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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 estimate the contributions to the solar-cycle variation of solar meridional and zonal flows near the surface expected solely from local magnetic regions. Maps of flows near magnetic regions are obtained using helioseismic holography and averaged over ensembles of such regions with similar magnetic flux. These averaged flows are assigned to the positions and times of all magnetic regions identified from daily magnetograms over an 11 yr period. Residuals are obtained after averaging both meridional and zonal-flow components over longitude for each Carrington rotation and subtracting the temporal mean at each latitude. Results indicate that magnetic regions produce solar-cycle variations of both components in the active latitude range with peak amplitudes of about 1 m s−1, which represents only a fraction of the known variations in either the residual zonal or meridional flow with amplitudes around 3 and 5 m s−1, respectively. Thus it is unlikely that active regions, and compact magnetic concentrations in general, are the primary source of the 11 yr variation in these global flows. A threshold magnetic flux of 5 × 1020 Mx, below which no significant flows are observed, is identified as a check on the completeness of the set of magnetic regions. We also find that inflows around most magnetic regions are confined to within 10°–12° of their centers, in contrast with recent evidence of more extended inflows. [ABSTRACT FROM AUTHOR]

Emerging active regions are associated with convective flows on the spatial scale and lifetimes of supergranules. To understand how these flows are involved in the formation of active regions, we aim to identify where active regions emerge in the supergranulation flow pattern. We computed supergranulation scale flow maps at the surface for all active regions in the Solar Dynamics Observatory Helioseismic Emerging Active Region Survey. We classified each of the active regions into four bins based on the amplitude of their average surface flow divergence at emergence. We then averaged the flow divergence over the active regions in each bin as a function of time. We also considered a corresponding set of control regions. We found that, on average, the flow divergence increases during the day prior to emergence at a rate independent of the amount of flux that emerges. By subtracting the averaged flow divergence of the control regions, we found that active region emergence is associated with a remaining converging flow at 0.5–1 d prior to emergence. This remnant flow, |$\Delta \, \mathrm{div} \, \boldsymbol {v}_{\rm h} = (-4.9 \pm 1.7) \times 10^{-6}\,{\rm s}^{-1}$|⁠ , corresponds to a flow speed of 10–20 m s−1 (an order of magnitude less than supergranulation flows) out to a radius of about 10 Mm. We show that these observational results are qualitatively supported by simulations of a small bipole emerging through the near-surface convective layers of the Sun. The question remains whether these flows are driving the emergence, or are caused by the emergence. [ABSTRACT FROM AUTHOR]

We report for the first time the detection of thermal free–free emission from post-flare loops at 34 GHz in images from the Nobeyama Radioheliograph. We studied eight loops, seven of which were from regions with an extremely strong coronal magnetic field reported by Fedenev et al. Loop emission was observed in a wide range of wavelength bands, up to soft X-rays, confirming their multitemperature structure and was associated with noise storm emission in metric λ. The comparison of the 17 GHz emission with that at 34 GHz, after a calibration correction of the latter, showed that the emission was optically thin at both frequencies. We describe the structure and evolution of the loops and we computed their density, obtaining values for the top of the loops between 1 and 6 × 1010 cm−3, noticeably varying from one loop to another and in the course of the evolution of the same loop system; these values have only a weak dependence on the assumed temperature, 2 × 106 K in our case, as we are in the optically thin regime. Our density values are above those reported from EUV observations, which go up to about 1010 cm−3. This difference could be due to the fact that different emitting regions are sampled in the two domains and/or due to the more accurate diagnostics in the radio range, which do not suffer from inherent uncertainties arising from abundances and non-LTE excitation/ionization equilibria. We also estimated the magnetic field in the loop tops to be in the range of 10–30 G. [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]

The article "Ionosphere Monitoring with Remote Sensing Vol II." provides an overview of the importance of studying the Earth's ionosphere and its influence on space weather. It discusses the use of remote sensing and in situ instruments to investigate ionospheric features and anomalies. The article presents original research papers that contribute to the understanding of the ionosphere and its relevance for signal transmission and space weather studies. It also explores the use of artificial intelligence technology in ionospheric prediction models and the study of ionospheric anomalies associated with natural phenomena. The research findings highlight the complex nature of the ionosphere and its impact on communication and navigation systems. [Extracted from the article]

