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An experiment measuring the solar far-ultraviolet-ultraviolet (FUV-UV) irradiance with spectral resolution better than 0.1 nm in the wavelength range from 170 to 400 nm was carried out by the "HongHu-6" high-altitude balloon that flew to the bottom region of the near-space in September 2022. This experiment was based on the fact that solar FUV-UV penetrates through a complex cross-section window of the upper atmosphere, from outer to near space. The solar FUV-UV deposits energy in the upper atmosphere, which provides a key to answer scientific questions on the most important energy contributor to overall heating sources of the near space and how the near-space environment responds to solar activities. In the wavelength range between 150 and 210 nm, irradiance maps from active regions of the solar corona, the comparative small cross-section of molecular oxygen allows certain wavelengths of the band to arrive at altitudes between 20 and 30 km above the ground, indicating solar flares could directly impact the bottom region of the near space. Solar UV irradiance in the wavelength range 210 – 400 nm is absorbed by the upper atmosphere as a function of wavelength, and energy is deposited vertically in the lower regions of the near space. This experiment historically provides measurement data to fill a gap in the wavelength shorter than 280 nm in the lower regions of the near space. The solar FUV-UV spectrometer (SUVS) is a compact instrument based on improved Roland circle optics to adapt to the "HongHu-6" balloon payload platform. In this paper, we introduce the scientific goals of the solar FUV-UV spectrum measurement experiment, provide information on the SUVS instrument preflight calibration, and present the first results from the flight data. [ABSTRACT FROM AUTHOR]

The Sun is a standard reference object for astrophysics and also a fascinating subject of study in its own right. X-ray and extreme ultraviolet movies of the Sun's atmosphere show an extraordinary diversity of plasma phenomena, from barely visible bursts and jets to coronal mass ejections that impact a large portion of the solar surface. The processes that produce these phenomena, heat the corona and power the solar wind remain actively studied and accurate atomic data are essential for interpreting observations and making model predictions. For the Sun's interior intense effort is focused on resolving the "solar problem," (a discrepancy between solar interior models and helioseismology measurements) and atomic data are central to both element abundance measurements and interior physics such as opacity and nuclear reaction rates. In this article, topics within solar interior and solar atmosphere physics are discussed and the role of atomic data described. Areas of active research are highlighted and specific atomic data needs are identified. An image of a solar active region obtained with the 193 A channel of SDO/AIA, showing plasma at around 1.5 million degrees. [ABSTRACT FROM AUTHOR]

The accuracy of the M. Aschwanden's (2020) model for short-term (24 h) prediction of the maximum X-ray class of solar flares based on the power-law dependence on the energy of the potential magnetic field of active regions is checked and assessed. For this purpose, a sample of 275 flares (253 M-class and 22 X-class) in isolated active regions on the solar disk in 2010−2023 is analyzed. Magnetic field extrapolations are made in the nonlinear force-free and potential approximations using the GX Simulator based on photospheric vector magnetograms from the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). It is found that in 6% of cases, Aschwanden's model underestimates the predicted maximum flare class with respect to the observed one (maximal underestimation by 4.4 times). The accuracy of this model (the average ratio of the observed to predicted maximum flare class) is 0.31 ± 0.47. Four other statistical models are proposed, two of which, like Aschwanden's model, are based on the power-law dependence of the maximum flare class on the energy of potential magnetic field, and the other two are based on the power-law dependence on the free magnetic energy of active regions. These models give fewer (or no) underestimations of the maximum flare class, but two to three times lower forecast accuracy, ranging from 0.11 to 0.17. Additionally, based on the obtained statistical sample, estimates of the limiting X-ray class of solar flares are made. The five models give different limits ranging from ~X14 to ~X250. The realism of these values and the possibility of refining the models by expanding the sample of events is briefly discussed. [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]

We present a statistical study on the relationship of solar dynamic events (solar flares and coronal mass ejections) with active regions during Solar Cycle 24 (December 2008–December 2019). Combining data from NOAA Space Weather Prediction Center and observations of Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO) spacecraft, we found that more than a half of the coronal mass ejections were generated inside active regions. Geostationary Operational Environmental Satellite (GOES) soft X-ray flare listing data completed our study showing that almost 83% of Solar Cycle 24 flares are connected with active regions. Finally, we summarize the details for the related phenomena into an online catalog based on a list of all 1533 active regions that produced at least one flare and/or coronal mass ejection during Solar Cycle 24 and explore their properties like flare class, coronal mass ejection speed, and angular width paying special attention to the most powerful and threatful to Earth solar events. [ABSTRACT FROM AUTHOR]

Solar active regions serve as the primary energy sources of various solar activities, directly impacting the terrestrial environment. Therefore precise detection and tracking of active regions are crucial for space weather monitoring and forecasting. In this study, a total of 4577 HMI and MDI longitudinal magnetograms are selected for building the dataset, including the training set, validating set, and ten testing sets. They represent different observation instruments, different numbers of activity regions, and different time intervals. A new deep learning method, ReDetGraphTracker, is proposed for detecting and tracking the active regions in full-disk magnetograms. The cooperative modules, especially the redetection module, NSA Kalman filter, and the splitter module, better solve the problems of missing detection, discontinuous trajectory, drifting tracking bounding box, and ID change. The evaluation metrics IDF1, MOTA, MOTP, IDs, and FPS for the testing sets with 24-h interval on average are 74.0%, 74.7%, 0.130, 13.6, and 13.6, respectively. With the decreasing intervals, the metrics become better and better. The experimental results show that ReDetGraphTracker has a good performance in detecting and tracking active regions, especially capturing an active region as early as possible and terminating tracking in near-real time. It can well deal with the active regions whatever evolve drastically or with weak magnetic field strengths, in a near-real-time mode. [ABSTRACT FROM AUTHOR]

Low and Lou (Astrophys. J.352, 343, 1990) presented a family of nonlinear force-free magnetic fields that have established themselves as the gold standard for extrapolating force-free magnetic fields in solar physics. Building upon this important work, our study introduces a novel grid-free machine-learning-based method to effectively solve the equilibria proposed by Low and Lou. Through extensive numerical experiments, our results unequivocally demonstrate the efficient capability of the machine-learning algorithm in deriving numerical solutions for Low and Lou's equilibria. Furthermore, we explore the opportunities and challenges of applying artificial-intelligence technology to real observed solar active regions. [ABSTRACT FROM AUTHOR]

