We present metadata (date and time of observations) extracted from the digitized (scanned) photographic images of solar disk observations in neutral hydrogen Hα spectral line made at the U.S. National Solar Observatory at Sacramento Peak during 1966-2002. Successful extraction of metadata enables a more effective use of this historical data set in scientific research. About 4.8 million of images were processed using machine-learning models trained to detect the text fields and digitize text information in these fields. We report digitized data along with confidence estimates than can be used to filter out less reliable data. We discuss the complications in digitizing this historical dataset, and suggest additional steps that could be used to improve the extraction of metadata.

Total solar eclipses (TSEs) offer a unique opportunity to observe the solar atmosphere, detect limb phenomena, and accurately measure the solar radius. Following the TSE in 1733, Wassenius first reported the existence of prominences to the scientific community. Wassenius' original manuscript is held in the Royal Academy Archives of Sweden; this study translates his report and documents the associated source materials and local eclipse visibility. The solar radius (R_Sun) during the TSE in 1733 are 696250 +/- 170 km and 959.99 +/- 0.24" in the absolute and apparent scales, respectively. This result contrasts with the modern standard (helioseismic) R_Sun of 695780 +/- 160 km and 959.34 +/- 0.22"; however, it is consistent with the solar radius recorded in 1715. The observed prominences are located at +23.5 +/- 22.5°, +66.5 +/- 22.5°, and -68.5 +/- 22.5° in the heliographic latitude. The appearance of prominences at such high latitudes contrasts with the sunspot butterfly diagram for 1725-1750, confirming 1733 as a solar minimum. These high-latitude prominences can potentially be attributed to the so-called 'polar rush' prominences that appear a few years after a solar minimum. If they are categorised as 'polar rush' prominences, the solar minimum must be re-dated to before 1733 May. Furthermore, the latitudes of at least two of the prominences reported by Wassenius enable their classification as quiescent prominences, suggesting the presence of a polarity inversion line in the polar regions in early 1733.

The U.S. National Science Foundation’s (NSF) Global Oscillation Network Group (GONG) began observing the Sun in 1995, focusing initially on helioseismic full-disk Doppler measurements over a planned three-year period. The network utilizes six strategically located sites around the world. GONG has undergone significant upgrades and remains operational after three decades. It continues to deliver near-continuous, high-cadence observations of various solar observables. These observations have been pivotal in advancing numerous scientific and space-weather studies, unveiling details about the Sun’s internal dynamics, structural properties, and surface magnetic activity. We provide an overview of the instrumentation, data products, and key findings derived from GONG observations, highlighting their contributions to understanding the solar interior and its interactions with the atmosphere and heliosphere. Additionally, we will discuss the implications of these findings for theoretical models, summarize space-weather applications, and outline the network’s future plans.

The 2024 April 8 total solar eclipse provides a unique opportunity to study the solar corona. This work presents our simulations of the solar corona at the time of the eclipse based on magnetohydrodynamic modeling performed with the Alfvén Wave Solar atmosphere Model in the Space Weather Modeling Framework, developed at the University of Michigan. We performed multiple simulations based on photospheric magnetic maps from four sources, i.e., ADAPT-GONG, Lockheed Martin ESFAM-HMI, HipFT-HMI, and NSO-NRT-HMI maps. Our study focuses on how differences in the magnetic field maps affect the coronal magnetic field structure and coronal heating properties in the simulation. The synthesized observables show remarkable differences due to the distinct magnetic coronal topologies, which stem from the different local magnetic flux distributions. We analyze the properties of the open magnetic flux regions of the models. We also study the coronal heating rate in the models. The total volume integrated heating rate yields a difference of 20% across the models. The results also show that the differential emission measure in the high-temperature regions is sensitive to the magnetic field maps. Our findings underscore the importance of comprehensive photospheric magnetic field data in improving future solar coronal models.

The Solar Close Observations and Proximity Experiments(SCOPE)mission will send a spacecraft into the solar atmosphere at a low altitude of just 5 R☉ from the solar center.It aims to elucidate the mechanisms behind solar eruptions and coronal heating,and to directly measure the coronal magnetic field.The mission will perform in situ measurements of the current sheet between coronal mass ejections and their associated solar flares,and energetic particles produced by either reconnection or fast-mode shocks driven by coronal mass ejections.This will help to resolve the nature of reconnections in current sheets,and energetic particle acceleration regions.To investigate coronal heating,the mission will observe nano-flares on scales smaller than 70 km in the solar corona and regions smaller than 40 km in the photosphere,where magnetohydrodynamic waves originate.To study solar wind acceleration mechanisms,the mission will also track the process of ion charge-state freezing in the solar wind.A key achievement will be the observation of the coronal magnetic field at unprecedented proximity to the solar photosphere.The polar regions will also be observed at close range,and the inner edge of the solar system dust disk may be identified for the first time.This work presents the detailed background,science,and mission concept of SCOPE and discusses how we aim to address the questions mentioned above.