Studying the hemispheric distribution of active regions (ARs) with different magnetic morphologies may clarify the features of the dynamo process that is hidden under the photospheric level. The magnetic flux data for 3047 ARs from the CrAO catalogue (https://sun.crao.ru/databases/catalog-mmc-ars), between May 1996 and December 2021 (cycles 23 and 24) were used to study ARs cyclic variations and perform correlation analysis. According to the magneto-morphological classification (MMC) of ARs proposed earlier, subsets of the regular (obeying empirical rules for sunspots) and irregular (violating these rules) ARs were considered separately. Our analysis shows the following: For ARs of each MMC type, in each of the hemispheres, time profiles demonstrate a multipeak structure. The double-peak structure of a cycle is formed by ARs of both MMC types in both hemispheres. For the irregular ARs, the pronounced peaks occur in the second maxima (close to the polar field reversal). Their significant hemispheric imbalance might be caused by a weakening of the toroidal field in one of the hemispheres due to the interaction between the dipolar and quadrupolar components of the global field, which facilitates the manifestation of the turbulent component of the dynamo. The similarity of the irregular ARs activity that was found in adjacent cycles in different hemispheres also hints at realization of the mix-parity dynamo solution. For the quadrupolar-like component of the flux (compiled in the simple axisymmetric approximation), signs of oscillations with a period of about 15 years are found, and they are pronounced specifically for the irregular groups. This MMC type ARs might also contribute in |$\alpha$| -quenching. [ABSTRACT FROM AUTHOR]

Studies of active region (AR) lifetimes are mostly restricted to short-lived ARs. The aim of this paper is to include recurrent ARs, which should be identified unambiguously. The first step is the algorithmic listing of possible returns; then, the candidates are visually checked using the unique HTML-feature of the Debrecen sunspot database. The final step is application of an asymmetric Gaussian function, introduced in previous articles, for short-lived ARs. This function has a surprisingly good fit to the data on correctly identified recurrent sunspot groups over several rotations enabling the reconstruction of the development on the far side of the sun. The Gnevyshev–Waldmeier rule for the area–lifetime relationship is not applicable for recurrent ARs; however, as a novel approach, a linear regression analysis extended to long lifetimes made it possible to recognize two populations of sizes for which two different area–lifetime relationships can be obtained. The lifetimes exhibit weak dependencies on the heliographic latitude and solar cycle phase. If an asymmetric Gaussian cannot be fit to the data, then they presumably belong to consecutive members of an active nest. [ABSTRACT FROM AUTHOR]

The Huairou Solar Observing Station (HSOS) has conducted solar vector magnetic field observations for 40 years and developed multiple vector magnetographs (including one space magnetic field observation instrument). Using these accumulated magnetic field observation data, HSOS has achieved significant progress in solar physics research, including important advancements in the helicity sign rule of solar active regions, the helicity characteristics of strong and weak magnetic fields in active regions, the chromospheric magnetic field characteristics of the Sun, the evolution of magnetic fields in active regions, and the extraction of magnetic field characteristics for flare precursors. However, due to historical reasons, the calibration of vector magnetic field data in HSOS are not standardized. Therefore, this paper summarizes past historical experiences and introduces the standardized calibration procedure for vector magnetic field processing in detail. These calibration procedures are the basic steps of the calibration process for the space vector magnetograph (Full-Disk Vector MagnetoGraph, abbreviated as FMG) observation data, and are also applicable to the calibration of other instrument observation data at HSOS. They mainly include basic processing of polarization data and in-depth processing of vector magnetic fields. We believe that such calibration processing of the historical data collected by HSOS over the past 40 years will help us to accurately measure and analyze the solar magnetic field, further revealing the laws of solar activity and its impact on the Earth's environment. [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]

