollaston prism as beam splitter and linear polarizer combined with a retractable super‐achromatic λ/4$$ \lambda /4 $$ retarder. The Wollaston beam diameter is 14 mm and large enough that it does not require a classical telescopic feed anymore. Both polarimetric beams are re‐imaged into respective integration spheres from which two fibers feed the scrambled light to the spectrograph. The system is fully automated in the sense that it finds the Sun in the morning, closes the guider loop, observes a predefined number of individual spectra, and moves to a home position at the end of the day. Among the scientific aims is Zeeman–Doppler imaging of the Sun as a star over the next activity cycle. Our first‐light application detects a clear Stokes‐V/I profile with a full amplitude of 1×10−4$$ 1\times {10}^{-4} $$ on, for example, October 13, 2023, suggesting a solar disk‐averaged line‐of‐sight net magnetic field of +0.37±0.02 G. Comparison of this value with a contemporary full‐disk line‐of‐sight magnetogram suggests an unsigned mean field of about ≈13 G. [ABSTRACT FROM AUTHOR]

Effective data compression technology is essential for addressing data storage and transmission needs, especially given the escalating volume and complexity of data generated by contemporary astronomy. In this study, we propose utilizing deep learning-based lossless compression techniques to improve compression efficiency. We begin with a qualitative and quantitative analysis of the temporal and spatial redundancy in solar observation data. Based on this analysis, we introduce a novel deep learning-based framework called AstroDLLC for the lossless compression of astronomical solar images. AstroDLLC first segments high-resolution images into blocks to ensure that deep learning model training does not rely on high-computation power devices. It then addresses the non-normality of the partitioned data through simple reversible computational methods. Finally, it utilizes Bit-swap to train deep learning models that capture redundant features across multiple image frames, thereby enhancing compression efficiency. Comprehensive evaluations using data from the New Vacuum Solar Telescope reveal that AstroDLLC achieves a maximum compression ratio of 3.00 per image, surpassing Gzip, RICE, and other lossless technologies. The performance of AstroDLLC underscores its potential to address data compression challenges in astronomy. [ABSTRACT FROM AUTHOR]

The pointing accuracy of large radio telescopes is affected by antenna structure errors, installation errors, servo errors, gravity, and varying environmental factors. Although an on-line pointing model had been developed in the past for several large radio telescopes to correct these effects, it is also valuable to explore new ways to correct these effects. Therefore, in order to estimate the dynamic pointing error of large radio telescopes under solar radiation, a new way based on digital twin (DT) is used in this paper. Digital models of the Wuqing 70-m radio telescope (WRT70) are created based on the DT operational framework. Finally, the reliability of the WRT70's DT system is verified, and the dynamic pointing error of WRT70 (staying at the position of the elevation angle of 90° and the azimuth angle of 180°) on two sunny days in different seasons is estimated by the created DT system. The research results show that the pointing error of large radio telescopes under solar radiation fluctuates greatly and the seasonal differences are very striking. The maximum elevation pointing error of WRT70 under solar radiation around the winter solstice is over 40 arcsec, which is equal to 4/11 of the beam width for WRT70 operating at 8 GHz. In this paper, it is feasible to estimate the time-varying pointing errors of large radio telescopes at rest under solar radiation by constructing a DT system, but it is not effective to estimate the dynamic pointing errors of large radio telescopes in operation. It is expected to establish a dynamic pointing error model and calibrate the dynamic pointing errors of large radio telescopes in operation by DT technology in the future. [ABSTRACT FROM AUTHOR]

The design and usability of a fully autonomous robotic control system (SunbYte - Sheffield University Balloon “lYfted” TElescope) for solar tracking and observational applications onboard high-altitude balloons are addressed here. The design is based on a six-step development plan balancing scientific objectives and practical engineering requirements. The high-altitude solar observational system includes low-cost components such as a Cassegrain-type telescope, stepper motors, harmonic drives, USB cameras and microprocessors. OpenCV installed from ROS (Robotic Operating System), python and C facilitated the collection, compression, and processing of housekeeping and scientific data. This processed data was then transmitted to the ground station through the launch vehicle’s telecommunication link. The SunbYte system allows the brightest spot in the sky, the sun, to be identified, and a telescope pointed towards it with high enough accuracy that a scientific camera can capture images. This paper gathers and presents the results from primarily two missions with the High-Altitude Student Platform (HASP, NASA Balloon Program office and LaSpace). Additionally, a discussion will be made comparing these with an earlier iteration flown with the German-Swedish “REXUS/BEXUS” programme coordinated by the European Space Agency. By capturing and analysing a series of tracking images with the location of the Sun at the calibrated centre, the system demonstrated the tracking capabilities on an unstable balloon during ascent. Housekeeping sensor data was collected to further analyse the thermal and mechanical performance. The low temperature increased friction in the drive train and reduced the responsiveness of the harmonic drive actuation system. This caused some issues which require further work in future missions, for example, with SunbYte 4 and its work when flying with the HEMERA ZPB (Zero Pressure Balloon) program. [ABSTRACT FROM AUTHOR]

Large ground-based solar telescopes are equipped with adaptive optics systems to correct wavefront distortions induced in the turbulent atmosphere. The design of the adaptive optics system strongly depends on the vertical profiles of the optical turbulence. In particular, the characteristics of the optical turbulence determine the design of tomographic adaptive optics systems, which provide image correction within a wide field of view. In the article, a new method to estimate reference optical turbulence characteristics from Era-5 reanalysis assimilated data is presented. This method is based on the dependence of the air refractive index structure constant |$C_n^2$| on the vertical shears of wind speed as well as the outer scale of turbulence L0. The L0 parameter is estimated by minimization of the dispersion between the modeled and measured values of the refractive index structure constant |$C_n^2$| within the surface layer. For the first time, parametrization coefficients and reference profiles of optical turbulence averaged for the period 1940–2022 are calculated for the Large Solar Vacuum Telescope (LSVT) site. The calculated optical turbulence profiles are representative; these profiles correspond to typical changes of the measured values of the Fried parameter, the isoplanatic angle, and the outer scale of turbulence at the LSVT site. The model turbulence profiles are verified taking into account the Shack–Hartmann wavefront sensor measurements at the LSVT. The higher accuracy of estimation of the optical turbulence characteristics makes it possible to refine parameters relevant to the LSVT adaptive optics system. The obtained results can be used in order to develop high-resolution solar adaptive optics technologies as applied to ground-based telescopes including those using the principles of atmospheric tomography. [ABSTRACT FROM AUTHOR]