Long-term reconstructions of sunspot number (SSN) and group number (GN) often tacitly assume that the basic characteristics of solar activity remain unchanged even over long times, e.g., that the number of sunspots and the number of sunspot groups now and, say, 100 or 500 years ago have the same relation. However, this assumption needs examination, especially as the long-term homogeneity between sunspots and several other solar activity parameters has recently been challenged (Mursula et al. 2024). Here we use long series of sunspot observations to study if and how the number of sunspots per group varies at different time scales. We use observations from the Kislovodsk Mountain Astronomical Station (KMAS) and the Solar Observing Optical Network (SOON) to create a reference series for the overlapping period of 1982 – 2016. We then scale other historical data to this reference series in order to have a unified time series for 1749 – 2024. We find that the yearly mean number of sunspots per group varies between about 2 and 10 (average of 6.8), closely in phase with the solar cycle over the whole 270-year time interval. We also find that the number of sunspots per group depicts a very similar secular variation as the sunspot number.

In this work we compare the full-disk line-of-sight magnetic field measurements of the Helioseismic and Magnetic Imager (HMI) with the magnetograms from multiple sources including the Global Oscillation Network Group (GONG), the Michelson Doppler Imager (MDI), the Synoptic Optical Long-term Investigations of the Sun (SOLIS) instrument, the Mount Wilson Observatory (MWO), and the Wilcox Solar Observatory (WSO). A scaling factor is then derived that matches the magnetograms from these instruments to those of HMI. The scaling factors are 1.438± 0.000 for GONG, 0.701± 0.001 for MDI, 1.038± 0.001 for SOLIS, 2.795± 0.002 for MWO, and 3.582± 0.006 for WSO. This scaling factor varies with the center-to-limb distance and field strength. Distortion maps for these instruments are also determined using the HMI data as a reference.

We compare two candidate nonlinearities for regulating the solar cycle within the Babcock-Leighton paradigm: tilt quenching (whereby the tilt of active regions is reduced in stronger cycles) and latitude quenching (whereby flux emerges at higher latitudes in stronger solar cycles). Digitized historical observations are used to build a database of individual magnetic plage regions from 1923 to 1985. The regions are selected by thresholding in Ca ii K synoptic maps, with polarities constrained using Mount Wilson Observatory sunspot measurements. The resulting data show weak evidence for tilt quenching, but much stronger evidence for latitude quenching. Further, we use proxy observations of the polar field from faculae to construct a best-fit surface flux transport model driven by our database of emerging regions. A better fit is obtained when the sunspot measurements are used, compared to a reference model where all polarities are filled using Hale's Law. The optimization suggests clearly that the "dynamo effectivity range" of the Sun during this period should be less than 10 degrees; this is also consistent with latitude quenching being dominant over tilt quenching.

The Sun experienced a period of unprecedented activity during solar cycle 19 in 1950s and 1960s, now called the Modern Maximum (MM). The decay of the MM has changed the Sun, the heliosphere and the planetary environments in many ways. However, this decay may not have proceeded synchronously in all solar parameters. One of the related key issues is if the relation between the two long parameters of solar activity, sunspot number and the solar 10.7cm radio flux, has remained the same during this decay. While a number of studies agree that this relation has indeed changed, no consensus on its validity exists. A recent study argues that there is an inhomogeneity in the 10.7cm radio flux in 1980, which led to a step-like jump ("1980 jump") in this relation. If true, this would imply that the 10.7cm radio flux is ineligible for long-term studies, which would seriously impede versatile studies of the Sun during the MM. Here we use the 10.7cm radio flux and four other, independent radio flux measurements, the sunspot number, the MgII index and the number of solar active regions in order to study their mutual relations during the decay of MM. We find that all the five radio fluxes depict an increasing trend with respect to the sunspot number from 1970s to 2010s. This excludes the interpretation of the "1980 jump" as an inhomogeneity in the 10.7cm flux, and re-establishes the 10.7cm flux as a reliable and homogeneous long-term measure of solar activity. We find that the fluxes of longer radio waves increased with respect to the shorter waves, which implies a long-term change in the solar spectrum at radio frequencies. We also find that both the MgII index and the number of active regions increased with respect to the sunspot number, indicating a difference in the long-term evolution in chromospheric and photospheric parameters. Our results give evidence for important structural changes in solar magnetic fields and solar atmosphere during the decay of the MM when solar activity weakened considerably. These changes have not been reliably documented so far. We also emphasise that the changing relation between the different (e.g. photospheric and chromospheric) parameters should be taken into account when using sunspot number or any single parameter in long-term studies of solar activity.

