Group Sunspot Number Research Articles

Reconstructing daily group sunspot numbers since the maunder minimum with objective inter-calibration algorithms

We present a new daily Group Sunspot Numbers (GSN) reconstruction from the Maunder minimum to Solar Cycle 24/25. This new reconstruction was obtained as a result of rectifying the raw and uncalibrated GSN database of Vaquero et al. (2016) and incorporating data from previously unconsidered GSN sources. Notably, a pivotal challenge encountered during the reconstruction of this GSN activity record, lay in the inter-calibration of different databases reported by multiple observers. The challenge is further compounded by data gaps that are particularly prevalent during the Maunder minimum. We have applied a novel methodology to solve these obstacles, entailing the calibration and standardization of all GSN databases relative to a reference observer. Our novel approach employs calibration techniques, ensuring standardized visual acuity between all individual observers. Moreover, our methodology accommodates variations in climatic, geographic, and instrumental conditions encountered by observers during their respective sunspot observations, thus establishing a uniform comparative framework. Furthermore, a comparative analysis was conducted with a total of four other different GSN reconstructions, revealing notable distinctions and resemblances that can be explained by the implementation of diverse methodologies in the respective GSN reconstruction processes and steps. In addition to the temporal differences found, both the wavelet and multi-cross wavelet analyses were carried out, which allowed a quantification of the differences and similarities in the spectral contents of various GSN records. The wavelet analysis results show the similarities and differences of the new GSN reconstruction with the inter-calibration algorithm when comparing it both temporally and spectrally with other GSN reconstructions. The periodicities of this analysis are 1.2, 5.5, 10.9, 20.6, 30.9, 58.4, and 119.8 years and 3.47, 5.4, 9.8, 27.8, 66.1, 117.8, and 160.5 days respectively.

Automatic sunspot detection through semantic and instance segmentation approaches

The solar influence on space weather and terrestrial environment is substantial. Strong geomagnetic storm activity can significantly affect astronauts in orbit, communications and GPS systems and disrupt Earth’s power distribution networks, making continuous monitoring and forecasting of solar activity vital. Sunspots are magnetic disturbances in the photosphere characterized by their dark appearance in the solar disk, being directly related to phenomena that contribute to these intense storms, namely solar flares and coronal mass ejections. This article lies at the intersection between solar surveillance and computer vision by applying state-of-the-art deep learning algorithms in the automatic detection of sunspots and sunspot groups. Based on two techniques, semantic segmentation and instance segmentation, two algorithms are implemented to tackle both purposes, U-Net and Mask R-CNN respectively. The ground-truth dataset was built from the available Debrecen Heliographic Observatory (DHO) space-borne sunspot catalogues from 2010 to 2014. The best U-Net implemented model presented a 74.2% IoU, surpassing the detection results evidenced by the Automated Solar Activity Prediction System (ASAP). The instance segmentation approach, a novelty application technique for sunspot group detection and still a challenging task in computer vision, achieved 51.7 bounding box AP and 78.6% accuracy in predicting the number of sunspot groups.

Evolutionary Relationship between Sunspot Groups and Soft X-Ray Flares over Solar Cycles 21–25

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.

Sunspot Observations at the Eimmart Observatory: Revision and Supplement

Digital images of sunspot drawings of the archives of Georg Christoph Eimmart stored at the National Library of Russia, St. Petersburg, are analyzed to obtain sunspot-group numbers and sunspot areas as well as heliographic positions. Overall, more than a hundred drawings were processed. The impact of drawing and reproduction uncertainties and the aims of historical observations are considered. The sunspot positions are compared to those reported by contemporary observers of the Maunder minimum. The restored sunspot-group numbers and latitudes are compared to those extracted by Hoyt and Schatten (Solar Phys. 179, 189, 1998) as well as Hayakawa et al. (Solar Phys. 296, 154, 2021b) and Hayakawa et al. (Astrophys. J. 909, 166, 2021d). The persistence of long-lived sunspots over several solar rotations is discussed.