We utilized the Forbush decreases (magnitude > 1.5 % ) detected in cosmic ray neutron monitor data during continuous five solar cycles, viz., 20, 21, 22, 23 and 24 (1965 to 2019) and subjected them to wavelet analysis in order to obtain the possible periodicities in their occurrence. We also studied the periodicities separately during the odd and even solar activity cycles. In addition to solar activity, the solar magnetic polarity and its extension into the interplanetary space makes significant difference in the cosmic ray modulation in the helisphere, we have also applied the wavelet analysis procedure separately during positive (A > 0) and negative (A < 0) polarity states of the heliospheric magnetic fields. Observed periodicities in Forbush decreases have been discussed and compared with earlier detected periodicities in solar and geomagnetic activity indices, e.g., sunspot numbers, sunspot areas, sunspot groups, solar flares, coronal mass ejections, and various geomagnetic activity indices. Significant short-term periodic behaviour detected in the occurrence of Forbush decreases, which in general, corroborates the observed behaviour in solar (in particular, solar eruptive activity) and geomagnetic activity. Understanding the quasi-periodic process in magnetic field emergence from solar active regions and solar eruptive activity, as well as solar-terrestrial coupling and space weather effects, requires comparing the quasi-periodic behaviour between parameters representing solar and geomagnetic activity along with cosmic ray variability. [ABSTRACT FROM AUTHOR]

The article gives a brief description of the Sun-Terahertz space experiment scheduled for 2024–2025 aboard the Russian segment of the International Space Station. This experiment is aimed at studying the Sun within the previously unexplored terahertz range, obtaining data on terahertz solar radiation, as well as studying solar active regions and solar flares. The authors consider scientific equipment being developed for the Sun-Terahertz experiment, which comprises eight detection channels that are sensitive to radiation of different frequencies within the range of 0.4–12.0 THz. Each channel includes an optical telescope, a system of series filters, an optical chopper, and a radiation receiver with an opto-acoustic detector—Golay cell. The change in the sensitivity of Golay cells with a change in their intrinsic temperature (temperature effect) was studied. By means of auxiliary equipment—a test bench comprising a single-channel mockup and a blackbody simulator—the linear section of the temperature dependence of Golay cells was determined. A procedure was developed to compensate for the temperature effect in Golay cells, and the results of piloting the procedure on the test bench were presented. The proposed procedure can be useful in the development of scientific equipment comprising Golay cells, which is expected to be used under ambient temperature fluctuations. [ABSTRACT FROM AUTHOR]

The article briefly describes the goals and tasks of the planned "Sun–Terahertz" space experiment on board the Russian segment of the ISS. The experiment was aimed at studying radiation from the Sun in the unexplored terahertz range at frequencies of 1012–1013 Hz and obtaining new data on the terahertz radiation of the Sun, solar active regions, and solar flares. The scientific equipment being developed is a set of eight detectors sensitive to radiation of various frequencies: 0.4, 0.7, 1.0, 3.0, 5.0, 7.0, 10.0, and 12.0 THz. The main components of the electronics unit of the scientific equipment are considered: amplifiers, optical-chopper drivers, the power board, and the electronics board. The accuracy of signal measurements using an ADC on the electronics board was calculated, and the sensitivity of the scientific equipment was assessed. [ABSTRACT FROM AUTHOR]

Type-II radio bursts are believed to occur as a result of the shock driven by flares or coronal mass ejections (CMEs). While the shock waves are important for the acceleration of electrons necessary for the generation of the radio emission, the exact nature of the shock and coronal conditions necessary to produce type-II radio emission is still under debate. In this investigation, we probe the relationship of kinematic characteristics of the type-II radio bursts with the magnetic-field complexity (Mj) of the active regions visible on the photosphere. Our investigation of 64 type-II solar radio bursts, which are associated with flares and CMEs, reveals that Mj is linearly correlated in the logarithmic scale with the starting frequency (fs) and drift-rate (Δ f / Δ t ) of type-II radio burst. Further, Mj exhibits a linear correlation with the shock height (r) and electron density () in logarithmic scale. This indicates that high frequency (fs ≥ 100 ) bursts, which occur at the reconnection site near the solar surface, are produced from a strong magnetically complex region. Further, strong and complex magnetic-field regions produce shocks of higher speeds. Based on the derived plasma parameters of the radio bursts and their relationship with fs as well as with Mj, we propose that the high-frequency type-II bursts were generated in a special situation when the shock is produced due to magnetic reconnection occurring in the low-lying coronal loops. We conclude that type-II radio bursts can occur even in the inner corona as well as in the outer corona; however, it depends on the magnetic complexity of the active region in which the event occurs. [ABSTRACT FROM AUTHOR]

The study of evolution of magnetic field and electric currents in active regions of the Sun over a long-time interval is of interest for understanding the processes of energy accumulation and release in them, leading to various phenomena that affect space weather. In this study, based on the photospheric vector magnetograms of the helioseismic and magnetic imager instrument aboard the Solar Dynamics Observatory, the authors analyzed the evolution of a number of characteristics of the magnetic field and vertical electric current in three active regions, 11 158, 11 675, and 12 673, that produced M and X class flares, during the time from their origin in the eastern hemisphere, during passage through the solar disk, and until their disappearance near the western limb with a step of 2 h. The characteristics considered included: the power-law exponent of the probability density function of the absolute value of the vertical electric current density, the maximum absolute value of the vertical current density, the signed and unsigned total vertical currents and the unsigned total vertical and horizontal magnetic fluxes, the energy of the nonlinear force-free and potential magnetic fields, the free magnetic energy, and the number of islands with strong vertical current. Some regularities in the behavior of the characteristics considered are found, in particular, regarding the occurrence of solar flares. The correlation coefficients between pairs of these characteristics are calculated. Additionally, M. Aschwanden's approach is shown to be promising for predicting the maximum X-ray class of a flare by calculating the energy of the potential magnetic field in active regions. The results can be used to predict powerful solar flares. [ABSTRACT FROM AUTHOR]

We analyze the magnetic evolution of solar active region (AR) NOAA 11476 that, between May 9 and 10, 2012, produced a series of surge-type eruptions accompanied by GOES X-ray class-M flares. Using force-free models of the AR coronal structure and observations at several wavelengths, in previous works we studied the detailed evolution of those eruptions, relating them to the characteristic magnetic topology of the AR and reconstructing the involved reconnection scheme. We found that the eruptions were due to the ejection of minifilaments, which were recurrently ejected and reformed at the polarity-inversion line of a bipole that emerged in the middle of the positive main AR magnetic polarity. The bipole was observed to rotate for several tens of hours before the events. In this article we analyze, for the full AR and the rotating bipole, the evolution of a series of magnetic parameters computed using the Helioseismic and Magnetic Imager (HMI) vector magnetograms. We combine this analysis with estimations of the injection of magnetic energy and helicity obtained using the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) method that determines, from vector magnetograms, the affine velocity field constrained by the induction equation. From our results, we conclude that the bipole rotation was the main driver that provided the magnetic energy and helicity involved in the minifilament destabilizations and ejections. The results also suggest that the observed rotation is probably due to the emergence of a kinked magnetic flux rope with negative writhe helicity. [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]