A strong correlation between the intensity of chromospheric emissions and the (unsigned) photospheric magnetic field strength has been established in several studies. These studies have typically been based on line-of-sight (LOS) observations of the magnetic field, while measurements of the full 3D magnetic vector, which provide the true field strength and the orientation of magnetic field line, have not been studied in this context. Thus, the possible effect of magnetic field inclination on chromospheric emissions has remained hidden so far. We study here how the inclination of the photospheric magnetic field, as measured by the full 3D magnetic vector from the Solar Dynamics Observatory (SDO) Helioseismic Magnetic Imager (HMI), affects the FUV emission at around 1600 Å from SDO Atmospheric Imaging Assembly (AIA). We analyze 1168 co-temporal observations by the two instruments from 2014 to 2017. We focus on magnetically active regions outside the sunspots (e.g., plages and network) close to the solar disk center.We find that the AIA 1600 Å emission typically decreases with increasing (more horizontal) inclination. For all inclinations, AIA 1600 Å emission increases with increasing magnetic field to a maximum emission and then slowly decreases for larger field strengths. Maximum emission and the related field strength decrease with inclination. Above this field strength of maximum emission, the emission decouples from the field strength and is mainly governed by inclination. For fixed AIA emission level the associated magnetic field strength decreases with inclination. The difference in the median magnetic field strength can be more than 200 G (about a factor of two) for the same emission level between almost radial (γLoc ≈ 15°) and nearly-horizontal (γLoc ≈ 60°) fields.AIA 1600 Å emission and magnetic field inclination are bimodally distributed with constant magnetic field strength below 1000 G. One population has a high AIA emission and a roughly vertical magnetic field, the other a lower emission and a horizontal field. The population consisting of less bright pixels with horizontal field is typically found at the border of active region, while the population with bright pixels with a vertical field occupy the bulk of an active region.Our results show that the chromospheric FUV emission at around 1600 Å is strongly influenced by the inclination of the magnetic field. These results are important, for example, for models aiming to reconstruct the solar spectral irradiance or the past solar activity based on chromospheric emissions. These models would be more accurate if they took into account the effect of inclination of the magnetic field on FUV emissivity.

The plethora of space-borne and ground-based observatories has provided astrophysicists with an unprecedented volume of data, which can only be processed at scale using advanced computing algorithms. Consequently, ensuring the quality of data fed into machine learning (ML) models is critical. The Hα observations from the GONG network represent one such data stream, producing several observations per minute, 24/7, since 2010. In this study, we introduce a lightweight (non-ML) anomaly-detection algorithm, called H-Alpha Anomalyzer, designed to identify anomalous observations based on user-defined criteria. Unlike many black-box algorithms, our approach highlights exactly which regions triggered the anomaly flag and quantifies the corresponding anomaly likelihood. For our comparative analysis, we also created and released a dataset of 2,000 observations, equally divided between anomalous and non-anomalous cases. Our results demonstrate that the proposed model not only outperforms existing methods but also provides explainability, enabling qualitative evaluation by domain experts.

In 2024 May, the Sun exhibited intense magnetic activity, marked by numerous high-intensity flares resulting from the interaction and merging of NOAA active regions (ARs) 13664 and 13668 in the southern hemisphere and AR 13663 in the northern hemisphere. Notably, AR 13664 displayed an extended lifetime, remaining visible after a full solar rotation and continuing to produce significant flaring activity. In this study, we investigate the evolution of subphotospheric plasma flows associated with these ARs during their disk passage using ring-diagram analysis of Solar Dynamics Observatory/Helioseismic and Magnetic Imager Dopplergrams. We analyze flow divergence, vorticity, and kinetic helicity across depths from the surface to 25 Mm, revealing pronounced temporal and depth-dependent variations. Our observations indicate that the majority of flares occur on the days when the Normalized Helicity Gradient Variance, a measure of kinetic helicity spread, peaks or on the following day. Furthermore, we examine the relationship between subsurface flow dynamics and surface magnetic properties of these complex ARs to understand the interaction between them.