Solar activity cycle 25: the first three years

We analyze features of current solar activity cycle 25 for the first three years of its development (2020–2022). Compared to cycle 24, the current cycle is shown to exceed the previous one in the number of sunspot groups (1.5 times), the number of flares (1.8 times), and the total flare index (1.5 times). We have found that distributions of sunspot groups during cycles 24 and 25 differ in maximum area. Solar cycle 25, unlike cycle 24, exhibits the most significant increase in the number of sunspot groups with areas up to 30 pmh and in the interval from 570 to 1000 pmh. In contrast to cycle 24, the degree of north-south asymmetry in cycle 25 is significantly reduced. This allows us to predict an increased height of cycle 25, as compared to cycle 24 (by 20–50 %), in accordance with the Gnevyshev—Ol rule, as well as the possible unimodal nature of the cycle.

25-й цикл солнечной активности: первые три года

We analyze features of current solar activity cycle 25 for the first three years of its development (2020–2022). Compared to cycle 24, the current cycle is shown to exceed the previous one in the number of sunspot groups (1.5 times), the number of flares (1.8 times), and the total flare index (1.5 times). We have found that distributions of sunspot groups during cycles 24 and 25 differ in maximum area. Solar cycle 25, unlike cycle 24, exhibits the most significant increase in the number of sunspot groups with areas up to 30 pmh and in the interval from 570 to 1000 pmh. In contrast to cycle 24, the degree of north-south asymmetry in cycle 25 is significantly reduced. This allows us to predict an increased height of cycle 25, as compared to cycle 24 (by 20–50 %), in accordance with the Gnevyshev—Ol rule, as well as the possible unimodal nature of the cycle.

Hemispheric Sunspot Numbers from the Astronomical Observatory of the University of Valencia (1940 – 1956)

In this work, we present hemispheric sunspot number counts made in the Astronomical Observatory of the University of Valencia for the period 1940 – 1956. We found these data in a secondary source (the bulletins of the Madrid Astronomical Observatory). We digitized the hemispheric sunspot number and group number recorded by the observers. Unfortunately, the sunspot positions or areas were not published in this secondary source. A machine-readable version of the dataset is available as supplementary material. We compare the Valencia hemispheric series with others from other observatories, such as the Madrid Astronomical Observatory, finding similar hemispheric behaviors. We also computed the asymmetry index from the hemispheric data. We found that the northern hemisphere was predominant in our study period (declining phase of Solar Cycle 17, Solar Cycle 18, and rising phase of Solar Cycle 19), and the southern hemisphere was leading. These results agree with those obtained in previous studies. Therefore, the Valencia hemispheric records are one more piece of information to know the behavior of the northern and southern hemispheres during the 20th century.

Karl von Lindener’s sunspot observations during 1800–1827: Another long-term dataset for the Dalton Minimum

On a centennial timescale, solar activity oscillates quasi-periodically and also tends to occasionally get into a low-activity period. The Dalton Minimum (circa 1790s–1820s) was one of such low-activity periods that had been captured in telescopic sunspot observations. However, it has been challenging to analyse the Dalton Minimum, as contemporary source records remained mostly unpublished and almost inaccessible to the scientific community. Recent studies have established reliable datasets for sunspot group number, sunspot number, and sunspot positions. This study further analyzes independent Silesian sunspot observations from 1800 to 1827 in a manuscript from the Library of Wrocław University (Ms AKC.1985/15) and complements it with the metadata for the observer: Karl Christian Reinhold von Lindener. We identified 547 days of sunspot observations in these records and derived the sunspot group number, individual sunspot number, and sunspot positions between 1800 and 1827. The results of this study have significantly revised von Lindener’s sunspot group number, which was previously known for only 517 days in scientific databases, and removed contamination from general descriptions. Using our results, we extended investigations into individual sunspot counts and derived their positions. In our analysis, we locate von Lindener’s sunspot positions in both solar hemispheres and contrast the Dalton Minimum with the Maunder Minimum, adding further independent credits to the previous results for Derfflinger’s and Prantner’s datasets. Sunspot positions are also slightly biased towards the northern solar hemisphere in early Solar Cycle 6 (1812–1813). The high-latitude sunspot positions indicate the onset of Solar Cycle 7 as early as June 1822.