Image super-resolution is an important subject in image processing and recognition. Here, we present an attention-aided convolutional neural network for solar image super-resolution. Our method, named SolarCNN, aims to enhance the quality of line-of-sight (LOS) magnetograms of solar active regions (ARs) collected by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). The ground-truth labels used for training SolarCNN are the LOS magnetograms collected by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Solar ARs consist of strong magnetic fields in which magnetic energy can suddenly be released to produce extreme space-weather events, such as solar flares, coronal mass ejections, and solar energetic particles. SOHO/MDI covers Solar Cycle 23, which is stronger with more eruptive events than Cycle 24. Enhanced SOHO/MDI magnetograms allow for better understanding and forecasting of violent events of space weather. Experimental results show that SolarCNN improves the quality of SOHO/MDI magnetograms in terms of the structural similarity index measure, Pearson's correlation coefficient, and the peak signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

Solar flares are releases of electromagnetic energy generally occurring in active solar regions with magnetic fields, known as sunspots. The burst of radiation released by a solar flare can reach Earth's atmosphere in a few minutes. High-intensity solar flares, M- or X-class flares, can significantly impact some of Earth's activities and technologies, such as satellites, telecommunications, and electrical power systems. Therefore driving efforts in high-intensity solar flare forecasting systems is crucial. A forecasting model that observes the evolution of active regions may analyze a set of attributes that indicate which active regions can be precursors to solar flares. Recent work has focused on deep-learning models that consider the evolution of active regions in the Sun. However, M- and X-class flares are spurious in the solar-cycle period. That situation leads to an imbalanced dataset, increasing the effort to develop machine-learning models for forecasting. Therefore we propose transformers-based models to forecast ≥M-class flares, taking sequences of line-of-sight magnetogram images as input. In addition, we apply data augmentation techniques and other methods to deal with training on imbalanced datasets. Our fine-tuned models outperformed state-of-the-art work using image processing to forecast ≥M-class flares in the next 48 h with an approximate True Skill Statistic (TSS ) of 0.8. Moreover, the data augmentation techniques applied to the training set kept the TSS stable and improved most of the secondary performance metrics analyzed. [ABSTRACT FROM AUTHOR]

We used magnetograms acquired with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to calculate and analyze spatial correlation functions and the multifractal spectra in solar active regions (ARs). The analysis was performed for two very different types of ARs: i) simple bipolar magnetic structures with regular orientation (the magnetomorphological class A1), and ii) very complex multipolar ARs (the magnetomorphological class B3). All ARs were explored at the developed phase during flareless periods. For correlation functions, the power-law and exponential approximations were calculated and compared. It was found that the exponential law holds for the correlation functions of both types of ARs within spatial scales of 1 – 36 Mm, while the power law failed to approximate the observed correlation functions. The property of multifractality was found in all ARs, being more pronounced for the complex B3-class ARs. Our results might imply that the photospheric magnetic field of an AR is a self-organized system, which, however, does not exhibit properties of self-organized criticality (SOC), and its fractal properties are an attribute of a broader (than SOC only) class of nonlinear systems. [ABSTRACT FROM AUTHOR]

The article investigates the statistical relations between the values of geomagnetic indices and the characteristics of cosmic rays and interplanetary disturbances for Forbush decreases with a sudden and gradual commencement associated with different types of solar sources: (a) coronal mass ejections from active regions accompanied by solar flares; (b) filament eruptions outside active regions; (c) high-speed streams from coronal holes; and (d) multiple sources. Using statistical methods, we also compare the dependence of geomagnetic indices on cosmic ray and solar wind parameters for Forbush decreases in solar cycles 23 and 24. The results show that (a) interplanetary disturbances associated with coronal mass ejections from active regions cause mainly magnetic storms with a sudden commencement, (b) interplanetary disturbances associated with high-speed streams from coronal holes cause mainly storms with a gradual commencement, and (c) interplanetary disturbances associated with filament eruptions outside active regions cause equally likely storms with a sudden and gradual commencement. For sporadic Forbush decreases, the cosmic ray and geomagnetic activity parameters are, on average, larger for sudden commencement events; for recurrent Forbush decreases, the nature of the event commencement does not affect the magnitude of these parameters. For all types of solar sources, the disturbed solar wind parameters are, on average, larger in events with sudden commencement. The geoeffectiveness of interplanetary disturbances is significantly higher in cycle 23 for events associated with ejections from active regions; for other types of disturbances, the difference between cycles is weak. [ABSTRACT FROM AUTHOR]

The Atacama Large Millimeter/submillimeter Array (ALMA) is a general purpose telescope that performs a broad program of astrophysical observations. Beginning in late 2016, solar observations with ALMA became available, thereby opening a new window onto solar physics. Since then, the number of solar observing capabilities has increased substantially but polarimetric observations, a community priority, have not been available. Weakly circularly polarized emission is expected from the chromosphere where magnetic fields are strong. Hence, maps of Stokes V provide critical new constraints on the longitudinal component of the chromospheric magnetic field. Between 2019 and 2022, an ALMA solar development effort dedicated to making solar polarimetry at millimeter wavelengths a reality was carried out. Here, we discuss the development effort to enable solar polarimetry in the 3 mm band (ALMA Band 3) in detail and present a number of results that emerge from the development program. These include tests that validate polarization calibration, including evaluation of instrumental polarization: both antenna-based "leakage" terms and off-axis effects (termed "beam squint" for Stokes V). We also present test polarimetric observations of a magnetized source on the Sun, the following sunspot in a solar active region, which shows a significant Stokes V signature in line with expectations. Finally, we provide some cautions and guidance to users contemplating the use of polarization observations with ALMA. [ABSTRACT FROM AUTHOR]

We present a study of the frequency of occurrence of quasi-periodic fluctuations (QPFs) of microwave radiation from solar active regions at the pre-flare stage. We used correlation curves obtained with the Nobeyama Radioheliograph (NoRH). We analyzed 529 flares of classes X, M, and C (according to the GOES classification). In a significant number of considered events (approximately 75% of the powerful flares and 50% of the weaker flares), preflare fluctuations of microwave radiation with a duration of up to ten pulses are observed. The character of preflare fluctuations is different in different cases. In some cases, almost harmonic oscillations are observed. In most cases, the fluctuations are sporadic and look like a sequence of weak flares. It is possible that the nature of the preflare QPFs is different in different cases. [ABSTRACT FROM AUTHOR]