The Simplest Magnetograph is a novel alternative to the standard filter-based magnetograph for solar observations. The instrument exploits an asymmetry in Stokes profiles, which results in a broadband net polarization. The latter enables measurements of magnetic fields without resolving individual spectral lines. The instrument is designed to take broadband measurements of solar polarization. The chosen spectral band covers several photospheric lines with significant Lande g-factors. While the method can be used to measure a full vector magnetic field, in our initial application we concentrated on measurements of the longitudinal magnetic field. We discuss the feasibility, design, lab testing and validation, and solar observation measurements.

Intense solar eruptions occasionally trigger extreme geomagnetic storms, expand the boundaries of the auroral oval, and facilitate equatorward extensions of the auroral visibility. It is important to analyse such events, to better understand the extremity of space weather and its impact on the technological infrastructure of the modern civilization. However, unlike other extreme geomagnetic storms, little is known about the auroral activity associated with the extreme geomagnetic storm on 15/16 July 1959, the second largest geomagnetic storm in the space age. This study acquired and analysed two Chinese accounts and one Russian account of auroral visibility at low (>= 40 degrees) magnetic latitudes (MLATs). These records allowed us to conservatively reconstruct the equatorward boundaries of the auroral visibility and the auroral oval at 27.4 degrees MLAT and 35.4 degrees invariant latitude, respectively. Our analysis chronologically contextualized these auroral records slightly before the peak of the extreme geomagnetic storm. Moreover, their coloration indicates the excitations of, at least, nitrogen at 427.8 nm and oxygen at 557.7 nm at these low MLATs. Our results allow us to contextualize this extreme geomagnetic storm within other extreme events, based on the equatorward boundary of the auroral oval, thereby facilitating the improvement in existing empirical models for correlations of the auroral extension and the storm intensity.

We have developed a comprehensive catalog of the variable differential rotation measured near the solar photosphere. This catalog includes measurements of these flows obtained using several techniques: direct Doppler, granule tracking, magnetic pattern tracking, global helioseismology, as well as both time-distance and ring-diagram methods of local helioseismology. We highlight historical differential rotation measurements to provide context, and thereafter provide a detailed comparison of the MDI-HMI-GONG-Mt. Wilson overlap period (April 2010 – Jan 2011) and investigate the differences between velocities obtained from different techniques and attempt to explain discrepancies. A comparison of the rotation rate obtained by magnetic pattern tracking with the rotation rates obtained using local and global helioseismic techniques shows that magnetic pattern tracking measurements correspond to helioseismic flows located at a depth of 25 to 28 Mm. In addition, we show the torsional oscillation from Sunspot Cycles 23 and 24 and discuss properties that are consistent across measurement techniques. We find that acceleration derived from torsional oscillation is a better indicator of long-term trends in torsional oscillation compared to the residual velocity magnitude. Finally, this analysis will pave the way toward understanding systematic effects associated with various flow measurement techniques and enable more accurate determination of the global patterns of flows and their regular and irregular variations.

We advocate for the development of a ground-based network of robotic instruments provisionally called ngGONG to maintain critical observing capabilities for synoptic research in solar physics and for the operational space weather forecast. ngGONG will consist of 6 geographically-distributed stations, with longitudes and weather patterns selected to provide nearly continuous observations of the Sun. ngGONG instruments will include: spectropolarimeters for precise measurements of vector magnetic fields at multiple heights in the solar atmosphere; an instrument for line-of-sight Doppler velocity measurements required for studies of the solar interior and farside; rapid narrow-band images; sun-as-a-star instruments; and tunable Ha imager and limited coronagraph capabilities to monitor the violent ejecta of magnetized plasma from the Sun's atmosphere and determine coronal magnetic topologies and plasma properties. We will discuss the requirements for such an observing system, and present its conceptual design.