An Overview of Sunspot Observations in 1727–1748

Solar activity generally exhibits cyclic behavior in terms of sunspot group number and sunspot positions every ≈11 yr. These sunspot data have therefore played key roles in numerous analyses of solar–terrestrial physics. However, their reconstructions prior to the 1830s have remained controversial and included significant data gaps, especially from the 1720s to the 1740s. Therefore, this study reviewed contemporary sunspot observations for 1727–1748 to add several forgotten records by Van Coesfelt in 1728–1729, Dûclos in 1736, Martin in 1737, and Cassini and Maraldi in 1748. On the basis of these records, this study revised the sunspot group number and newly derived the sunspot positions in this interval. The results show clearer solar cycles in sunspot group number than those of previous studies and indicate regular solar cycles with limited hemispheric asymmetry over Solar Cycles 0 to −2. The sunspot positions also show sunspot groups mostly at heliographic latitude φ fulfilling ∣φ∣ < 35° in both solar hemispheres, with slight equatorward motions. Furthermore, the solar minima between Solar Cycles −2 and −1 and between Solar Cycles −1 and 0 have been located around 1733.5 ± 0.5 and 1743 ± 0.5, indicating cycle lengths of 11.7 ± 0.5 yr and 10.0 ± 1.0 yr, respectively. Our results provide a chronological missing link between the Maunder Minimum and the regular solar cycles observed since Staudach’s observations from 1749 onward. This lets us better understand the transition of solar activity from the grand minimum to the regular solar cycles.

Iterative Construction of the Optimal Sunspot-Number Series

The relative number of sunspots represents the longest evidence describing the level of solar activity. As such, its use goes beyond solar physics, e.g. towards climate research. The construction of a single representative series is a delicate task which involves a combination of observation of many observers. We propose a new iterative algorithm that allows to construct a target series of relative sunspot number of a hypothetical stable observer by optimally combining series obtained by many observers. We show that our methodology provides us with results that are comparable with recent reconstructions of both sunspot number and group number. Furthermore, the methodology accounts for the possible non-solar changes of observers' time series such as gradually changing observing conditions or slow change in the observers vision. It also provides us with reconstruction uncertainties. We apply the methodology to a limited sample of observations by \v{C}ESLOPOL network and discuss its properties and limitations.

A novel Bayesian approach for disentangling solar and geomagnetic field influences on the radionuclide production rates

Cosmogenic radionuclide records (e.g., 10Be and 14C) contain information on past geomagnetic dipole moment and solar activity changes. Disentangling these signals is challenging, but can be achieved by using independent reconstructions of the geomagnetic dipole moment. Consequently, solar activity reconstructions are directly influenced by the dipole moment uncertainties. Alternatively, the known differences in the rates of change of these two processes can be utilized to separate the signals in the radionuclide data. Previously, frequency filters have been used to separate the effects of the two processes based on the assumption that millennial-scale variations in the radionuclide records are dominated by geomagnetic dipole moment variations, while decadal-to-centennial variations can be attributed to solar activity variations. However, the influences of the two processes likely overlap on centennial timescales and possibly millennial timescales as well, making a simple frequency cut problematic. Here, we present a new Bayesian model that utilizes the knowledge of solar and geomagnetic field variability to reconstruct both solar activity and geomagnetic dipole moment from the radionuclide data at the same time. This method allows for the possibility that solar activity and geomagnetic dipole moment exhibit variations on overlapping timescales. The model was tested and evaluated using synthetic data with realistic noise and then used to reconstruct solar activity and the geomagnetic dipole moment from the 14C production record over the last two millennia. The results agree with reconstructions based on independent geomagnetic field models and with solar activity inferred from the Group Sunspot number. Our Bayesian model also has the potential to be developed further by including additional confounding factors, such as climate influences on the radionuclide records.Graphical

Relationship between the Sunspot Number and Active Day Fraction: An Application for the Maunder Minimum

Long-term solar activity can be studied using several parameters. Some of the most used are based on the sunspot counting. The active day fraction (ADF) is the simplest index derived from this counting. It is reliable in periods of low solar activity such as the Maunder minimum (MM). In this work, we study the relationship between the ADF and the sunspot number. We have obtained that the optimal fit of that relationship is an exponential function whose exponent is a degree 3 polynomial including all data except those with ADF equal to 100%. Then, we use that fit to estimate the sunspot number during the MM from the ADF calculated from the most recent sunspot group number database. Our estimations of the annual sunspot numbers are below 15, except that for 1656, which is 40.8, whereas our estimations of the triennial sunspot numbers are below 10 from 1648 to 1714. We have found peaks of the solar cycle in the middle of the 1650s, 1670s, 1680s, and 1700s but no clear evidence of solar cycle in the 1660s and 1690s, likely due to the scarcity of the available data. Our results agree with previous works obtaining values significantly higher than those of the group sunspot number derived by Hoyt and Schatten in 1998 but still fully compatible with a grand minimum period.