In this paper we study the statistical relations between geomagnetic indices and the characteristics of cosmic rays and interplanetary disturbances for Forbush decreases associated with (a) coronal mass ejections from active regions accompanied by solar flares, (b) filament eruptions outside active regions, (c) high-speed streams from coronal holes, and (d) multiple sources. For sporadic Forbush decreases, the dependence of geomagnetic indices on cosmic ray and solar wind parameters in the presence or absence of a magnetic cloud is compared using statistical methods. The results show that (a) the highest geoeffectiveness is characteristic of interplanetary disturbances associated with solar matter eruptions from active regions in the presence of a magnetic cloud (and the lowest one for recurrent disturbances), (b) sporadic and recurrent events differ not only in the magnitude of geomagnetic indices and the southern component of the magnetic field but also in the nature of the relationship between them, (c) the geoeffectiveness of transient solar wind disturbances depends more strongly on the presence or absence of a magnetic cloud than on the type of solar source, and (d) for interplanetary disturbances associated with filament eruptions outside active regions, in the presence of a magnetic cloud, the geoeffectiveness depends only on the southern component of the magnetic field, while for other types of disturbances it depends on other solar wind parameters. [ABSTRACT FROM AUTHOR]

The emergence of active regions on the Sun is an integral feature of the solar dynamo mechanism. However, details about the generation of active-region-scale magnetism and the journey of this magnetic flux from the interior to the photosphere are still in question. Shifting paradigms are now developing for the source depth of the Sun's large-scale magnetism, the organization of this magnetism into fibril flux tubes, and the role of convection in shaping active-region observables. Here we review the landscape of flux emergence theories and simulations, highlight the role flux emergence plays in the global dynamo process, and make connections between flux emergence on the Sun and other cool stars. As longer-term and higher fidelity observations of both solar active regions and their associated flows are amassed, it is now possible to place new constraints on models of emerging flux. We discuss the outcomes of statistical studies which provide observational evidence that flux emergence may be a more passive process (at least in the upper convection zone); dominated to a greater extent by the influence of convection and to a lesser extent by buoyancy and the Coriolis force acting on rising magnetic flux tubes than previously thought. We also discuss how the relationship between stellar rotation, fractional convection zone depth, and magnetic activity on other stars can help us better understand the flux emergence processes. Looking forward, we identify open questions regarding magnetic flux emergence that we anticipate can be addressed in the next decade with further observations and simulations. [ABSTRACT FROM AUTHOR]

The lifetimes and length-scales for supergranular cells in active and quiescent regions of the solar chromosphere, and the relation between the two, were studied using a time series of Ca II K filtergrams. The lifetimes, in contrast to supergranular length scale and fractal dimension, show no significant dependence on solar latitude, suggesting that cell lifetimes are independent of the differential rotation and a possible supergranular super-rotation. The functional form of the relation was obtained guided by a comparison of the distributions of the two supergranular parameters. We infer a linear dependence of cell lifetime on area, which can be understood by the assumption of the network's evolution via a diffusion of the magnetic field. Our analysis suggests that the diffusion rate in quiet regions is about 10 greater than in active regions. [ABSTRACT FROM AUTHOR]

In this dataset we provide a comprehensive collection of line-of-sight (LOS) solar photospheric magnetograms (images quantifying the strength of the photospheric magnetic field) from the National Aeronautics and Space Administration's (NASA's) Solar Dynamics Observatory (SDO). The dataset incorporates data from three sources and provides SDO Helioseismic and Magnetic Imager (HMI) magnetograms of solar active regions (regions of large magnetic flux, generally the source of eruptive events) as well as labels of corresponding flaring activity. This dataset will be useful for image analysis or solar physics research related to magnetic structure, its evolution over time, and its relation to solar flares. The dataset will be of interest to those researchers investigating automated solar flare prediction methods, including supervised and unsupervised machine learning (classical and deep), binary and multi-class classification, and regression. This dataset is a minimally processed, user configurable dataset of consistently sized images of solar active regions that can serve as a comprehensive image dataset of LOS photospheric magnetograms for solar flare prediction research. [ABSTRACT FROM AUTHOR]

Solar flares are explosions on the Sun. They happen when energy stored in magnetic fields around solar active regions (ARs) is suddenly released. Solar flares and accompanied coronal mass ejections are sources of space weather, which negatively affects a variety of technologies at or near Earth, ranging from blocking high-frequency radio waves used for radio communication to degrading power grid operations. Monitoring and providing early and accurate prediction of solar flares is therefore crucial for preparedness and disaster risk management. In this article, we present a transformer-based framework, named SolarFlareNet, for predicting whether an AR would produce a γ -class flare within the next 24 to 72 h. We consider three γ classes, namely the ≥ M5.0 class, the ≥ M class and the ≥ C class, and build three transformers separately, each corresponding to a γ class. Each transformer is used to make predictions of its corresponding γ -class flares. The crux of our approach is to model data samples in an AR as time series and to use transformers to capture the temporal dynamics of the data samples. Each data sample consists of magnetic parameters taken from Space-weather HMI Active Region Patches (SHARP) and related data products. We survey flare events that occurred from May 2010 to December 2022 using the Geostationary Operational Environmental Satellite X-ray flare catalogs provided by the National Centers for Environmental Information (NCEI), and build a database of flares with identified ARs in the NCEI flare catalogs. This flare database is used to construct labels of the data samples suitable for machine learning. We further extend the deterministic approach to a calibration-based probabilistic forecasting method. The SolarFlareNet system is fully operational and is capable of making near real-time predictions of solar flares on the Web. [ABSTRACT FROM AUTHOR]

Magnetic helicity has been used widely in the analysis and modelling of solar active regions. However, it is difficult to evaluate and interpret helicity in spherical geometry since coronal magnetic fields are rooted in the photosphere and helicity is susceptible to gauge choices. Recent work extended a geometrical definition of helicity from Cartesian to spherical domains, by interpreting helicity as the average, flux-weighted pairwise winding of magnetic-field lines. In this paper, by adopting the winding-based definition of helicity, we compute helicity and winding in spherical coordinates for SHARP (Spaceweather HMI Active Region Patches) magnetograms. This is compared with results obtained in Cartesian coordinates to quantitatively investigate the effect of spherical geometry. We find that the Cartesian approximations remain mostly valid, but for active regions with large spatial extents or strong field strengths (usually leading to flares and coronal mass ejections) there are significant deviations due to surface curvature that must be accounted for. [ABSTRACT FROM AUTHOR]

This document is an editorial introducing a topical collection titled "Solar and Stellar Dynamos: A New Era." The collection consists of 15 comprehensive review papers that summarize the outcomes of an international workshop on solar and stellar dynamos. The workshop brought together researchers from 13 countries to discuss observations, simulations, and models related to the generation of magnetic fields in the Sun and other stars. The review papers cover a wide range of topics, including observations of the solar cycle, magnetic fields in cool stars, formation and evolution of magnetic structures, small-scale dynamo action, and dynamo models. The editors express their gratitude to the authors, coordinators, and the International Space Science Institute (ISSI) for their contributions and support. [Extracted from the article]