Updated summaries of the August 1972 and March 1989 space weather events have been constructed. The features of these two events are compared to the Carrington 1859 event and a few other major space weather events. It is concluded that solar active regions release energy in a variety of forms (X‐rays, EUV photons, visible light, coronal mass ejection (CME) plasmas and fields) and they in turn can produce other energetic effects (solar energetic particles (SEPs), magnetic storms) in a variety of ways. It is clear that there is no strong one‐to‐one relationship between these various energy sinks. The energy is often distributed differently from one space weather event to the next. Concerning SEPs accelerated at interplanetary CME (ICME) shocks, it is concluded that the Fermi mechanism associated with quasi‐parallel shocks is relatively weak and that the gradient drift mechanism (electric fields) at quasi‐perpendicular shocks will produce harder spectra and higher fluxes. If the 4 August 1972 intrinsic magnetic cloud condition (southward interplanetary magnetic field instead of northward) and the interplanetary Sun to 1 au conditions were different, a 4 August 1972 magnetic storm and magnetospheric dawn‐to‐dusk electric fields substantially larger than the Carrington event would have occurred. Under these special interplanetary conditions, a Miyake et al. (2012), https://doi.org/10.1038/nature11123‐like extreme SEP event may have been formed. The long duration complex 1989 storm was probably greater than the Carrington storm in the sense that the total ring current particle energy was larger. Plain Language Summary: All the main information for the August 1972 and March 1989 space weather events have been gathered together for the readership. Various features of these two events have been compared and contrasted to the Carrington event and several other recent major events. It is thought that major energetic MeV to GeV particle acceleration occurs at interplanetary shocks instead of acceleration at the solar flare site. Two popular mechanisms for shock particle acceleration are: (a) Fermi acceleration between waves upstream and downstream of a shock, and (b) particle gradient drift in the direction of an electric field along the shock surface. The second mechanism is concluded to form a harder particle spectrum with higher fluxes. If the intrinsic conditions of the 4 August 1972 coronal mass ejection were different (southward interplanetary magnetic fields) and that propagated to the Earth without major evolution or distortion, a magnetic storm equal to or more intense than the Carrington event would have occurred. We have also constructed a scenario involving altered interplanetary space for the August 1972 event which could lead to a high Mach number quasiperpendicular shock and possibly a solar particle event comparable to the Miyake et al. (2012, https://doi.org/10.1038/nature11123) tree ring event. Key Points: The August 1972 and March 1989 space weather events are summarized and compared to the Carrington plus a few other recent eventsWith different solar and interplanetary conditions, the August 1972 ICME could have caused a storm greater than the Carrington stormThe complex March 1989 storm was a "stealth magnetic storm", probably greater ring‐current energy‐wise than the Carrington storm [ABSTRACT FROM AUTHOR]

Composite analyses of NOAA satellite‐based outgoing longwave radiation data and ERA5 reanalysis data for nearly six solar maximum periods support the existence of a response of tropical convection and precipitation to short‐term (∼27‐day) solar ultraviolet variations. Following solar UV peaks, the response consists of an increase in average convection and precipitation in the equatorial Indian Ocean and a decrease in the western and central tropical Pacific, with maximum amplitude at a lag of 4 to 8 days. The opposite occurs following short‐term solar UV minima. The observed responses are most detectable when the Madden‐Julian oscillation (MJO) is active and appear to be related to a reduced ability of the MJO to propagate across the Maritime Continent barrier following solar UV peaks relative to UV minima. A similar behavior has previously been found when the stratospheric quasi‐biennial oscillation is in its westerly phase relative to its easterly phase. Plain Language Summary: Under solar maximum conditions, active regions on the sun produce strong variations of solar ultraviolet radiation on the solar rotational time scale (∼27 days). We have analyzed data from NOAA satellites and ERA5 reanalysis for almost six solar maximum periods. The results indicate that these short‐term solar ultraviolet variations affect tropical convection and precipitation. Following a peak in solar UV radiation, there is an increase in convection and precipitation in the equatorial Indian Ocean, and a decrease in the western and central tropical Pacific, with the most significant changes occurring 4 to 8 days later. The opposite happens after solar UV minima. These effects are most noticeable when the Madden‐Julian Oscillation is active and seem linked to its inability to move across the Maritime Continent barrier after solar peaks compared to solar minima. Similar patterns have been observed with the stratospheric quasi‐biennial oscillation in its westerly phase versus its easterly phase. Key Points: Evidence of lagged responses of tropical convection and precipitation to short‐term solar UV variations is found over six solar cyclesThe responses are due to solar effects on eastward propagation of the Madden‐Julian oscillation across the Maritime Continent barrierAverage precipitation rate differences following solar peaks and minima can be more than 20% of mean daily rates [ABSTRACT FROM AUTHOR]

The solar cycle is generated by a magnetohydrodynamic dynamo mechanism which involves the induction and recycling of the toroidal and poloidal components of the Sun's magnetic field. Recent observations indicate that the Babcock–Leighton (BL) mechanism – mediated via the emergence and evolution of tilted bipolar active regions – is the primary contributor to the Sun's large-scale dipolar field. Surface flux transport models and dynamo models have been employed to simulate this mechanism, which also allows for physics-based solar cycle forecasts. Recently, an alternative analytic method has been proposed to quantify the contribution of individual active regions to the Sun's dipole moment (DM). Utilizing solar cycle observations spanning a century, here, we test the efficacy of this algebraic approach. Our results demonstrate that the algebraic quantification approach is reasonably successful in estimating DMs at solar minima over the past century – providing a verification of the BL mechanism as the primary contributor to the Sun's dipole field variations. We highlight that this algebraic methodology serves as an independent approach for estimating DMs at the minima of solar cycles, relying on characteristics of bipolar solar active regions. We also show how this method may be utilized for solar cycle predictions; our estimate of the Sun's dipole field at the end of cycle 24 using this approach indicates that solar cycle 25 would be a moderately weak cycle, ranging between solar cycle 20 and cycle 24. [ABSTRACT FROM AUTHOR]