Long-term North–South Asymmetry of the Heliospheric Current Sheet

Abstract In this paper, we evaluate the heliospheric current sheet (HCS) north–south asymmetry using the ecliptical sector structure of the interplanetary magnetic field (IMF) reconstructed since the second half of the 19th century. During the last five solar cycles, the inferred IMF polarities fairly reproduce the observed dominance of the sectors with the polarity of the northern solar hemisphere, i.e., the prolonged southward shift of the HCS. For the presatellite era, we found that the northward shift of the HCS was more common in cycles 10, 15, and 17–19, and the southward HCS shift was more common in cycles 9, 11–14, and 16. We also analyzed the north–south asymmetry in sunspot group numbers since 1749 and found that the northern hemisphere dominated in cycles 2–3, 7–9, and 15–20, and the southern hemisphere activity was stronger in cycles 4, 9–14, and 21–24. Moreover, other solar phenomena bear similar long-term asymmetry variations. The regularity of these variations is not clear. According to the available proxies of the solar data, the dominance of the northern hemisphere is found in the ascending phase of the secular solar cycle, and the dominance of the southern hemisphere coincides with the descending phase.

Revisiting Christoph Scheiner’s Sunspot Records: A New Perspective on Solar Activity of the Early Telescopic Era

Abstract Christoph Scheiner was one of the most outstanding astronomers in the history of sunspot observations. His book, Rosa Ursina, is the reference work regarding the study of the earliest sunspot records. The sunspot observations compiled by Scheiner in Rosa Ursina and Prodomus, including records made by other observers, forms one of the main references of the observations known for that period—particularly around the 1620s. Thus, his work is crucial to determine the solar activity level of the first solar cycles of the telescopic era. The number of sunspot groups recorded in Scheiner’s documentary sources has been included in the existing sunspot group number databases. However, we have detected significant errors in the number of groups currently assigned to Scheiner’s records. In this work, we reanalyze the information in Scheiner’s source documents. Consequently, the standard 11 yr solar cycle shape for the second solar cycle of the telescopic era, which is not clear in previous studies, now becomes evident. In addition, the highest daily number of groups recorded during this cycle (eight groups) is 20% less than in the one included in the existing sunspot group number databases. Using the hypergeometrical probability distribution, we find that solar minima in 2008–2009 and 2018–2019 are comparable to the most probable solar activity level of the minimum around 1632. In particular, the estimated lower limit for the solar activity in 1632 is even comparable with the solar activity level in 2008 and 2018.

Group Sunspot Numbers: A New Reconstruction of Sunspot Activity Variations from Historical Sunspot Records Using Algorithms from Machine Learning

Group Sunspot Numbers: A New Reconstruction of Sunspot Activity Variations from Historical Sunspot Records Using Algorithms from Machine Learning

Number of Sunspot Groups and Individual Sunspots Recorded by Tevel for the Period 1816–1836 in the Dalton Minimum

Abstract Cornelis Tevel made sunspot observations during the period 1816–1836, including the Dalton Minimum. In this work, the first revision of these observations since Wolf incorporated them into his database is presented. On the one hand, the number of individual sunspots from Tevel’s drawings was counted. This is of special interest for the sunspot number reconstruction because this kind of information is not as common in historical sunspot records as the number of groups. Thus, Tevel could be considered for the future reconstruction of the sunspot number index. On the other hand, the number of groups counted according to modern sunspot group classifications finding significant misinterpretations with the number of groups assigned to Tevel in the existing databases. Tevel was a relevant sunspot observer in the Dalton Minimum. In fact, he was the observer with the highest number of groups observed in Solar Cycles 6 and 7 according to the existing sunspot group number databases. According to the raw group number recount in this work, the maximum amplitudes for Solar Cycles 6 and 7 are, respectively, 27% and 7% lower than those previously determined. Moreover, Solar Cycle 6 is the weakest solar cycle since the Maunder Minimum after applying these new counts. Group counts from Tevel’s observations were compared with those from relevant contemporary astronomers, demonstrating that Schwabe and Tevel systematically recorded a higher number of groups than Flaugergues and Derfflinger. In addition, sunspot areas and positions recorded by Tevel should be used with caution for scientific purposes.