In this paper, we study the similarities and differences of Forbush decreases in solar cycles 23 and 24. Groups of events associated with various types of solar sources were analyzed: coronal mass ejections from active regions accompanied by solar flares (CME1 group), filament ejections outside active regions (CME2 group), and high-velocity streams from coronal holes (CH group). The distributions and relations of various parameters were studied: the amplitude of Forbush decreases, the maximum values of the hourly decrease in the cosmic ray density, the equatorial cosmic ray anisotropy, the solar wind velocity, and the magnetic field strength, as well as the values of the solar wind velocity and the magnetic field strength 1 hour before the onset of the Forbush decrease during the event. The results showed that the number of events, parameter values, and their relations depend on the phase and cycle of solar activity. In the 24th cycle, the number of events in the CME1 group decreased, did not change in CME2, and increased in CH. The values of the parameters and the difference between them in various groups of events are higher in cycle 23, which is characterized by greater asymmetry and long "tails" of distributions. The magnitude of the Forbush decreases in the CME1 group in cycle 23 depends more strongly on the solar wind velocity while, in cycle 24, on the magnetic field strength, as in the CME2 group in both solar cycles. Multiple linear regression describes the dependences of the parameters of Forbush decreases in the CME1 and CME2 groups in the 23rd cycle and in the CME1 group in the 24th cycle well. [ABSTRACT FROM AUTHOR]

Solar activity is often caused by the evolution of solar magnetic fields. Magnetic field parameters derived from photospheric vector magnetograms of solar active regions (ARs) have been used to analyze and forecast eruptive events, such as solar flares and coronal mass ejections. Unfortunately, the most recent Solar Cycle 24 was relatively weak with few large flares, though it is the only solar cycle in which consistent time-sequence vector magnetograms have been available through the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) since its launch in 2010. In this work, we look into another major instrument, namely the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) from 1996 to 2010. The data archive of SOHO/MDI covers a more active Solar Cycle 23 with many large flares. However, SOHO/MDI only has line-of-sight (LOS) magnetograms. We propose a new deep learning method, named MagNet, to learn from combined LOS magnetograms, B x and B y , taken by SDO/HMI, along with H α observations collected by the Big Bear Solar Observatory (BBSO), and to generate synthetic vector components B x ′ and B y ′ of ARs. These generated vector components, together with observational LOS data, would form vector magnetograms for SOHO/MDI. In this way, we can expand the availability of vector magnetograms to the period from 1996 to present. Experimental results demonstrate the good performance of the MagNet method. To our knowledge, this is the first time that deep learning has been used to generate photospheric vector magnetograms of ARs for SOHO/MDI using SDO/HMI and H α data. [ABSTRACT FROM AUTHOR]

In this paper, we develop five machine learning models, neural network (NN), long short-term memory (LSTM), LSTM based on attention mechanism (LSTM-A), bidirectional LSTM (BLSTM), and BLSTM based on attention mechanism (BLSTM-A), for predicting whether a ≥C class or ≥M class flare will occur in an active region in the next 24 hr. We use the data base provided by the Space-weather Helioseismic and Magnetic Imager Active Region Patches of Solar Dynamic Observatory, including 10 magnetic field features of active regions from 2010 May 1 to 2018 September 13. The samples are labeled flare information (i.e. No-flare/C/M/X) using solar flare events catalogue provided by the Geostationary Operational Environmental Satellite and Solar Geophysical Data solar event reports. In addition, we generated 10 cross-validation sets from these data using the cross-validation method. Then, after training, validating, and testing our models, we compare the results with the true skill statistics (TSS) as the assessment metric. The main results are as follows. (1) The TSS scores for ≥C class are 0.5472 ± 0.0809, 0.6425 ± 0.0685, 0.6904 ± 0.0575, 0.6681 ± 0.0573, and 0.6833 ± 0.0531 for NN, LSTM, LSTM-A, BLSTM and BLSTM-A, respectively. The TSS scores for ≥M class are 0.5723 ± 0.1139, 0.6579 ± 0.0758, 0.5943 ± 0.0712, 0.6493 ± 0.0826, and 0.5932 ± 0.0723, respectively. (2) For the first time, we add an attention mechanism to BLSTM for flare prediction, which improves the performance of the model for ≥C class. (3) Among the five models, the prediction model based on deep learning algorithms is generally superior to the model based on the traditional machine learning algorithm. The performance of the LSTM models is comparable to that of the BLSTM models. In general, LSTM-A for ≥C class performs better than other models. In addition, we also discuss the influence of 10 features on LSTM-A, and we find that removing the least significant feature will result in better performance than using all 10 features together, and the TSS score of the model will improve to 0.7059 ± 0.0440. [ABSTRACT FROM AUTHOR]

This paper poses the problem of studying the role of large-scale electric currents propagating in the upper layers of the solar atmosphere in processes of coronal heating of the sun. For detecting and calculating the magnitude of the large-scale electric current, data on the distribution of the components of the magnetic field vector in the photosphere provided by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) were used. Photoheliograms of the sun's corona in the ultraviolet radiation channels at 131, 171, 193, and 211 Å provided by the Atmospheric Imaging Assembly (AIA/SDO) were used to estimate the temperature in the corona above active regions (ARs). The dynamics of the large-scale current and the average temperature in 9 regions with different levels of flare activity of the corona above the ARs have been studied and charts of the spatial distribution of the temperature in the corona above the ARs have been constructed. The following results have been obtained: 1. Heating of the coronal matter owing to ohmic dissipation of large-scale electric currents proceeds in a stationary regime. 2. The increase in the average temperature in the corona above an AR during solar flares to < log T ¯ > = 6.3 - 6.5 (2.0-3.2 MK) is caused, not only by heating of coronal structures by large-scale electric currents, but also by other processes at coronal elevations. 3. For the NOAA 11899 and 12494 regions a reduction in the average temperature of the corona to < log T ¯ > = 5.7 (0.5-0.6 MK) was observed with a simultaneous drop in the values of the large-scale electric current to zero. These observations indicate that the mechanism for heating of the corona by ohmic dissipation of electric currents is shut off at zero values (within the computational errors) of the large-scale electric current. 4. In the NOAA regions 12192 and 12371, when constructing charts of the temperature distribution in the corona outside flare events, hot structures with temperatures ≥ 10 MK were observed outside the flare events which appear to mark the location of the channel of a large-scale electric current at coronal elevations. For the NOAA region 12192 this assumption is confirmed by a numerical simulation carried out in 2016. [ABSTRACT FROM AUTHOR]

