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Solar activity is an important driver of long-term climate trends and must be accounted for in climate models. Unfortunately, direct measurements of this quantity over long periods do not exist. The only observation related to solar activity whose records reach back to the seventeenth century are sunspots. Surprisingly, determining the number of sunspots consistently over time has remained until today a challenging statistical problem. It arises from the need of consolidating data from multiple observing stations around the world in a context of low signal-to-noise ratios, non-stationarity, missing data, non-standard distributions and errors of different kind. The data from some stations experience therefore severe and various deviations over time. In this paper, we apply a systematic statistical approach for monitoring these complex and important series. It consists of three steps essential for successful treatment of the data: smoothing on multiple time-scales, monitoring using block bootstrap calibrated CUSUM charts and classifying of out-of-control situations by support vector techniques. This approach allows us to detect a wide range of anomalies (such as sudden jumps or more progressive drifts), unseen in previous analyses. It helps us to identify the causes of major deviations, which are often observer or equipment related. Their detection and identification will contribute to improve future observations. Their elimination or correction in past data will lead to a more precise reconstruction of the world reference index for solar activity: the International Sunspot Number.

The $F_{10.7cm}$ radio flux and the Sunspot Number are the most widely used long-term indices of solar activity. They are strongly correlated, which led to the publication of many proxy relations allowing to convert one index onto the other. However, those existing proxies show significant disagreements, in particular at low solar activity. Our aim is to bring a global clarification of those many issues. We compute new polynomial regressions up to degree 4, in order to obtain a more accurate proxy. We also study the role of temporal averaging on the regression, and we investigate the issue of the all-quiet $F_{10.7}$ background flux. Finally, we check for any change in the $F_{10.7}$ -- sunspot number relation over the entire period 1947 -- 2015. We find that, with a $4^{th}$-degree polynomial, we obtain a more accurate proxy relation than all previous published ones, and we derive a formula giving standard errors. The relation is different for daily, monthly and yearly mean values, and it proves to be fully linear for raw non-averaged daily data. By a simple two-component model for daily values, we show how temporal averaging leads to non-linear proxy relations. We also show that the quiet-Sun $F_{10.7}$ background is not absolute and actually depends on the duration of the spotless periods. Finally, we find that the $F_{10.7cm}$ time series is inhomogeneous, with an abrupt 10.5% upward jump occurring between 1980 and 1981. Our new proxy relations show the importance of temporal scale for choosing the appropriate proxy and the $F_{10.7}$ quiet-Sun background level. From historical evidence, we conclude that the 1981 jump is most likely due to a unique change in the $F_{10.7}$ scientific team and the data processing, and that the newly re-calibrated sunspot number (version2) will probably provide the only possible reference to correct this inhomogeneity., 51 pages, 27 figures, 10 tables

The recalibration of the sunspot number series, the primary long-term record of the solar cycle, requires the recovery of the entire collection of raw sunspot counts collected by the Zurich Observatory for the production of this index between 1849 and 1980. Here, we report about the major progresses accomplished recently in the construction of this global digital sunspot number database, and we derive global statistics of all the individual observers and professional observatories who provided sunspot data over more than 130 years. First, we can announce the full recovery of long-lost source-data tables covering the last 34 years between 1945 and 1979, and we describe the unique information available in those tables. We then also retrace the evolution of the core observing team in Zurich and of the auxiliary stations. In 1947, we find a major disruption in the composition of both the Zurich team and the international network of auxiliary stations. This sharp transition is unique in the history of the Zurich Observatory and coincides with the main scale-jump found in the original Zurich sunspot number series, the so-called "Waldmeier" jump. This adds key historical evidence explaining why methodological changes introduced progressively in the early $20^{th}$ century could play a role precisely at that time. We conclude on the remaining steps needed to fully complete this new sunspot data resource., Comment: 22 pages, 9 figures

We will present the international effort that led to the first-ever revision of the Sunspot and Group Numbers (let us call them "the Sunspot Series"), a well-known series that had never been put into question since its creation by Rudolf Wolf in 1849. We will review the process and its challenges (ISSI review paper, https://www.issibern.ch/teams/sunspotnoser/). At this point in time, the number of Group Number series available is still based on different techniques that enable the stitching of datasets of various quality over wildly different periods, and although they present an undeniable improvement over the original Group Number from Hayt & Schatten (1998), the resulting series remain to be extensively tested in order to be clearly validated by the scientific community.Since this first revision in 2015 (Solar Physics Topical Issue, 2016), the Sunspot Series have become living datasets that require constant monitoring since more source data are being recovered regularly (Arlt & Vaquero, 2020). With the team from ISSI, we are currently driving a large effort to gather raw data from all around the world. At the Royal Observatory of Belgium, within the WDC-SILSO (https://wwwbis.sidc.be/silso/) where the original Mittheilungen have been digitized (2017-2019) we also have 2 PhD students working on stitching historical and modern sunspot numbers and evaluating the quality of the reconstructed series through advanced statistical techniques.After this review of previous efforts, we will focus more specifically on the reconstruction of the International Sunspot Number from raw sunspot data (Bhattacharya et al., 2021, 2022, FARSUN belgian project) end present the potential impact of these revisions on end users (e.g. F10.7, Clette, 2021).