The cross-correlation of two-dimensional digital images is fundamental to solar adaptive optics computations. It can be used in a simple tip-tilt correction system to identify the relative shift between consecutive images and correlating sub-aperture images of a Shack–Hartmann wave-front sensor. The typical frequency of computation is about 1 kHz. While the software-based optimized cross-correlations may be sufficient when a small number of sub-apertures are used in a wave-front sensor, hardware-accelerated (FPGA), correlations may be required when a large number of sub-aperture images are involved, e.g., in the case of the proposed National Large Solar Telescope in India. This paper presents SolarAccel: An FPGA-based acceleration of a basic two-dimensional cross-correlation of two images. We accelerate the FPGA-based design by pipelining the individual components of the cross-correlation process. We implemented our RTL logic on a few sets of 128 × 128 pixel images and 32 × 32 pixel images on a Xilinx Zynq Ultrascale + MPSoC on the ZCU104 FPGA evaluation platform. SolarAccel performs a 2D FFT on a 128 × 128 image faster than existing work. The cross-correlation on a 32 × 32 image is also faster than the existing work. This demonstrates that FPGA acceleration is beneficial in solar adaptive optics applications. [ABSTRACT FROM AUTHOR]

One of the key factors of space weather is solar flare activity, the monitoring and prediction of which is an important task of specialized dedicated groups of space experts and solar astronomers. Solar flare forecasts are based on identifying and detecting the so-called precursors, specific processes in solar activity events that occur before flares. Collecting data for space weather analysis and prediction comes down to several types of measurements performed by more than a dozen spacecraft. Ground-based observations and monitoring nowadays are becoming more or less complimentary. One of the reasons for this is the limitation of observation time with ground-based telescopes due to adverse Earth weather conditions. However, solar radio astronomy is immune to almost any weather activity, and the main question here is what new quality it can bring. Observational data accumulated in the 20th century show that solar radio bursts can be associated with flare activity. In addition, the existing network of solar radio telescopes is already well established. As an example, in this paper, we describe the possibilities of a fully steerable 32-meter radio telescope of Ventspils International Radio Astronomy Centre (VIRAC), Latvia, which can be useful for searching for new precursors of solar flares. [ABSTRACT FROM AUTHOR]

In the forefront of quantitative solar physics research using large-aperture ground-based solar optical telescopes, high-contrast observation along with high-accuracy polarimetric measurement in the solar active region are required. In this paper, we propose a novel high-contrast imaging telescope construction with a 60 cm medium aperture, namely, the PAthfinder in Solar Adaptive Telescope (PASAT), in which a deformable secondary mirror is used as the adaptive optical correction device and a symmetrical optical path design is employed, leading to the least Muller matrix polarization instruments. The telescope can provide a high-resolution magnetic field with high accuracy for the solar active regions, as well as high-contrast images with a superior signal-to-noise ratio and photometric accuracy of the solar photosphere and chromosphere. These data will be directly used for a better understanding of the evolution and release of magnetic energy, which will help in improving space weather forecasting. Meanwhile, PASAT will accumulate the relevant techniques for constructing similar, larger solar telescopes in the future. [ABSTRACT FROM AUTHOR]

Context. Solar flare ribbons are intense brightenings of primarily chromospheric material that are responsible for a large fraction of the chromospheric emission in solar and stellar flares. We present an on-disc observation of flare ribbon substructures in an X9.3-class flare observed by the Swedish 1-m Solar Telescope. Aims. We aim to identify categories of ribbon substructures seen in the Ca II 8542 Å, Hα, and Ca II K lines, focusing on their spatial locations and their (spectro-)polarimetric properties. Methods. We used COlor COllapsed Plotting (COCOPLOT) software to assist in identifying areas of interest. Results. We present five categories of spectral profiles within the general body of the flare ribbon: (1) extremely broadened spectral line profiles, where the standard Fabry–Perot interferometer wavelength windows (≈70 km s−1) are not sufficiently wide to allow for a complete analysis of the dynamics and atmospheric conditions. The mechanisms causing this degree of this broadening are not yet clearly understood; (2) long-lived, dense kernels that manifest as more saturated chromospheric line profiles with lower signal in both Stokes parameters. They are interpreted as footpoints of bunched magnetic field loops, whose chromospheric lines form at greater heights than the nearby areas; (3) Doppler-shifted leading edges of the flare ribbon in regions that transiently display lower Stokes signals due to the emission dominating at greater heights in the atmosphere; (4) condensed coronal rain overlapping the flare ribbons in the line of sight, producing exceptionally high Doppler shifts near the footpoints; and (5) compact blueshifted areas close to areas with coronal rain down-flows, which are understood to be material that has been thrown up as a result of the down-flowing material impacting the chromosphere. Additionally, a ribbon formation height of about 700 km with respect to penumbral features is estimated using correlating structures on the ribbon and the underlying photosphere. Conclusions. When selecting areas of the flare ribbon for more general analysis (especially small regions consisting of a few pixels or low-resolution averages), it is important to be aware of the variety of substructures present within a flare ribbon and of the spatial context that can produce these differences. General behaviors across the ribbon should not be inferred from regions that show localized differences. [ABSTRACT FROM AUTHOR]