Analyses of Early Sunspot Records by Jean Tarde (1615\u2009\u2013\u20091617) and Jan Smogulecki (1621\u2009\u2013\u20091625)

Jean Tarde and Jan Smogulecki carried out sunspot observations in the 1610s and 1620s at the dawn of the telescopic era. We analysed their original observational records to revise their sunspot-group numbers in the existing database. In this study, we provide a new counting as a basis for future scientific discussions. Furthermore, we compared Smogulecki’s sunspot observations with those of Scheiner and Schönberger on the same observation days. We also detected a big sunspot group on 2 – 3 February 1622 in Smogulecki’s sunspot drawings and estimated its area to be approximately 1600 millionths of the solar disc. In addition, we measured the sunspot positions in Tarde’s and Smogulecki’s sunspot drawings to construct a butterfly diagram for this early period.

Stephan Prantner’s Sunspot Observations during the Dalton Minimum

Abstract In addition to regular Schwabe cycles (≈11 yr), solar variability also shows longer periods of enhanced or reduced activity. Of these, reconstructions of the Dalton Minimum provide controversial sunspot group numbers and limited sunspot positions, partially due to limited source record accessibility. In this context, we analyzed Stephan Prantner’s sunspot observations spanning from 1804 to 1844, the values of which had only been known through estimates despite their notable chronological coverage during the Dalton Minimum. We identified his original manuscript in Stiftsarchiv Wilten, near Innsbruck (Austria). We reviewed his biography (1782–1873) and located his observational sites at Wilten and Waidring, which housed the principal telescopes for his early and late observations: a 3.5 inch astronomical telescope and a Reichenbach 4 foot achromatic erecting telescope, respectively. We identified 215 days of datable sunspot observations, which is almost twice as much data as his estimated data in the existing databases (=115 days). In Prantner’s records, we counted up to seven to nine sunspot groups per day and measured sunspot positions, which show their distributions in both solar hemispheres. These results strikingly emphasize the difference between the Dalton Minimum and the Maunder Minimum as well as the similarity between the Dalton Minimum and the weak solar cycles in the modern observations.

Average annual total sunspot area in the last 410 yr: the most probable values and limits of their uncertainties

ABSTRACT The aim of this work is to create a long (410-yr) series of average annual total sunspot areas AR – a physically based index of sunspot activity. We use telescopic observations of the AR index in 1832–1868 and 1875–2020, as well as the relationship between AR and long series of sunspot indices SN (international sunspot numbers, version 2.0) and sunspot groups GN (Svalgaard and Schatten’s version of group sunspot numbers). The Royal Greenwich Observatory series after 1976 is extended by the Kislovodsk Mountain Astronomical Station data. When reconstructing AR from SN, it is taken into account that the function AR = f(SN) has a non-linear systematic character and uncertainty associated with the heterogeneity of these indices. Therefore, in addition to modelling the most probable AR values, predictive limits of reconstruction uncertainty are determined. In the interval 1610–1699 we carried out the reconstruction on the basis of the GN series using the previously proposed decomposition in pseudo-phase space (DPS) method. The resulting series NO21y is freely available online. We show that for this series the empirical Gnevyshev–Ohl rule and Waldmeier effect are fulfilled. Wavelet analysis reveals periodicities of 8.4–13.8 yr for the main cycle (with a sharp decrease of the period before the global Maunder and Dalton minima) and a two-component Gleissberg cycle with typical periods of 50–60 and 90–110 yr.

A Reanalysis of the Number of Sunspot Groups Recorded by Pierre Gassendi in the Cycle Before the Maunder Minimum

Pierre Gassendi (1592 – 1655) carried out sunspot observations during the 1630s. This period is particularly interesting because it occurred a few years before the Maunder Minimum. Gassendi was the second most active sunspot observer in that decade, surpassed only by Christoph Scheiner. Moreover, the sunspot observations made by Gassendi are of interest because most of them were made in days when no other sunspot observations were available. We have analyzed the sunspot drawings included in his Complete Works (Opera Omnia) and the textual reports with sunspot information after translating them from the original Latin. Thus, we have detected mistakes in previous works that studied these observations, such as incorrect dates and incorrect daily number of groups assigned to Gassendi. In addition, we found some observation days recorded by Gassendi not included in the current sunspot-group-number database. In this work, we provide a new account of the number of groups recorded by Gassendi in addition to the translations of his relevant texts on observed sunspots. Our main objective is to include these new recounts into the current sunspot-group-number database, which is the basis of the group-number index, to have a more accurate knowledge of solar activity of the third solar cycle observed with the aid of the telescope.