Solar flares are characterized by sudden bursts of electromagnetic radiation from the Sun's surface, and are caused by the changes in magnetic field states in active solar regions. Earth and its surrounding space environment can suffer from various negative impacts caused by solar flares, ranging from electronic communication disruption to radiation exposure-based health risks to astronauts. In this paper, we address the solar flare prediction problem from magnetic field parameter-based multivariate time series (MVTS) data using multiple state-of-the-art machine learning classifiers that include MINImally RandOm Convolutional KErnel Transform (MiniRocket), Support Vector Machine (SVM), Canonical Interval Forest (CIF), Multiple Representations Sequence Learner (Mr-SEQL), and a Long Short-Term Memory (LSTM)-based deep learning model. Our experiment is conducted on the Space Weather Analytics for Solar Flares (SWAN-SF) benchmark data set, which is a partitioned collection of MVTS data of active region magnetic field parameters spanning over nine years of operation of the Solar Dynamics Observatory (SDO). The MVTS instances of the SWAN-SF dataset are labeled by GOES X-ray flux-based flare class labels, and attributed to extreme class imbalance because of the rarity of the major flaring events (e.g., X and M). As a performance validation metric in this class-imbalanced dataset, we used the True Skill Statistic ( T S S ) score. Finally, we demonstrate the advantages of the MVTS learning algorithm MiniRocket, which outperformed the aforementioned classifiers without the need for essential data preprocessing steps such as normalization, statistical summarization, and class imbalance handling heuristics. [ABSTRACT FROM AUTHOR]

Modelling the early evolution of magnetic flux ropes (MFRs) in the solar atmosphere is crucial for understanding their destabilization and eruption mechanism. Identifying the relevant magnetic field lines in simulation data, however, is not straightforward. In previous work an extraction and tracking method was developed to facilitate this task. Here, we present the corresponding graphical user interface (GUI), called GUITAR (GUI forTracking and Analysing flux Ropes), with the aim to offer a variety of tools to the community for identifying and tracking MFRs. The starting point is a map of a selected proxy parameter for MFRs, e.g., a map of the twist-parameter Tw, current density, etc. We showcase how the GUITAR tools can be used to disentangle a multi-MFR system and facilitate in-depth analysis of their properties and evolution by applying them on a time-dependent data-driven magnetofrictional model (TMFM) simulation of solar active region AR12473. We show the MFR extraction using Tw maps, together with targeted use of mathematical morphology algorithms and discuss the evolution of the system. [ABSTRACT FROM AUTHOR]

An analysis of the observational polarization properties of the decameter spikes is presented in the paper. It is shown that decameter spikes possess high degree of circular polarization with average value of about 60%. In the frames of "leading spot" theory we associated the spikes activity with a certain active region on the solar disk and determined the mode of the emission. Supposing plasma emission mechanism we link and determine coronal plasma and fast electron beam parameters. [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. The Sun experienced a period of unprecedented activity during the 20th century, now called the Modern Maximum (MM). The decay of the MM after its maximum in cycle 19 has changed the Sun, the heliosphere, and the planetary environments in many ways. However, studies disagree on whether this decay has proceeded synchronously in different solar parameters or not. Aims. One of the related key issues is if the relation between two long parameters of solar activity, the sunspot number and the solar 10.7 cm radio flux, has remained the same during this decay. A recent study argues that there is an inhomogeneity in the 10.7 cm radio flux in 1980, which leads to a step-like jump ("1980 jump") in this relation. If true, this result would reduce the versatility of possible long-term studies of the Sun during the MM. Here we aim to show that the relation between sunspot number and 10.7 cm radio flux does indeed vary in time, not due to an inhomogeneous radio flux but due to physical changes in the solar atmosphere. Methods. We used radio flux measurements made in Japan at four different wavelengths, and studied their long-term relation with the sunspot number and the 10.7 cm radio flux during the decay of MM. We also used two other solar parameters, the MgII index and the number of solar active regions, in order to study the nature of the observed long-term changes in more detail. Results. We find that the 1980 jump is only the first of a series of 1–2-year "humps" that mainly occur during solar maxima. All five radio fluxes depict an increasing trend with respect to the sunspot number from the 1970s to 2010s. These results exclude the interpretation of the 1980 jump as an inhomogeneity in the 10.7 cm flux, and reestablish the 10.7 cm flux as a homogeneous measure of solar activity. The fluxes of the longer radio waves are found to increase with respect to the shorter waves, which suggests a long-term change in the solar radio spectrum. We also find that the MgII index of solar UV irradiance and the number of active regions also increased with respect to the sunspot number, further verifying the difference in the long-term evolution in chromospheric and photospheric parameters. Conclusions. Our results provide evidence for important structural changes in solar magnetic fields and the solar atmosphere during the decay of the MM, which have not been reliably documented so far. We also emphasize that the changing relation between the different (e.g., photospheric and chromospheric) solar parameters should be taken into account when using the sunspot number or any single parameter in long-term studies of solar activity. [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]