We discuss the chosen concepts, detailed design, implementation and calibration of the 85-electrode adaptive optics (AO) system of the Swedish 1-meter Solar Telescope (SST), which was installed in 2013. The AO system is unusual in that it uses a combination of a monomorph mirror with a Shack-Hartmann (SH) wavefront sensor (WFS) and a second high-resolution SH microlens array to aid the characterization, calibration, and modal control of the deformable mirror. An Intel PC workstation performs the heavy image processing associated with cross-correlations and real-time control at a 2 kHz update rate with very low latency. The computer and software continue the successful implementation since 1995 of earlier generations of correlation tracker and AO systems at SST and its predecessor, the 50-cm Swedish Vacuum Solar Telescope, by relying entirely on work-station technology and an extremely efficient algorithm for implementing cross-correlations with the large field of view of the WFS. We describe critical aspects of the design, calibrations, software, and functioning of the AO system. The exceptionally high performance is testified through the highest Strehl ratio (inferred from the measured granulation contrast) of existing meter-class solar telescopes, as demonstrated here at wavelengths shorter than 400 nm and discussed in more detail in a previous separate publication We expect that some aspects of this AO system may also be of interest outside the solar community. [ABSTRACT FROM AUTHOR]

Multiconjugate adaptive optics (MCAO) stands as an essential technology for the development of future large-aperture solar telescopes. Its primary objective is to empower telescopes to achieve nearly diffraction-limited performance while substantially extending the correction field of view (FoV). Conventional solar MCAO relies on the combination of adaptive optics and high-altitude correction (AO + HAC) modules for multiconjugate correction. However, this architectural approach excels in correction performance primarily at the central position, with performance deteriorating as one moves farther from the centre. Consequently, it results in poor consistency of FoV correction performance. To address these limitations, a new architectural approach was introduced, which combines ground layer AO with HAC (GLAO + HAC). Preliminary results have shown that, compared to AO + HAC, this approach significantly enhances FoV correction uniformity. Building upon these initial findings, this paper undertakes a more extensive research of the GLAO + HAC system. Its objective is to compare various solar MCAO system architectures, including AO + HAC, GLAO + HAC, and general MCAO, to finally propose optimization tailored to GLAO + HAC. Through this analysis, the paper conducts the performance comparison between GLAO + HAC and general MCAO. It underscores that, under equivalent configuration parameters, the differences between these two systems are marginal. However, due to the advantage of the independent control of dual correction modules in GLAO + HAC, it can introduce an optimization strategy by increasing the number of subapertures at the cost of reducing the GLAO guide star sensing FoV. Finally, the results of this strategy demonstrate an obvious enhancement in performance and FoV correction consistency within the GLAO + HAC system. [ABSTRACT FROM AUTHOR]

Since the concept of adaptive optics(AO) was proposed in 1953, AO has become an indispensable technology for large aperture ground-based optical telescopes aimed at high resolution observations. This paper provides a comprehensive review of AO progress for large aperture astronomical optical telescopes including both night-time and day-time solar optical telescopes. The recent AO technological advances, such as Laser Guide Star, Deformable Secondary Mirror, Extreme AO, and Multi-Conjugate AO are focused. [ABSTRACT FROM AUTHOR]

The strongly coupled hydrodynamic, magnetic, and radiation properties of the plasma in the solar chromosphere make it a region of the Sun's atmosphere that is poorly understood. We use data obtained with the high-resolution Visible Broadband Imager (VBI) equipped with an H β filter and the Visible Spectro-Polarimeter (ViSP) on the Daniel K. Inouye Solar Telescope to investigate the fine-scale structure of the plage chromosphere. To aid in the interpretation of the VBI imaging data, we also analyze spectra from the CHROMospheric Imaging Spectrometer on the Swedish Solar Telescope. The analysis of spectral properties, such as enhanced line widths and line depths, explains the high contrast of the fibrils relative to the background atmosphere demonstrating that H β is an excellent diagnostic for the enigmatic fine-scale structure of the chromosphere. A correlation between the parameters of the H β line indicates that opacity broadening created by overdense fibrils could be the main reason for the spectral line broadening frequently observed in chromospheric fine-scale structures. Spectropolarimetric inversions of the ViSP data in the Ca ii 8542 Å and Fe i 6301/6302 Å lines are used to construct semiempirical models of the plage atmosphere. Inversion outputs indicate the existence of dense fibrils in the Ca ii 8542 Å line. The analyses of the ViSP data show that the morphological characteristics, such as orientation, inclination, and length of fibrils, are defined by the topology of the magnetic field in the photosphere. Chromospheric maps reveal a prominent magnetic canopy in the area where fibrils are directed toward the observer. [ABSTRACT FROM AUTHOR]

Solar radio bursts are intense radio radiation sources that occur during the energy-release process and represent a hot topic in solar-physics and space-weather research. In this paper, we present a multimode prediction model for daily solar radio bursts. The model uses deep learning and machine learning to obtain data information from different dimensions and to establish the relationship between the characteristics of the solar active region on the solar surface and solar radio bursts. For this model, we use data from the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI) total solar magnetic map, the Royal Observatory of Belgium World Data Centre in Brussels, and NOAA sunspot parameters (including number, area, and type of sunspots) as inputs. The output results are then compared with the list of solar radio bursts recorded by the Radio Solar Telescope Network (RSTN) to determine whether solar radio bursts are present and to determine the key parameters for determining radio bursts. Based on 5449 days of observational data, we find that the prediction accuracy of the model is 0.898 ± 0.011, and that the number of sunspots is a key parameter in determining the occurrence of solar radio bursts. Specifically, when the number of sunspots is greater than 15, the probability of occurrence of solar radio bursts is greater than 90%. We have identified the key parameters and thresholds for determining solar radio bursts and highlighted the key parameters for space-weather prediction. In addition, the prediction model can also be used for predicting in other fields. [ABSTRACT FROM AUTHOR]