Active regions on the surface of the Sun are potential sources of solar flares. These include active regions that are not directly facing Earth, such as those on the Sun's far-side. However, unlike Earth-facing active regions, far-side active regions including the east limb are difficult to observe. Therefore, east limb active region observation and analysis capability are necessary to predict the occurrences of east limb flares. The purpose of this study is to develop an automatic detection system of active regions occurring in the solar east limb using EUV intensity data from the EUVI instrument onboard the STEREO-A and STEREO-B spacecraft. Random forest and support vector machine algorithms were used in the system development. Active regions will be detected as bright regions in these EUV data. This detection system supports routine space weather forecasting activities at the Research Center for Space, part of the Space Weather Information and Forecast Services (SWIFtS). The system combines STEREO/EUVI with SDO/AIA data into a single composite map in the heliographic coordinate system, and then automatically detects the presence of east limb bright regions based on their intensities in near real-time. It also provides the estimated time of arrival (ETA) of the east limb bright regions on the east limb of the solar disk. The system also calculates the bright regions' mean intensity, total intensity, area and angular width. A machine-learning model has also been developed based on eight years of data to classify bright regions as different types of photospheric entities according to the calculated parameters. A validation test using an operational dataset produced accuracy scores of 79% and 64% for the random forest and support vector machine algorithms, respectively. [ABSTRACT FROM AUTHOR]

Halo coronal mass ejections (CMEs) from the Sun are the subset of CMEs, which are more energetic than the normal CMEs. Only the more energetic CMEs are observable in white-light coronagraphs when they are traveling along the line of sight (LOS) and a CME is better observed when it is traveling perpendicularly to the LOS. We investigated the flare associated front side halo CMEs, their active regions (ARs) and flare ribbon properties observed during the period of 2010 – 2016 in Solar Cycle 24. The aim of this study is to investigate the statistical relationship between the kinematic properties of CMEs and their source active region and flare ribbon properties. The properties of the solar source region (sunspot classification according to Hale and McIntosh, sunspot area, and the number of spots) and flare ribbons (total unsigned magnetic flux, total unsigned reconnection flux, active region area, and ribbon area) are obtained for the selected events and analyzed to find the dependence of the halo CME properties (linear speed, kinetic energy, space speed, and mass) on the source region characteristics. From the preliminary analysis, 13% of halo CMEs are found to be associated with X-class flares, 52% with M-class flares, and 35% with C-class flares. It is found that many events (45/60) were produced by β γ δ and β γ Hale-type sunspot groups and Ekc/Dkc/Fkc McIntosh sunspot classes, which implies a degree of complexity of the involved sunspots. We found moderate correlations between the active region properties and CME properties, but strong correlation between the flare ribbons and CME properties. This result indicates that the kinematics of CMEs are determined by the role of magnetic reconnection and the flux related to it. We estimated the synthetic CME mass (using a synthetic CME generated based on a full ice-cream cone structure proposed recently by Na et al., Astrophys. J.906, 46, 2021). It is demonstrated that the estimated mass of halo CMEs is 1.44 times (without occulted area) and 2 times (with occulted area) that of the observed CME mass. Further, the relations between the halo CME synthetic mass with the active region and flare ribbon properties have been obtained. [ABSTRACT FROM AUTHOR]

Sunquakes are seismic emissions visible on the solar surface, associated with some solar flares. Although discovered in 1998, they have only recently become a more commonly detected phenomenon. Despite the availability of several manual detection guidelines, to our knowledge, the astrophysical data produced for sunquakes is new to the field of machine learning. Detecting sunquakes is a daunting task for human operators, and this work aims to ease and, if possible, to improve their detection. Thus, we introduce a dataset constructed from acoustic egression-power maps of solar active regions obtained for Solar Cycles 23 and 24 using the holography method. We then present a pedagogical approach to the application of machine-learning representation methods for sunquake detection using autoencoders, contrastive learning, object detection and recurrent techniques, which we enhance by introducing several custom, domain-specific data augmentation transformations. We address the main challenges of the automated sunquake-detection task, namely the very high noise patterns in and outside the active region shadow and the extreme class imbalance given by the limited number of frames that present sunquake signatures. With our trained models, we find temporal and spatial locations of peculiar acoustic emission and qualitatively associate them to eruptive and high energy emission. While noting that these models are still in a prototype stage, and there is much room for improvement in metrics and bias levels, we hypothesize that their agreement on example use cases has the potential to enable detection of weak solar acoustic manifestations. [ABSTRACT FROM AUTHOR]

Using simultaneous observations from the EUV Variability Experiment (EVE) and imaging from the Atmospheric Imaging Assembly (AIA), we characterise the temperature dependence of apparent hot flows in solar active regions. The EVE instrument performs Sun-as-a-star spectroscopy and is composed of two spectrographs: MEGS-A and MEGS-B (Multiple EUV Grating Spectrograph-A, -B). It is known that EVE can measure wavelength shifts and thus can observe relative Doppler velocities in solar atmospheric plasmas over an extended temperature range. However, MEGS-A is affected by a known astigmatism effect (Chamberlin: Solar Phys.291, 1665, 2016); inhomogeneities in EUV brightness on the solar surface result in purely instrumental wavelength errors. We validate our methods by independently quantifying this effect and comparing to Chamberlin's results, and we explore the wavelength dependence as an extension of his formula as derived for He ii 304 Å. MEGS-B is unaffected by this instrumental effect in any case, and this has allowed us to find evidence of hot prograde flows in active regions. Using our image-based models for the astigmatism and flows, we independently confirm our original MEGS-B result. We now extend our knowledge of the temperature dependence of these flows via the additional Fe emission lines available in MEGS-A. We find a monotonic increase of apparent flow speed with temperature up through lines of Fe xvi, nominally formed at about 6.4 MK. [ABSTRACT FROM AUTHOR]

We present a study of the spatial distribution of quasi-periodic oscillations (QPOs) of microwave emission in three solar active regions before flares. The following events are considered: SOL2011-09-06T01:50M5.3, SOL2011-09-25T02:33M4.4, and SOL2012-07-08T04:19C6.6. Daily observations on the Nobeyama Radioheliograph at a frequency of 17 GHz are used. In all three cases, pre-flare trains of the QPOs of microwave emission are detected. The length of the trains is 3–4 pulses. In all cases, the most strongly detected oscillations manifested themselves in a compact zone in an active region coinciding with the region of maximum microwave emission brightness during the flare. [ABSTRACT FROM AUTHOR]