Recent coronal loop modeling has emphasized the importance of combining both Coulomb collisions and turbulent scattering to characterize field-aligned thermal conduction, which invokes a hybrid loop model. In this work, we generalize the hybrid model by incorporating a nonuniform heating and cross section that are both formulated by a power-law function of temperature. Based on the hybrid model solutions, we construct scaling laws that relate loop-top temperature (T a ) and heating rate (H a ) to other loop parameters. It is found that the loop-top properties for turbulent loops are additionally power-law functions of the turbulent mean free path (λ T ), with the functional forms varying from situation to situation, depending on the specification of the heating and/or areal parameters. More importantly, both a sufficiently footpoint-concentrated heating and a cross-sectional expansion with height can effectively weaken (strengthen) the negative (positive) power-law dependence of T a (H a ) on λ T . The reason lies in a notable reduction of heat flux by footpoint heating and/or cross-sectional expansion in the turbulence-dominated coronal part, where turbulent scattering introduces a much weaker dependence of the conduction coefficient on temperature. In this region, therefore, the reduction of the heat flux predominately relies on a backward flattening of the temperature gradient. Through numerical modeling that incorporates more realistic conditions, this scenario is further consolidated. Our results have important implications for solar active region (AR) loops. With the factors of nonuniform heating and cross section taken into account, AR loops can bear relatively stronger turbulence while still keeping a physically reasonable temperature for nonflaring loops. [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]

The physics of solar flares occurring on the Sun is highly complex and far from fully understood. However, observations show that solar eruptions are associated with the intense kilogauss fields of active regions, where free energies are stored with field-aligned electric currents. With the advent of high-quality data sources such as the Geostationary Operational Environmental Satellites (GOES) and Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI), recent works on solar flare forecasting have been focusing on data-driven methods. In particular, black box machine learning and deep learning models are increasingly being adopted in which underlying data structures are not modeled explicitly. If the active regions indeed follow the same laws of physics, similar patterns should be shared among them, reflected by the observations. Yet, these black box models currently used in the literature do not explicitly characterize the heterogeneous nature of the solar flare data within and between active regions. In this paper, we propose two finite mixture models designed to capture the heterogeneous patterns of active regions and their associated solar flare events. With extensive numerical studies, we demonstrate the usefulness of our proposed method for both resolving the sample imbalance issue and modeling the heterogeneity for rare energetic solar flare events. [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]

An unusually low density solar wind event was observed in December 2022 moving past both Earth and Mars. The source was traced back to a coronal hole and active region on the Sun's surface. The resulting solar wind lead to the development of a co‐rotating interaction region (CIR) and trailing rarefaction region that lasted for multiple solar rotations. Within this structure, the solar wind conditions, including density, velocity, and magnetic field magnitude and orientation drastically changed. In this study we analyze the response of the Martian ionosphere using MAVEN data to these changing solar wind conditions. The low density solar wind region associated with the December event resulted in the expansion of the Martian ionospheric boundaries. We show that the ion composition boundary (ICB) is located at extreme altitudes that are beyond previously observed locations from the MAVEN mission between 2015 and 2018. Furthermore, the boundary between shocked solar wind and the Martian ionosphere identified using electron and ion data moved together on the dayside of the planet with the changing solar wind conditions. However, at the flank region these boundaries do not move together, and we show here that the decoupling of the two boundaries may be the result of a change in the interplanetary magnetic field azimuthal angle. Plain Language Summary: The Sun constantly emits fast moving charged particles into interplanetary space in what is known as the solar wind. In December 2022, a region of fast moving solar wind overtook a region of slower moving solar wind. The fast solar wind acted as a broom sweeping up and compressing the slower wind preceding it. This interaction of two solar wind speeds is observed to rotate with the Sun. Trailing this interaction region is a region of low density solar wind. In December 2022, this type of interplanetary solar wind structure interacted with Mars' electrically charged atmosphere, or ionosphere. Throughout this period, the ionosphere boundary characterized by a transition from solar wind ions to planetary ions moved up and down relative to the planet near dusk. As the rarefaction region passed through the system, the ionosphere expanded to unusually high heights as it was not compressed by the solar wind. However, the flapping of the ionospheric boundary seems to depend on the orientation that the solar wind interacts with Mars. Key Points: The disappearing solar wind event observed at Mars in December 2022 was caused by a co‐rotating interaction regionMars' ionosphere expanded further than previously observed locations between 2015 through 2018The flank ion composition boundary experiences large fluctuations in altitude after a change in the interplanetary magnetic field direction [ABSTRACT FROM AUTHOR]