From the beginning of January 2021 to the end of August 2023, the radiation monitor of the Spektr-RG spacecraft registered three enhancements in the count rate, which exceed the background variations during the solar activity cycle and have a comparable maximum value. These enhancements are associated with solar proton events (SPEs) from the flares X1.0 on October 28, 2021; M6.3 on February 25, 2023; and M5.7 on July 17, 2023. Using the example of these events, as well as smaller SPEs from the flares M3.7 on February 24, 2023, and M4.0 on July 16, 2023, threshold criteria for "proton" flares are discussed. In powerful SPEs, the contribution of solar protons to the radiation dose can exceed the total contribution of galactic cosmic rays (GCR) over a sufficiently long period of time. Therefore, such SPEs are sources of increased radiation hazard and require prediction based on real-time observations. It was shown that, in these five flares, thresholds were overcome according to three criteria: plasma temperature >12 MK (soft X-ray source), duration (>5 min) of microwave or hard X-ray (HXR) radiation (electron acceleration >100 keV), and height of flare development process >60 Mm (radio emission at plasma frequencies <610 MHz). The arrival of the first solar protons >100 MeV to the Earth's orbit was expected no earlier than 10 min relative to the beginning of HXR or microwave radiation, i.e., could have been predicted in advance. To study the relationship between solar flares and SPEs, we used data from the anticoincidence shield of the spectrometer on INTEGRAL (ACS SPI), which is an effective but uncalibrated detector of HXR >100 keV and protons >100 MeV, as well as patrol observations of radio emission at fixed frequencies (Radio Solar Telescope Network). It is noted that the X2.2 (N25E64) flare on February 17, 2023 satisfied all three "protonity" criteria and could become the source of a powerful SPE near the Earth in a case of favorable location on the Sun. In the M8.6 (N27W29) flare on February 28, 2023, the third criterion was not met, and it did not lead to an SPE as expected (it developed in a plasma with a density >2.5 × 1010 cm–3 and plasma frequency >1415 MHz). [ABSTRACT FROM AUTHOR]

Context. Transmission spectroscopy is one of the most powerful techniques used to characterize transiting exoplanets, since it allows for the abundance of the atomic and molecular species in the planetary atmosphere to be measured. However, stellar lines may bias the determination of such abundances if their center-to-limb variations (CLVs) are not properly accounted for. Aims. This paper aims to show that three-dimensional (3D) radiation hydrodynamic models and the assumption of non-local ther-modynamic equilibrium (non-LTE) line formation are required for an accurate modeling of the stellar CLV of the Na I D1 and K I resonance lines on transmission spectra. Methods. We modeled the CLV of the Na I D1 and K I resonance lines in the Sun with 3D non-LTE radiative transfer. The synthetic spectra were compared to solar observations with high spatial and spectral resolution, including new data collected with the CRISP instrument at the Swedish 1-m Solar Telescope between µ = 0.1 and µ = 1.0. Results. Our 3D non-LTE modeling of the Na I D1 resonance line at 5896 Å and the K I 7699 Å resonance line in the Sun is in good agreement with the observed CLV in the solar spectrum. Moreover, the simulated CLV curve for a Jupiter-Sun system inferred with a 3D non-LTE analysis shows significant differences from the one obtained from a 1D atmosphere. The latter does indeed tend to overestimate the amplitude of the transmission curve by a factor that is on the same order of magnitude as a planetary absorption depth (i.e., up to 0.2%). Conclusions. This work highlights the fact that to correctly characterize exoplanetary atmospheres, 3D non-LTE synthetic spectra ought to be used to estimate the stellar CLV effect in transmission spectra of solar-like planet hosts. Moreover, since different spectral lines show different CLV curves for the same geometry of the planet-star system, it is fundamental to model the CLV individually for each line of interest. The work will be extended to other lines and FGK-type stars, allowing for synthetic high-resolution spectra to mitigate the stellar contamination of low-resolution planetary spectra, for example, those drawn from JWST. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is believed to play an important role in the release and conversion of energy among magnetized plasma systems. So far, we have been unable to understand under what conditions magnetic reconnection can take place. Based on observations from the New Vacuum Solar Telescope and the Solar Dynamics Observatory (SDO), we study 16 magnetic reconnection events, and each event has a clear X-type configuration consisting of two sets of atmospheric structures. We focus on 38 footpoints that are relevant to these structures and can be clearly determined. By using SDO/Helioseismic and Magnetic Imager line-of-sight magnetograms, we track the field evolution of these footpoints. Prior to the occurrence of magnetic reconnection, the associated fields at the footpoints underwent convergence and shear motions, and thus became enhanced and complex. During the converging period, the rates of increase of the mean magnetic flux densities (MFDs) at these footpoints are 0.03–0.25 hr−1. While the unsigned mean MFDs are 70–300 G, magnetic reconnection in the solar atmosphere takes place. Subsequently, the photospheric fields of these footpoints diffuse and weaken, with rates of decrease of the MFDs from 0.03 to 0.18 hr−1. These results suggest that, due to the photospheric dynamical evolution at the footpoints, the footpoint MFDs increase from a small value to a large one, and the corresponding atmospheric magnetic fields become complicated and nonpotential; then reconnection happens and it releases the accumulated magnetic field energy. Our study supports the conjecture that magnetic reconnection releases free magnetic energy stored in the nonpotential fields. [ABSTRACT FROM AUTHOR]

Within the Universat program, a set of solar vacuum ultraviolet (VUV) telescopes has been developed for deployment on 6U nanosatellites. Telescopes are designed to get images of the solar corona. The spectral ranges of observations is considered, the characteristics of the nanosatellite from the point of view of the observations feasibility are opmized, the optical scheme of the telescope and VUV multilayer mirrors coatings and thin-film filters are modelled. [ABSTRACT FROM AUTHOR]

In solar observations, the vertical distribution of turbulence can be detected with a multidirectional Shack–Hartmann wavefront sensor, based on SLODAR or S-DIMM+. By expressing the measured cross-correlation as a linear combination of theoretical correlation functions for various height layers, the distribution of turbulence intensity can be obtained through fitting. The theoretical correlation functions in different heights are employed to describe the slope correlation and calculated based on the phase structure function corresponding to classical turbulence statistical theories such as Kolmogorov or von Karman. However, for turbulence that does not obey classical theory, this deviation of the statistical characteristics would result in the measurement accuracy degradation of seeing profiles. A method, so-called AutoCorrelation-SLODAR (AC-SLODAR), is proposed. The feasibility of transforming cross-correlation into autocorrelation is analysed theoretically. Then, the autocorrelation function is calculated based on the actual data to avoid the deviation introduced by the theoretical turbulence statistical model. Extracting statistical characteristics from data also simplifies measurements, without requiring the evaluation of whether the data conforms to a particular classical statistical model. AC-SLODAR was validated with simulation data generated by the open-source emulator SOAPY. The measured error was reduced by 10 per cent compared with SLODAR for the situation of turbulence model deviation. The performance of AC-SLODAR was further compared with those of SLODAR based on the actual data collected in 2016 and 2017 on the New Vacuum Solar Telescope. The corresponding results of AC-SLODAR are consistent with those of SLODAR using a pre-evaluated turbulent model. [ABSTRACT FROM AUTHOR]