This paper analyzes the observations of an original spectroheliograph telescope installed at the Kislovodsk Mountain Astronomical Station of the GAO RAS. The telescope is intended for continuous observation of the activity of the solar chromosphere. The tool makes it possible to register the full profile of the H-alpha spectral line in each pixel of the solar disk with a frequency of approximately 1 minute. We analyzed observational data from September 2015 to July 2020 and found downdrafts with average velocities Vav is approximately 0.5 km/s. The flows are directed predominantly downwards with a small horizontal component. The properties of the streams do not significantly depend on the phase of the solar cycle, the area, or the relative brightness of active regions. We assume that these streams can influence the speed of the solar wind over active regions. [ABSTRACT FROM AUTHOR]

In this paper, we study the development of Forbush decreases associated with coronal mass ejections from active regions accompanied by solar flares and filament eruptions from non-active regions using the database of Forbush effects and interplanetary disturbances created at IZMIRAN. We compared the development of two types of Forbush decreases during solar cycles 23–24, the maxima of these cycles, and the minimum between them. Using statistical methods, we studied the distributions of time intervals from the beginning of the Forbush decrease to registration: the minimum cosmic ray density, the maximum hourly decrease in density, the maximum cosmic ray anisotropy, the maximum solar wind velocity, the maximum strength of the interplanetary magnetic field, and the minimum of the Dst index. The difference in the development of two types of Forbush decreases was compared when the interplanetary disturbance contains or does not contain a magnetic cloud near the Earth. The results showed that flare-associated events develop faster than filament-associated events, even at close values of the solar wind parameters. The difference in the development of two types of Forbush decreases is more noticeable in the case of the presence of a magnetic cloud near the Earth's orbit. The largest difference between the time parameters in the two types of events is observed for the time of registration of the maximum intensity of the interplanetary magnetic field. The main phase of the two types of Forbush decreases is the same at the solar cycle 23 maximum and longer for filament-associated events at the cycle 24 maximum and 23–24 minimum. Considering all time parameters, the difference in the development of the two types of Forbush decreases is more noticeable at the maximum of cycle 23 and at the minimum of cycle 23–24 than at the maximum of cycle 24. [ABSTRACT FROM AUTHOR]

Active regions (ARs) on the Sun are discussed as a possible source of the slow solar wind (SW) whose origin is still a subject of debates. In this paper we present experimental evidence for the possible influence of ARs on the SW speed and temperature near the maximum of solar cycle 23 (2000–2002). We have studied separately the characteristics of the SW formed in the periods when ARs were on the central meridian (CM) of the Sun ( of the entire observing time) and the characteristics of the SW formed in the absence of ARs on the CM ( of the observing time). The SW speed in the former case (in the presence of ARs), on average, have turned out to be lower than the speed of the SW formed in the absence of ARs approximately by 1 (434.06 versus 438.09 km s with a measurement error km s ). For the SW temperature the corresponding difference is about 6 ( K versus K with an error K). This result confirms, on average, a lower speed and temperature of the SW forming in ARs in comparison with the SW component forming in coronal holes. [ABSTRACT FROM AUTHOR]

We study the association of solar Type IV radio bursts with the location of active regions on the Sun during Solar Cycle 24. The active regions associated with moving and stationary Type IV bursts are categorized as close to disk center and far from disk center, based on their location on the solar surface (i.e. ≤ 45 ∘ or ≥ 45 ∘ , respectively). The location of active regions associated with Type IV bursts accompanied with coronal mass ejections (CMEs) are also studied. We found that ≈ 30 – 40% of the active regions are located far from disk center for all the bursts. However, it is found that most of the active regions associated with stationary Type IV bursts are close to disk center (≈ 60 – 70%). The active regions associated with moving Type IV bursts are more evenly distributed across the surface, i.e. ≈ 56 % and ≈ 44 % , close to disk center and far from disk center, respectively. The fact that most of the bursts have active regions close to disk center indicates that these bursts can be used to obtain physical properties such as electron density and magnetic fields of the CMEs responsible for geomagnetic storms. [ABSTRACT FROM AUTHOR]

Hα flare index is one of the solar activity indicators to describe the physical properties of the active regions in the solar atmosphere. Sunspot number is a representative indicator of the magnetic activity in the photosphere, which is associated with the energy supply rate to the corona. Thus, it is significant to investigate the phase relationship between Hα flare activity and sunspot activity. In the present work, monthly Hα flare index (FIm), monthly sunspot activity (sunspot numbers (Rs) and group sunspot numbers (Rg)) compiled from the Purple Mountain Observatory are used to find their phase relationship using the cross-correlation analysis. For solar cycles 20–23, there is a very pronounced time lag of about 9, 4, 4.5 and 10 months between the monthly FIm and Rs, respectively. Furthermore, there is a noticeable time lag of about 7, 8, 4.5 and 9 months between FIm and Rg for the same cycles, respectively. If we take the whole time series together, we notice that FIm has a time lag of about 4 months with regard to both Rs and Rg. The possible physical mechanisms for their observed phase lag are discussed. [ABSTRACT FROM AUTHOR]

The Forbush decreases for the period from 1997 to 2020 were studied based on data from the database on Forbush effects and interplanetary disturbances created and maintained at the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN). Using statistical methods, we compared the Forbush decreases associated with coronal mass ejections from active regions of the Sun accompanied by solar flares; filament ejections outside active regions; high-speed streams from coronal holes; and several sources. The difference between Forbush decreases was related to coronal mass ejections when magnetic clouds in the interplanetary disturbances near the Earth were observed or not observed. It has been shown that the distributions of most of parameters are asymmetric for the sporadic Forbush decreases; for the recurrent Forbush decreases, they are nearly symmetric. The strongest correlations between the parameters of Forbush decreases and interplanetary disturbances are observed in the group of coronal ejections from active regions that are accompanied by solar flares and have a structure of magnetic cloud. [ABSTRACT FROM AUTHOR]

Necessary conditions for the appearance of decay instability of low-frequency kinetic Alfven waves in loop structures of an active region on the Sun before a flare are obtained. On the basis of the transcendental dispersion equation obtained using the synchronism conditions and additional conservation laws, an expression is obtained for the nonlinear growth rate of the decay instability of the initial kinetic Alfven wave (KAW) to the secondary KAW and the kinetic ion-acoustic wave (KIAW). Boundary values of the reduced amplitude of the initial KAW in the preflare atmosphere of an active region (AR) are obtained. It was assumed in the calculations that the waves involved in the process in the AR under study appear due to the development of instability caused by the presence of a weak large-scale (sub-Dreicer) electric field and drift motions of the plasma due to spatial inhomogeneities of its temperature and density. It is shown that, for a certain type of semiempirical model of the solar atmosphere, kinetic Alfven and kinetic ion-acoustic waves can be generated during the linear stage of instability development. In this case, the process of wave generation can take place both in plasma with purely Coulomb conductivity and in the presence of small-scale Bernstein turbulence in it. To "trigger" the instability, relatively low values of plasma nonisothermality and a very low threshold value of the reduced amplitude of the sub-Dreicer electric field are required. The possibility of the appearance of undamped kinetic waves of small amplitude in the region under study is proven. [ABSTRACT FROM AUTHOR]