A paradigm shift is taking place in the conception of solar cycles. In the previous conception, the changing numbers of sunspots over intervals of 9-14 years have been regarded as the fundamental solar cycle although two average 11-year cycles were necessary to account for the complete magnetic cycle. In the revised picture, sunspots are a phase in the middle of two 22-year overlapping solar cycles that operate continuously with clock-like precision. More than 20 researchers have contributed to the initial research articles from 2014 through 2021 which are dramatically altering the perception of solar cycles. The two 22-year cycles overlap in time by 11 years. This overlap is coincidentally the same average duration as the sunspot phase in each 22-year cycle. This coincidence and the relative lack of knowledge of the large numbers of small active regions without sunspots is what led to the previous paradigm in which the 11-year sunspot phases were misinterpreted as a single fundamental solar cycle. The combination of the two 22-year solar cycles, with their large numbers of short-lived active regions and ephemeral active regions are now understood to be the fundamental cycle with the proposed name "The Hale Solar Cycle." The two 22-year solar cycles each occupy separate but adjacent bands in latitude. The orientations of the majority of bipolar magnetic regions in the two adjacent bands differ from each other by ∼180°. Both bands continuously drift from higher to lower latitudes as has been known for sunspot cycles. However, the polarity reversal occurs at the start of each 22-year cycle and at higher latitudes than it does for the sunspot cycles. This paradigm shift in the concept of solar cycles has resulted in major reconsiderations of additional topics on solar cycles in this review. These are 1) the large role of ephemeral active regions in the origin of solar cycles, 2) the depth of the origin of active regions and sunspots, 3) the mechanisms of how areas of unipolar magnetic network migrate to the solar poles every 11 years, and 4) the nature of the polarity reversal in alternate 22-year cycles rather than 11-year cycles. [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]

It has been suggested that the ratio of photospheric direct to return current, ∣DC/RC∣, may be a better proxy for assessing the ability of solar active regions to produce a coronal mass ejection (CME) than others such as the amount of shear along the polarity inversion line (PIL). To test this conjecture, we measure both quantities prior to eruptive and confined flares of varying magnitude. We find that eruptive-flare source regions have ∣DC/RC∣ > 1.63 and PIL shear above 45° (average values of 3.°2 and 68°, respectively), tending to be larger for stronger events, while both quantities are on average smaller for confined-flare source regions (2.°2 and 68°, respectively), albeit with substantial overlap. Many source regions, especially those of eruptive X-class flares, exhibit elongated direct currents (EDCs) bracketing the eruptive PIL segment, which typically coincide with areas of continuous PIL shear above 45°. However, a small subset of confined-flare source regions have ∣DC/RC∣ close to unity, very low PIL shear (<38°), and no clear EDC signatures, rendering such regions less likely to produce a CME. A simple quantitative analysis reveals that ∣DC/RC∣ and PIL shear are almost equally good proxies for assessing CME-productivity, comparable to other proxies suggested in the literature. We also show that an inadequate selection of the current-integration area typically yields a substantial underestimation of ∣DC/RC∣, discuss specific cases that require careful consideration for ∣DC/RC∣ calculation and interpretation of the results, and suggest improving photospheric CME-productivity proxies by incorporating coronal measures such as the decay index. [ABSTRACT FROM AUTHOR]