Solar white-light flares (WLFs) are those accompanied by brightenings in the optical continuum or integrated light. The White-light Solar Telescope (WST), as an instrument of the Lyman-alpha Solar Telescope (LST) on the Advanced Space-based Solar Observatory (ASO-S), provides continuous solar full-disk images at 360 nm, which can be used to study WLFs. We analyze 205 major flares above M1.0 from October 2022 to May 2023 and identify 49 WLFs at 360 nm from WST observations, i.e. with an occurrence rate of 23.9%. The percentages of WLFs for M1 – M4 (31 out of 180), M5 – M9 (11 out of 18), and above X1 (7 for all) flares are 17.2%, 61.1%, and 100%, respectively, namely the larger the flares, the more likely they are WLFs at 360 nm. We further analyze 39 WLFs among the identified WLFs and investigate their properties such as white-light enhancement, duration, and brightening area. It is found that the relative enhancement of the white-light emission at 360 nm is mostly (>90%) less than 30% and the mean enhancement is 19.4%. The WLFs' duration at 360 nm is mostly (>80%) less than 20 minutes and its mean is 10.3 minutes. The brightening area at 360 nm is mostly (>75%) less than 500 arcsecond2 and the median value is 225. We find that there exist good correlations between the white-light enhancement/duration/area and the peak soft X-ray (SXR) flux of the flare, with correlation coefficients of 0.68, 0.58, and 0.80, respectively. In addition, the white-light emission in most WLFs peaks around the same time as the temporal derivative of SXR flux as well as the hard X-ray emission at 20 – 50 keV, indicative of the Neupert effect. It is also found that the limb WLFs are more likely to have a greater enhancement, which is consistent with numerical simulations. [ABSTRACT FROM AUTHOR]

The Full-disk MagnetoGraph (FMG) onboard the Advanced Space based Solar Observatory has obtained a series of line-of-sight magnetic-field measurements since its launch in October 2022. It is important to compare its observational data with other existing solar telescopes. In this paper, we make a detailed comparison of four active regions and a pore region simultaneously observed by FMG, the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamic Observatory, and the Solar Magnetism and Activity Telescope (SMAT) at Huairou Solar Observing Station. We find that the magnetic-field structure and spatial distribution are basically consistent among these three instruments. The initial correlation coefficient of magnetic field is approximately 0.90. The coefficient slightly increases when sunspot umbra regions are excluded, and it increases significantly up to 0.98 for the magnetic field in penumbra regions. The magnetic field observed by FMG tends to be weaker than the HMI in strength in sunspot umbra due to saturation effect, whereas larger outside sunspot. The differences are probably due to different noise levels, seeing conditions (SMAT is affected by the Earth's atmosphere) and observational and calibration methods. [ABSTRACT FROM AUTHOR]

Aims. Recurring jets are observed in the solar atmosphere. They can erupt intermittently over a long period of time. By the observation of intermittent jets, we wish to understand what causes the characteristics of the periodic eruptions. Methods. We report intermittent jets observed by the Goode Solar Telescope (GST) with the TiO Broadband Filter Imager (BFI), the Visible Imaging Spectrometer (VIS) in Hα, and the Near-InfraRed Imaging Spectropolarimeter (NIRIS). The analysis was aided and complemented by 1400 Å and 2796 Å data from the Interface Region Imaging Spectrograph (IRIS). These observational instruments allowed us to analyze the temporal characteristics of the jet events. By constructing the Hα dopplergrams, we found that the plasma first moves upward, but during the second phase of the jet, the plasma flows back. Working with time slice diagrams, we investigated the characteristics of the jet dynamics. Results. The jet continued for up to 4 h. The time-distance diagram shows that the peak of the jet has clear periodic-eruption characteristics (5 min) during 18:00 UT–18:50 UT. We also found a periodic brightening phenomenon (5 min) during the jet bursts in the observed bands in the transition region (1400 Å and 2796 Å), which may be a response to intermittent jets in the upper solar atmosphere. The time lag is 3 min. Evolutionary images in the TiO band revealed a horizontal movement of the granulation at the location of the jet. By comparison to the quiet region of the Sun, we found that the footpoint of the jet is enhanced at the center of the Hα spectral line profile, without significant changes in the line wings. This suggests prolonged heating at the footpoint of the jet. In the mixed-polarity magnetic field region of the jet, we observed the emergence of magnetic flux, its cancellation, and shear, indicating possible intermittent magnetic reconnection. This is confirmed by the nonlinear force-free field model, which was reconstructed using the magneto-friction method. Conclusions. The multiwavelength analysis indicates that the events we studied were triggered by magnetic reconnection that was caused by mixed-polarity magnetic fields. We suggest that the horizontal motion of the granulation in the photosphere drives the magnetic reconnection, which is modulated by p-mode oscillations. [ABSTRACT FROM AUTHOR]