The relation between the intensity of chromospheric emissions and the photospheric magnetic field strength has been examined in several studies, but the effect of the magnetic field inclination on chromospheric emissions remains almost unexplored. We study how the inclination of the photospheric magnetic field, as measured by the full 3D magnetic vector from the Helioseismic and Magnetic Imager (HMI), affects the relationship between the magnetic field strength and the far-ultraviolet emission at around 1600 Å observed by the Atmospheric Imaging Assembly (AIA). We also study how these parameters change spatially close to the active region perimeter. We analyzed the mutual dependence of 1168 co-temporal AIA and HMI observations from 2014 to 2017. We focused on magnetically active regions outside sunspots (e.g., plages and network) close to the solar disk center. We studied how the AIA and HMI parameters change with distance from the active region perimeter. The AIA 1600 emission typically decreases with increasing (more horizontal) inclination. For all inclinations, AIA 1600 emission increases with increasing magnetic field strength until saturating at some peak intensity, which depends on the cosine of the inclination, with horizontal regions saturating at lower intensities. In addition, we find that activity clusters have a narrow boundary (< 2 arcseconds) in which the AIA 1600 intensity, magnetic field strength, and inclination distributions and relations differ significantly from those in the inner layers. This study demonstrates the significant effect that magnetic field inclination and activity cluster border regions have on chromospheric emissions. Although the observed effects are likely reduced in low-resolution observations where different regions are averaged together, a detailed study is needed to examine the emission–magnetic field relation at different resolutions.

Filaments are omnipresent features in the solar atmosphere. Their location, properties and time evolution can provide information about changes in solar activity and assist the operational space weather forecast. Therefore, filaments have to be identified in full disk images and their properties extracted from these images. Manual extraction is tedious and takes much time; extraction with morphological image processing tools produces a large number of false-positive detections. Automatic object detection, segmentation, and extraction in a reliable manner allows to process more data in a shorter time. The Chromospheric Telescope (ChroTel), Tenerife, Spain, the Global Oscillation Network Group (GONG), and the Kanzelh\"ohe Observatory (KSO), Austria, provide regular full-disk observations of the Sun in the core of the chromospheric H-alpha absorption line. We present a deep learning method that provides reliable extractions of filaments from H-alpha filtergrams. First, we train the object detection algorithm YOLOv5 with labeled filament data of ChroTel. We use the trained model to obtain bounding-boxes from the full GONG archive. In a second step, we apply a semi-supervised training approach, where we use the bounding boxes of filaments, to learn a pixel-wise classification of filaments with u-net. Here, we make use of the increased data set size to avoid overfitting of spurious artifacts from the generated training masks. Filaments are predicted with an accuracy of 92%. With the resulting filament segmentations, physical parameters such as the area or tilt angle can be easily determined and studied. This we demonstrate in one example, where we determine the rush-to-the pole for Solar Cycle 24 from the segmented GONG images. In a last step, we apply the filament detection to H-alpha observations from KSO which demonstrates the general applicability of our method to H-alpha filtergrams.

Long-term synoptic observations of the Sun are critical for advancing our understanding of Sun as an astrophysical object, understanding the solar irradiance and its role in solar-terrestrial climate, for developing predictive capabilities of solar eruptive phenomena and their impact on our home planet, and heliosphere in general, and as a data provider for the operational space weather forecast. We advocate for the development of a ground-based network of instruments provisionally called ngGONG to maintain critical observing capabilities for synoptic research in solar physics and for the operational space weather forecast.

Studies of stellar magnetic fields mostly rely on proxies derived from chromospheric lines, typically forming in the UV and shorter wavelengths and therefore accessible only from space based observatories. Even Ca II K or H observations, forming in regions accessible from the ground, are not always available. As a result, there is a crucial need to explore alternative activity proxies to overcome the limitations posed by observational constraints. Using sun-as-a-star observations acquired with the ISS at SOLIS we investigated the correlation between the Ca II K emission index and indices derived from the Hα 656.3 nm and Ca II 854.2 nm lines, which are well known chromospheric diagnostics. We found that both the core intensities and widths of the two lines are positively correlated with the Ca II K emission index (ρ ≳ 0.8), indicating their suitability as reliable indicators of magnetic activity, the width of the Hα line showing the highest correlation (ρ = 0.9). We also found that such correlations vary with the activity cycle. Specifically, during the analyzed cycle 24, the correlations with the Ca II K index varied 14% for the Hα width, 33% for the Hα core intensity, and doubled for the two Ca II 854.2 nm line indices. These results suggest that, among the investigated indices, the Hα width best traces magnetic activity. Results are discussed at the light of current knowledge of the formation heights of the two lines, and of spatially resolved solar observations.