On 3 July 2021, an X1.5 solar flare from the National Oceanic and Atmospheric Administration solar Active Region AR12838 (24°N, 88°W) occurred at 14:18 UT, peaked at 14:29 UT, and decayed at 14:34 UT. The study of this X1.5 solar flare is significant due to its unique geomagnetic crochet feature at high latitudes and its effective signature on Earth. The study examined X‐rays, the extreme ultraviolet spectrum, ionospheric equivalent current (IEC), and geomagnetic field components. The study reveals a sudden increase in IEC during the X1.5 flare episode, forming a zonal current region and producing a geomagnetic crochet signature in geomagnetic field components at high latitudes (50°–80°N) along the 11°–26°E longitude sector during the flare peak time. All three geomagnetic field components show different sensitivity to the solar flare effect (sfe), and the amplitude and phase of the geomagnetic crochet across latitudes (for a given longitude) are consistent with the variations in the IEC. The present study is the first to appraise geomagnetic crochets of low magnitude (8–40 nT) and short duration (10–15 min) at high latitudes, particularly in the polar cusp region, during the X‐class limb flare. Plain Language Summary: The X1.5 solar flare at 14:18 UT on 3 July 2021, occurred in a newly formed solar active region AR12838 (24°N, 88°W) on the sun. The current study analyses the solar flare effect (sfe) signature at high latitude induced by an X1.5 flare, and compares the results from previous paper of Yamauchi et al. (2020, https://doi.org/10.5194/angeo-38-1159-2020). However, while that paper reported geomagnetic crochet of high amplitude (200 nT) and long duration (∼2 hr), the present work reports geomagnetic crochet of small amplitude (40 nT) and short duration (∼15 min). The current study is unique regarding observations of geomagnetic crochet in the polar cusp region. Key Points: An X1.5‐class solar flare was recorded on 3 July 2021, from the newly emerged solar active region NOAA AR12838 over the solar limbThe current study examines the solar flare effect at high latitude magnetometer stations, an aspect that has not been extensively studiedUnique signatures of Cusp geomagnetic crochet of small amplitude and short duration are observed [ABSTRACT FROM AUTHOR]

We present a magnetic configuration of a compound solar eruption observed on 2012 March 10, from NOAA AR 11429 near the disk center, which displayed a soft X-ray sigmoid before the eruption. We constructed a series of magnetic field models, including double-decker flux rope configurations, using the flux rope insertion method. This produces three-dimensional coronal magnetic field models constrained by the photospheric magnetogram and observed EUV coronal structures. We used different combinations of flux rope paths. We found that two flux ropes sharing the same path at different heights quickly experience a partial merging in the initial iteration of the magnetofrictional relaxation process. Different paths with less than 30% overlap allowed us to construct stable double-decker structures. The high spatial and temporal resolution of the Solar Dynamics Observatory/Atmospheric Imaging Assembly facilitated the selection of a best-fit model that matches the observations best. Moreover, by varying fluxes in this validated nonlinear force-free field double-decker configuration, we successfully reproduce all three scenarios of eruptions of double-decker configurations: (i) eruption due to the instability of higher flux rope; (ii) eruption due to rising lower flux rope and merging with higher flux rope; and (iii) eruption due to the instability of both flux ropes. This demonstrates that magnetofrictional simulation can capture the large-scale magnetic structure of eruptions for a realistic field configuration at eruption onset. [ABSTRACT FROM AUTHOR]

Solar eruptions, such as flares and coronal mass ejections, have serious impacts on the Earth's environment and human activity. Predicting the eruptions and therefore avoiding their damages is a main goal in the research community. Heliospheric current sheet (HCS), the boundary dividing the positive and negative magnetic fields that open up to the interplanetary space, is believed to be related with solar eruptions. The HCS consists of two types of boundaries: the Hale boundary and the non-Hale boundary, and the former is suggested in previous studies to be the region producing most of the solar flares. In this work, we explore relationships between Hale boundary, flares, and property of active regions (ARs). The HCS is determined from the magnetic field in the heliosphere calculated from the observed photospheric magnetic field using a potential field source surface model. We have analysed 8075 flares of C-class and above and 1652 ARs in the period from May 2010 to May 2019 in Solar Cycle 24. The following results are obtained: (1) 5957 flares (⁠|$74\pm 1~{{\ \rm per\ cent}}$|⁠) and 1212 ARs (⁠|$73\pm 2~{{\ \rm per\ cent}}$|⁠) are in the HCS; (2) among them, 70  per cent (⁠|$\pm 1~{{\ \rm per\ cent}}$|⁠) flares and 57  per cent (⁠|$\pm 3~{{\ \rm per\ cent}}$|⁠) ARs are in the Hale boundary; and (3) big ARs tend to emerge in the Hale boundary. We conclude that the HCS is related tightly to solar activity, and the Hale boundary is a region favoured to produce flares and to host large ARs. [ABSTRACT FROM AUTHOR]