Context. The bright heads of penumbral filaments, penumbral grains (PGs), show apparent horizontal motions inward, toward the umbra, or outward, away from the umbra. Aims. We aim to ascertain statistically whether the direction of PGs' apparent motion is related to the inclination of the surrounding magnetic field. Methods. We used spectropolarimetric observations of five sunspot penumbrae to compare magnetic inclinations inside PGs with those in their surroundings. The data were taken by three observatories: the Hinode satellite, the Swedish Solar Telescope, and the GREGOR solar telescope. The direction of PGs' motion was determined by feature tracking. The atmospheric conditions in PGs and their surroundings, including magnetic field information, were retrieved by means of height-stratified spectropolarimetric inversions. Results. Out of a sample of 444 inward-moving PGs and 269 outward-moving ones, we show that 43% of the inward-moving PGs have a magnetic inclination larger by 8° ±4° than the inclination in their surroundings and 51% of the outward-moving PGs have an inclination smaller by 13° ±7° than the surrounding one. The opposite relation of inclinations is observed in only one fifth of the inward- and outward-moving PGs. Conclusions. Rising hot plasma in PGs surrounded by a less inclined magnetic field may adapt its trajectory to be more vertical, causing an inward apparent motion of PGs. Conversely, it may be dragged by a more horizontal surrounding magnetic field such that an outward apparent motion is observed. [ABSTRACT FROM AUTHOR]

Context. Stellar flares cannot be spatially resolved, which complicates ascertaining the physical processes behind particular spectral signatures. Due to their proximity to Earth, solar flares can serve as a stepping stone for understanding their stellar counterparts, especially when using a Sun-as-a-star instrument and in combination with spatially resolved observations. Aims. We aim to understand the disk-integrated spectral behaviors of a confined X2.2 flare and its eruptive X9.3 successor, which had energies of 2.2 × 1031 erg and 9.3 × 1031 erg, respectively, as measured by Sun-as-a-star observations with the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N). Methods. The behavior of multiple photospheric (Na D1 & D2, Mg I at 5173 Å, Fe I at 6173 Å, and Mn I at 4031 Å) and chromospheric (Ca II H & K, Hα, Hβ, and He ID3) spectral lines were investigated by means of activity indices and contrast profiles. A number of different photospheric lines were also investigated by means of equivalent widths, and radial velocity measures, which were then related to physical processes directly observed in high-resolution observations made with the Swedish 1-m Solar Telescope (SST) and the Atmospheric Imaging Assembly (AIA) on board of the Solar Dynamics Observatory (SDO). Results. Our findings suggest a relationship between the evolving shapes of contrast profile time and the flare locations, which assists in constraining flare locations in disk-integrated observations. In addition, an upward bias was found in flare statistics based on activity indices derived from the Ca II H & K lines. In this case, much smaller flares cause a similar increase in the activity index as that produced by larger flares. Hα-based activity indices do not show this bias and are therefore less susceptible to activity jitter. Sodium line profiles show a strongly asymmetric response during flare activity, which is best captured with a newly defined asymmetrical sodium activity index. A strong flare response was detected in Mn I line profiles, which is unexpected and calls for further exploration. Intensity increases in Hα, Hβ, and certain spectral windows of AIA before the flare onset suggest their potential use as short-term flare predictors. [ABSTRACT FROM AUTHOR]

Context. Synthetic spectra from 3D models of the solar atmosphere have become increasingly successful at reproducing observations, but there are still some outstanding discrepancies for chromospheric spectral lines, such as Ca II and Mg II, particularly regarding the width of the line cores. It has been demonstrated that using sufficiently high spatial resolution in the simulations significantly diminishes the differences in width between the mean spectra in observations and simulations, but a detailed investigation into how this impacts subgroups of individual profiles is currently lacking. Aims. We compare and contrast the typical shapes of synthetic Ca II 854.2 nm spectra found in Bifrost simulations having different magnetic activity with the spectral shapes found in a quiet-Sun observation from the Swedish 1-m Solar Telescope (SST). Methods. We used clustering techniques to extract the typical Ca II 854.2 nm profile shapes synthesized from Bifrost simulations with varying amounts of magnetic activity. We degraded the synthetic profiles to observational conditions and repeated the clustering, and we compared our synthetic results with actual observations. Subsequently, we examined the atmospheric structures in our models for some select sets of clusters, with the intention of uncovering why they do or do not resemble actual observations. Results. While the mean spectra for our high resolution simulations compare reasonably well with the observations, we find that there are considerable differences between the clusters of observed and synthetic intensity profiles, even after the synthetic profiles have been degraded to match observational conditions. The typical absorption profiles from the simulations are both narrower and display a steeper transition from the inner wings to the line core. Furthermore, even in our most quiescent simulation, we find a far larger fraction of profiles with local emission around the core, or other exotic profile shapes, than in the quiet-Sun observations. Looking into the atmospheric structure for a selected set of synthetic clusters, we find distinct differences in the temperature stratification for the clusters most and least similar to the observations. The narrow and steep profiles are associated with either weak gradients in temperature or temperatures rising to a local maximum in the line wing forming region before sinking to a minimum in the line core forming region. The profiles that display less steep transitions show extended temperature gradients that are steeper in the range−3 ≲ log τ5000 ≲ −1. [ABSTRACT FROM AUTHOR]

We perform a statistical analysis of 112 proton events from 24 November 2000 to 20 December 2014, accompanied by an increase in the intensity of solar energetic particles (SEPs) with energy E p > 1 –850 MeV using GOES data. All events were accompanied by metric type II radio bursts in the frequency range of 25–180 MHz observed with the Radio Solar Telescope Network. A correlation in the peak proton integral intensity I p with the intensity of type II radio bursts I i and the frequency drift rate V is shown. Taking into account the helio-longitudinal weakening, i.e. the dependence of SEP intensity on the heliographic longitude of the flare, we find that the correlation coefficients between I p and I i , as well as between I p and V for protons with E p > 30 MeV are 0.79 and 0.71, respectively. This suggests that non-thermal electrons, which drive type II radio bursts, and energetic protons are generated at the front of the same shock wave. The correlation coefficients mentioned above decrease for E p ≳ 100 MeV. Therefore, the contribution of high energetic protons to the integral intensity I p is rather determined by accelerated processes in the flare energy release region. The weak dependence of SEP intensity on the helio-longitudinal weakening is discussed. [ABSTRACT FROM AUTHOR]