The Sun is an active star that often produces numerous bursts of electromagnetic radiation at radio wavelengths. Low frequency radio bursts have recently been brought back to light with the advancement of novel radio interferometers. However, their polarisation properties have not yet been explored in detail, especially with the Low Frequency Array (LOFAR), due to difficulties in calibrating the data and accounting for instrumental leakage. Here, using a unique method to correct the polarisation observations, we explore the circular polarisation of different sub-types of solar type III radio bursts and a type I noise storm observed with LOFAR, which occurred during March–April 2019. We analysed six individual radio bursts from two different dates. We present the first Stokes V low frequency images of the Sun with LOFAR in tied-array mode observations. We find that the degree of circular polarisation for each of the selected bursts increases with frequency for fundamental emission, while this trend is either not clear or absent for harmonic emission. The type III bursts studied, that are part of a long–lasting type III storm, can have different senses of circular polarisation, occur at different locations and have different propagation directions. This indicates that the type III bursts forming a classical type III storm do not necessarily have a common origin, but instead they indicate the existence of multiple, possibly unrelated acceleration processes originating from solar minimum active regions. [ABSTRACT FROM AUTHOR]

Precisely localising solar Active Regions (AR) from multi-spectral images is a challenging but important task in understanding solar activity and its influence on space weather. A main challenge comes from each modality capturing a different location of the 3D objects, as opposed to typical multi-spectral imaging scenarios where all image bands observe the same scene. Thus, we refer to this special multi-spectral scenario as multi-layer. We present a multi-task deep learning framework that exploits the dependencies between image bands to produce 3D AR localisation (segmentation and detection) where different image bands (and physical locations) have their own set of results. Furthermore, to address the difficulty of producing dense AR annotations for training supervised machine learning (ML) algorithms, we adapt a training strategy based on weak labels (i.e. bounding boxes) in a recursive manner. We compare our detection and segmentation stages against baseline approaches for solar image analysis (multi-channel coronal hole detection, SPOCA for ARs) and state-of-the-art deep learning methods (Faster RCNN, U-Net). Additionally, both detection and segmentation stages are quantitatively validated on artificially created data of similar spatial configurations made from annotated multi-modal magnetic resonance images. Our framework achieves an average of 0.72 IoU (segmentation) and 0.90 F1 score (detection) across all modalities, comparing to the best performing baseline methods with scores of 0.53 and 0.58, respectively, on the artificial dataset, and 0.84 F1 score in the AR detection task comparing to baseline of 0.82 F1 score. Our segmentation results are qualitatively validated by an expert on real ARs. [ABSTRACT FROM AUTHOR]

The magnetic evolution of the active region of the Sun at the level of the photosphere has been studied. Magnetic synoptic maps of the 2007 Kitt Peak Observatory were used for the analysis. The observed patterns can be interpreted as the formation of an active region as a result of the action of a magnetoactive plasma, which is transported by convective cells (the process of magnetoconvection) from a level of 0.90 Rs (solar radius) from the center of the sun. The radial size of such a convective cell is ≈55 000 km and ≥85 000 km in longitude. The magnetic configuration of the moving magnetoactive plasma has a significant octupole magnetic harmonic. During the phase of its motion to the photosphere, the dipole and octupole magnetic harmonics are equally oriented and perpendicular to the solar radius. After reaching the maximum distance from the center of the Sun (0.98 Rs), the magnetoactive plasma is probably redistributed between two convective cells and moves back to the initial level of 0.90 Rs. [ABSTRACT FROM AUTHOR]

In this study, a comparative analysis was carried out on the preflare and flare conditions for six flares accompanied by coronal mass ejections (CMEs) and five events not accompanied by CMEs. All of the events occurred in the period 2011–2014. The features of the spatial dynamics of microwave emission of active regions (data from the Nobeyama Radioheliograph) prior to the selected events and during flares were studied. For the same 11 events, the fine structure of active ultraviolet (UV) sources was examined based on SDO/AIA data. Certain differences were found in the behavior of microwave emission for events with CMEs. The features of the frequency radio spectra from the RATAN-600 data were found for some active regions. [ABSTRACT FROM AUTHOR]

The paper presents an analysis of a series of observations of the full H-alpha line profile in the solar chromosphere. The analysis is based on data from the spectroheliograph patrol telescope developed at the Kislovodsk Mountain Astronomical Station, which has been in operation since 2015. The telescope conducts continuous observations with a scanning time of the entire solar disk of about 1 min. The problem of measurement of the parameters of the H-alpha line with various methods of profile approximation is considered. The possibility of determining the Doppler shift in the H-alpha line with the use of a convolutional, artificial neural network has been studied. Also considered are the characteristic forms of the H-alpha profile in active regions of the Sun and the evolution of the line profile and Doppler velocities during the formation of a solar flare on the example of an X-class flare. [ABSTRACT FROM AUTHOR]

Based on data obtained from numerical modeling of an active region (AR) of the Sun with the codes of radiation magnetohydrodynamics (MURaM), the horizontal gradient of the vertical component of the magnetic field on three layers in the vicinity of the photosphere and the horizontal component of the electric current density for the AR of the flare were calculated. It was found that ≈90% of the calculated pixels contain and of the same order, but these quantities are weakly correlated. The correlation coefficient is calculated for several groups of pixels that are distinguished by the magnitude of the horizontal current and the magnetic-field configuration. The correlation coefficients for these cases did not exceed 0.34 for the lower layer and 0.54 for the upper one. The contours of the strong and (in units of electric current density) are also weakly correlated. The result may indicate that caution must be taken when the horizontal-field gradient is used for approximation of the horizontal current in the photosphere (in particular, in the development of methods for the forecasting of flares). In additional, the probability-density functions in AR photosphere are constructed. They show that the median value of the ratio of the horizontal current to the vertical current is 3.5. This is consistent with the few estimates of various components of currents in real solar ARs and indicates the importance of the further study of horizontal currents. [ABSTRACT FROM AUTHOR]

It has been a prevailing picture that active regions on the solar surface originate from a strong toroidal magnetic field stored in the overshoot region at the base of the solar convection zone, generated by a deep seated solar dynamo mechanism. This article reviews the studies in regard to how the toroidal magnetic field can destabilize and rise through the convection zone to form the observed solar active regions at the surface. Furthermore, new results from the global simulations of the convective dynamos, and from the near-surface layer simulations of active region formation, together with helioseismic investigations of the pre-emergence active regions, are calling into question the picture of active regions as buoyantly rising flux tubes originating from the bottom of the convection zone. This article also gives a review on these new developments. [ABSTRACT FROM AUTHOR]