The plasma composition of the solar corona is different from that of the solar photosphere. Elements that have a low first ionization potential (FIP) are preferentially transported to the corona and therefore show enhanced abundances in the corona compared to the photosphere. The level of enhancement is measured using the FIP bias parameter. In this work, we use data from the EUV Imaging Spectrometer on Hinode to study the plasma composition in an active region following an episode of significant new flux emergence into the preexisting magnetic environment of the active region. We use two FIP bias diagnostics: Si x 258.375 Å/S x 264.233 Å (temperature of approximately 1.5 MK) and Ca xiv 193.874 Å/Ar xiv 194.396 Å (temperature of approximately 4 MK). We observe slightly higher FIP bias values with the Ca/Ar diagnostic than Si/S in the newly emerging loops, and this pattern is much stronger in the preexisting loops (those that had been formed before the flux emergence). This result can be interpreted in the context of the ponderomotive force model, which proposes that the plasma fractionation is generally driven by Alfvén waves. Model simulations predict this difference between diagnostics using simple assumptions about the wave properties, particularly that the fractionation is driven by resonant/nonresonant waves in the emerging/preexisting loops. We propose that this results in the different fractionation patterns observed in these two sets of loops. [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]

We review observations of solar activity, geomagnetic variation, and auroral visibility for the extreme geomagnetic storm on 1872 February 4. The extreme storm (referred to here as the Chapman–Silverman storm) apparently originated from a complex active region of moderate area (≈ 500 μ sh) that was favorably situated near disk center (S19° E05°). There is circumstantial evidence for an eruption from this region at 9–10 UT on 1872 February 3, based on the location, complexity, and evolution of the region, and on reports of prominence activations, which yields a plausible transit time of ≈29 hr to Earth. Magnetograms show that the storm began with a sudden commencement at ≈14:27 UT and allow a minimum Dst estimate of ≤ −834 nT. Overhead aurorae were credibly reported at Jacobabad (British India) and Shanghai (China), both at 19.°9 in magnetic latitude (MLAT) and 24.°2 in invariant latitude (ILAT). Auroral visibility was reported from 13 locations with MLAT below ∣20∣° for the 1872 storm (ranging from ∣10.°0∣–∣19.°9∣ MLAT) versus one each for the 1859 storm (∣17.°3∣ MLAT) and the 1921 storm (∣16.°2∣ MLAT). The auroral extension and conservative storm intensity indicate a magnetic storm of comparable strength to the extreme storms of 1859 September (25.°1 ± 0.°5 ILAT and −949 ± 31 nT) and 1921 May (27.°1 ILAT and −907 ± 132 nT), which places the 1872 storm among the three largest magnetic storms yet observed. [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]

Studying the interaction between solar flares and sunspot groups (SGs) is crucial for understanding and predicting solar activity. We examined the distribution, correlation, and flaring rates in the northern and southern hemispheres to reveal the relationship between different classes of soft X-ray (SXR) flares and different magnetic classifications of SGs. We discovered a significant north–south asymmetry in SXR flares and SG distribution over Solar Cycles (SC) 21–25. In the rising phase of SC24, the northern hemisphere's activity is significantly excessive. In the declining phase of SC24, the southern hemisphere's activity becomes significantly excessive. The total numbers of various SXR flares and SGs vary between the northern and southern hemispheres over the solar cycle. B-class flares are negatively correlated with all SGs at maximum but positively correlated at minimum. C-class flares correlate best with α and β SGs. M-class flares correlate best with β γ δ and β SGs. X-class flares correlate highest with β γ δ SGs. The flaring rate of each flare class is lowest for α SGs and highest for β γ δ SGs. The flaring rates are higher in the southern hemisphere than in the northern hemisphere. Our results demonstrate that solar flares originate from different sources of solar active regions; the high-energy flares tend to be caused by more complex magnetic fields. [ABSTRACT FROM AUTHOR]

Photospheric bright points (BPs) fast movement is more effective than the slow movement that occurs most of the time in transmitting energy to the corona. The splitting and merging BPs tend to produce relatively large velocities. Therefore, our aim is to detect and track BP groups. After that, the attribute distribution of BP groups and the attribute evolution under possible high-speed movement are studied and analysed. We use a new BPs detection model (HBD-model) that can effectively detect weak BPs and layered BPs and then use similarity measures to judge the splitting and merging of BPs for tracking. The average accuracy of the BPs detection algorithm is 0.74, and the average accuracy of tracking is more than 85 per cent. For the BPs in the quiet Sun (QS) and active region (AR), a total of 62 388 evolutionary BPs were identified and tracked. The average velocities are more than 2 km s−1 in the QS and ARs. For the QS and ARs, the evolution process of splitting or merging BPs is basically the same. The results show that for the merged BPs, the area and velocity of the BPs are tiny in the initial stage, reach a peak in the middle stage, and then become smaller and disappear. The area, intensity contrast, and velocity of the BPs with splitting are relatively tiny at the beginning, become larger at the middle stage, reach the peak after splitting, and then become smaller and disappear rapidly. [ABSTRACT FROM AUTHOR]