The Wang–Shelley–Arge (WSA) solar wind (SW) model is based on the idea that weakly expanding coronal magnetic field tubes are associated with fast SW sources and vice versa. The Flux-Tube Expansion factor (FTE) is used to determine the degree of expansion of magnetic tubes. The FTE is calculated based on a model of the coronal magnetic field, usually in the potential approximation. The second input parameter of the WSA model is the distance from the base of the magnetic tube on the photosphere to the boundary of the corresponding coronal hole (Distance to the Coronal Hole Boundary, DCHB). These two parameters of the coronal magnetic field are related empirically to the speed of the solar wind near the Sun. The WSA model has shortcomings and does not fully explain the mechanisms of SW formation. This paper presents an analysis of the degree of correlation of various parameters of the magnetic field (length of field lines, latitude of the base of field lines, etc.) with the observed velocity SW. The parameters are calculated in potential (PFSS) and nonpotential approximations based on three synoptic series of magnetographic observations: the Kislovodsk Solar Telescope for Operative Predictions (STOP), SDO/HMI, and WSO. We found that the FTE correlates relatively weakly with solar wind speed, in contrast to field line lengths and DCHB. We propose an alternative relation to the WSA model that relates the length of field lines, the DCHB, and the amplitude of the magnetic field at the source surface with the velocity SW. The presented relationship is not based on the FTE and shows a better correlation with observations compared to the WSA model. We also optimized the formula in the WSA model for the STOP magnetograph. [ABSTRACT FROM AUTHOR]

Recently, several new solar radio telescopes have been put into operation and provided spectral-imaging observations with much higher resolutions in decimeter (dm) and centimeter (cm) wavelengths. These telescopes include the Mingantu Spectral Radioheliograph (MUSER, at frequencies of 0.4–15 GHz), the Expanded Owens Valley Solar Array (EOVSA, at frequencies of 1–18 GHz), and the Siberian Radio Heliograph (SRH, at frequencies of 3–24 GHz). These observations offer unprecedented opportunities to study solar physics and space weather, especially to diagnose the coronal magnetic fields, reveal the basic nature of solar eruptions and the related non-thermal energy release, particle accelerations and propagation, and the related emission mechanisms. These results might be the important input to the space weather modeling for predicting the occurrence of disastrous powerful space weather events. In order to provide meaningful reference for other solar physicists and space weather researchers, this paper mainly focus on discussing the potential scientific problems of solar radio spectral-imaging observations in dm-cm wavelengths and its possible applications in the field of space weather. These results will provide a helpful reference for colleagues to make full use of the latest and future observation data obtained from the above solar radio telescopes. [ABSTRACT FROM AUTHOR]

The solar physics community is entering a golden era that is ripe with next-generation ground- and space-based facilities, advanced spectral inversion techniques, and realistic simulations that are becoming more computationally streamlined and efficient. With ever-increasing resolving power stemming from the newest observational telescopes, it becomes more challenging to obtain (near-)simultaneous measurements at high spatial, temporal and spectral resolutions, while operating at the diffraction limit of these new facilities. Hence, in recent years there has been increased interest in the capabilities integral field units (IFUs) offer towards obtaining the trifecta of high spatial, temporal and spectral resolutions contemporaneously. To date, IFUs developed for solar physics research have focused on mid-optical and infrared measurements. Here, we present an IFU prototype that has been designed for operation within the near-ultraviolet to mid-optical wavelength range, which enables key spectral lines (e.g., Ca ii H/K, H β , Sr ii, Na i D1/D2, etc.) to be studied, hence providing additional spectral coverage to the instrument suites developed to date. The IFU was constructed as a low-budget proof-of-concept for the upcoming 2 m class Indian National Large Solar Telescope and employs circular cross-section fibres to guide light into a Czerny–Turner configuration spectrograph, with the resulting spectra captured using a high quantum efficiency scientific CMOS camera. Mapping of each input fibre allows for the reconstruction of two-dimensional spectral images, with frame rates exceeding 20 s − 1 possible while operating in a non-polarimetric configuration. Initial commissioning of the instrument was performed at the Dunn Solar Telescope, USA, during August 2022. The science verification data presented here highlights the suitability of fibre-fed IFUs operating at near-ultraviolet wavelengths for solar physics research. Importantly, the successful demonstration of this type of instrument paves the way for further technological developments to make a future variant suitable for upcoming ground-based and space-borne telescope facilities. [ABSTRACT FROM AUTHOR]

We analyze the magnetism of the quiet Sun internetwork (IN) using high-spatial-resolution data obtained by the spectropolarimeter (SP) of the Solar Optical Telescope aboard the Hinode satellite near the disk center of the Sun. The SP data were inverted using the Stokes Inversion based on Response functions (SIR) inversion code with a single-component atmosphere with depth dependent in the solar photosphere, assuming gradients in physical parameters along the line of sight (LOS). To avoid the effect of noise, only pixels with Stokes U and/or Q signals above 4.5 times the noise level are considered. The inversion results show that the magnetic field of the IN has mainly hG field strength and the inclination distribution is quasi-isotropic at the solar surface. The field strength decreases with height and becomes predominantly horizontal at the upper layers. At the mid photosphere, the distributions of field strength and inclination are consistent with those derived by Milne–Eddington inversion. The mean transverse and longitudinal flux densities are 66 Mx cm−2 and 13 Mx cm−2 at log τ = −1.0; we also study the ratio between the transverse and longitudinal components in the IN region as a function of depth in the photosphere, finding that the ratio is almost 2.7 in the deep layer, increasing to 5.1 in the upper layer. The mean field strength is greater than 100 G in the upper photosphere, which is consistent with the results based on the Hanle effect. We present the LOS velocity probability distribution function for IN at different optical depths, where its distribution is mainly associated with upflow velocities of V LOS = 1.2 and 0.6 km s−1 in the deeper and upper layers, respectively. In addition, there exists a reliable inversion analysis, which is obvious from the comparison between the observed and calculated area asymmetries in both Fe  i lines of Hinode SP data. [ABSTRACT FROM AUTHOR]

Pin-hole camera as a soft X-ray solar telescope is presented. The instrument is aimed on imaging of flares in the solar corona with angular resolution up to 40″. It will allow to register temporal profiles of flares and determine spectra in soft X-ray. The telescope consist on pin hole 0.1 mm in diameter in tungsten disk, thin film Al/maylar filter and back illuminated CMOS based 2d detector. The telescope designed for 6 U cubesat. The instrument itself has dimension of about 0.5 U with extended tube from 20 to 50 cm length. [ABSTRACT FROM AUTHOR]

We present the results of the optical identification and spectroscopic redshift measurements of 216 galaxy clusters detected in the SRG/eROSITA all-sky X-ray survey. The spectroscopic observations were performed in 2020–2023 with the 6-m BTA telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences, the 2.5-m telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University, the 1.6-m AZT-33IK telescope at the Sayan Solar Observatory of the Institute of Solar–Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences, and the 1.5-m Russian–Turkish telescope (RTT-150) at the TÜBİTAK Observatory. For all of the galaxy clusters presented here the spectroscopic redshift measurements have been obtained for the first time. Of these, 139 galaxy clusters have been detected for the first time in the SRG/eROSITA survey and 22 galaxy clusters are at redshifts , including three at . Deep direct images with the rizJK filters have also been obtained for four distant galaxy clusters at . For these observations we chose the most massive clusters and, therefore, most of the galaxy clusters presented here with the spectroscopic redshifts measured by us will most likely enter in future into the cosmological samples of galaxy clusters from the SRG/eROSITA survey. [ABSTRACT FROM AUTHOR]

High-resolution spectral observations of the lower solar atmosphere (chromosphere and transition region) during coronal heating events, in combination with predictions from models of impulsively heated loops, provide powerful diagnostics of the properties of the heating in active region cores. Here, we analyze the first coordinated observations of such events with the Interface Region Imaging Spectrograph (IRIS) and the CHROMospheric Imaging Spectrometer (CHROMIS), at the Swedish 1 m Solar Telescope (SST), which provided extremely high spatial resolution and revealed chromospheric brightenings with spatial dimensions down to ∼150 km. We use machine-learning methods (k -means clustering) and find significant coherence in the spatial and temporal properties of the chromospheric spectra, suggesting, in turn, coherence in the spatial and temporal distribution of the coronal heating. The comparison of IRIS and CHROMIS spectra with simulations suggests that both nonthermal electrons with low energy (low-energy cutoff ∼5 keV) and direct heating in the corona transported by thermal conduction contribute to the heating of the low atmosphere. This is consistent with growing evidence that nonthermal electrons are not uncommon in small heating events (nanoflare to microflares), and that their properties can be constrained by chromospheric and transition region spectral observations. [ABSTRACT FROM AUTHOR]

Solar radio flux is an important indicator of solar activity and solar UV burst. Accurate prediction of solar radio flux plays a crucial role in preventing and mitigating the impact of solar activity on human productivity. We propose a novel approach for the first time to predict short-term radio flux trends using a bidirectional long short-term memory (BLSTM) network. This approach aims to address the unique characteristics of temporality and nonlinearity observed in solar radio flux data. Our model takes into account various frequency characteristics that impact radio flux. This allows it to learn temporal patterns within the data, ultimately enabling accurate predictions of radio flux for the next 30 minutes. The proposed method is experimentally applied to the radio flux dataset of the US Radio Solar Telescope Network (RSTN). The results show that, in most frequency bands, the BLSTM model exhibits superior prediction accuracy and greater sensitivity to peak responses compared to the LSTM model, LSTM-Attention (LSTM-A) model, BLSTM-Attention (BLSTM-A) model, and persistence model (PM). Consequently, the BLSTM model is better equipped to accurately forecast changes in radio flux for the next 30 minutes. [ABSTRACT FROM AUTHOR]

We present the results of the study of the magnetic field in the active prominence on July 24, 1999 at 07:00 UT, using the observational material obtained on the Echelle spectrograph of the horizontal solar telescope of the Astronomical Observatory of Taras Shevchenko Kyiv National University. Our analysis is based on the study of I ± V profiles of the Hα line, which were related to heights in the range of 11–20 Mm. It was found that the bisectors of the I ± V profiles are non-parallel to each other in majority of places of this prominence. This indicates the inhomogeneity of the magnetic field: with a uniform magnetic field, the named bisectors should be parallel. Moreover, the maximum splitting of bisectors is observed not only in the core of the line (which was found earlier by other authors), but also in its far wings, at distances of 1.5–2.5 Å from the line center. The specified maximum of splitting corresponds to magnetic field of about 3000 G, but this value should be considered only as a lower estimate of the true local magnetic fields. In particular, the second maximum of bisector splitting may indicate that the actual value of Zeeman splitting in small-scale structures with a small filling factor reaches the above value of 1.5–2.5 Å which corresponds to the field strength of almost 100 kG. From our study it follows that evidences on such extremely magnetic fields may not actually be a rare phenomenon, but a rather common one, which, however, can be recorded only under certain favorable observational conditions. [ABSTRACT FROM AUTHOR]

New instruments and telescopes covering the optical and ultraviolet spectral regions have revealed a range of small-scale dynamic features, many which may be related. For example, the range of spicule-like features hints towards a spectrum of features and not just two types; however, direct observational evidence in terms of tracking spicules across multiple wavelengths is needed in order to provide further insight into the dynamics of the Sun's outer atmosphere. This paper uses H α data obtained with the CRisp Imaging SpectroPolarimeter instrument on the Swedish 1-m Solar Telescope, and in the transition region using the Interface Region Imaging Spectrograph with the SJI 1400 Å channel plus spectral data via the Si iv 1394 Å line to track spicules termed rapid blueshifted excursions (RBEs). The RBEs as seen in the H α blue wing images presented here can be subdivided into two categories: a single or multithreaded feature. Based on the H α spectra, the features can be divided into events showing broadening and line core absorption, events showing broadening and line core emission, events with a pure blueshifted H α profile without any absorption in the red wing, and broadened line profile with the absorption in the blue stronger compared to the red wing. From the RBE-like events that have a Si iv 1394 Å line profile, 78  per cent of them show a Si iv line flux increase. Most of these features show a second broadened Si iv component that is slightly blueshifted. [ABSTRACT FROM AUTHOR]