The sun’s magnetic field drives the 11-year solar cycle, and predicting its strength has practical importance for many space weather applications. Previous studies have shown that analysing the solar activity of the two hemispheres separately instead of the full sun can provide more detailed information on the activity evolution. However, the existing Hemispheric Sunspot Number data series (1945 onwards) was too short for meaningful solar cycle predictions. Based on a newly created hemispheric sunspot number catalogue for the time range 1874-2020 (Veronig et al. 2021, http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/652/A56) that is compatible with the International Sunspot Number from World Data Centre SILSO, we investigate the evolution of the solar cycle for the two hemispheres and develop a novel method for predicting the solar cycle amplitude. We demonstrate a steady relationship between the maximal growth rate of activity in the ascending phase of a cycle and its subsequent amplitude and form a 3rd order regression for the predictions. Testing this method for cycles 12-24, we show that the forecast made by the sum of the maximal growth rate from the North and South Hemispheric Sunspot number is more accurate than the same forecast from the Total Sunspot Number: The rms error of predictions is smaller by 27%, the correlation coefficient r is higher by 11% on average reaching values in the range r = 0.8-0.9 depending of the smoothing window of the monthly mean data. These findings demonstrate that empirical solar cycle prediction methods can be enhanced by investigating the solar cycle dynamics in terms of the hemispheric sunspot numbers, which is a strong argument supporting regular monitoring, recording, and analysing solar activity separately for the two hemispheres.

The focus of this article is a re-count of Richard Carrington’s original sunspot observations from his book drawings (Carrington in Observations of the Spots on the Sun from November 9, 1853, to March 24, 1861 Made at Redhill, Williams and Norgate, London, 1863) by an observer from the World Data Center-SILSO (WDC-SILSO, http://www.sidc.be/silso/home) network, Thomas H. Teague (UK). This modern re-count will enable the recomputation of the entire Sunspot Number series in a way Carrington’s original counts (Casas and Vaquero in Solar Phys. 289(1), 79, 2014) did not. Here we present comparison studies of the new re-counted series with contemporary observations, new data extracted from the Journals of the Zürich Observatory and other sources of Carrington’s own observations and conclude that Carrington’s group counting is very close to the modern way of counting while his method for counting individual spots lags significantly behind modern counts. We also test the quality and robustness of the new recount with methods developed in Mathieu et al. (Astrophys. J. 886(1), 7, 2019)., SCOPUS: ar.j, info:eu-repo/semantics/published

Forecasting the solar cycle amplitude is important for a better understanding of the solar dynamo as well as for many space weather applications. We demonstrated a steady relationship between the maximal growth rate of sunspot activity in the ascending phase of a cycle and the subsequent cycle amplitude on the basis of four data sets of solar activity indices: total sunspot numbers, hemispheric sunspot numbers from the new catalogue from 1874 onwards, total sunspot areas, and hemispheric sunspot areas. For all the data sets, a linear regression based on the maximal growth rate precursor shows a significant correlation. Validation of predictions for cycles 1-24 shows high correlations between the true and predicted cycle amplitudes reaching r = 0.93 for the total sunspot numbers. The lead time of the predictions varies from 2 to 49 months, with a mean value of 21 months. Furthermore, we demonstrated that the sum of maximal growth rate indicators determined separately for the north and the south hemispheric sunspot numbers provides more accurate predictions than that using total sunspot numbers. The advantages reach 27% and 11% on average in terms of rms and correlation coefficient, respectively. The superior performance is also confirmed with hemispheric sunspot areas with respect to total sunspot areas. The maximal growth rate of sunspot activity in the ascending phase of a solar cycle serves as a reliable precursor of the subsequent cycle amplitude. Furthermore, our findings provide a strong foundation for supporting regular monitoring, recording, and predictions of solar activity with hemispheric sunspot data, which capture the asymmetric behaviour of the solar activity and solar magnetic field and enhance solar cycle prediction methods., 11 pages, 11 figures, accepted for publication in the Astronomy & Astrophysics

The 11-Year solar cycle is driven by the sun's magnetic field. The sunspot number is the most-common long term index of solar activity and prediction of its amplitude can help to understand the physics of the solar dynamo and the effects of space weather. Previous studies have shown that analysing the solar activity of the two hemispheres separately instead of the full suncan provide more detailed information on the activity evolution. However, the existing Hemispheric Sunspot Number data series (1945 onwards) is too short for the purpose of solar cycle predictions. Based on a newly created hemispheric sunspot number catalogue for the time range 1874-2020 (Veronig et al. 2021) that is compatible with the International Sunspot Number from World Data Center SILSO, we investigate the evolution of the solar cycle for the two hemispheres, and demonstrate that empirical solar cycle prediction methods can be enhanced by investigating the solar cycle dynamics in terms of the hemispheric sunspot numbers. We develop a method for predictingthe solar cycle amplitude based on the peak growth rate in the ascending phase of the cycle using the Hemispheric Sunspot numbers for cycles 12-24. We show that using this technique, the sum of the predictions (North+South) of the two hemisphere give better estimates of the cycle amplitude than the Total Sunspot Numbers. In addition, we estimate the cycle peaks with 1st order and 3rd order regressions and find that the hemispheric sunspot numbers provide better estimates of cycle peak than total sunspot numbers for both the cases with the obtained correlations lying in the range r =0.8 to 0.9 depending on the applied smoothing window

Due to its unique 400-year duration, the sunspot number is a central reference for understanding the long-term evolution of solar activity and its influence on the Earth environment and climate. Here, we outline current data recovery work. For the sunspot number, we find historical evidence of a disruption in the source observers occurring in 1947–48. For the sunpot group number, recent data confirm the clear southern predominance of sunspots during the Maunder Minimum, while the umbra-penumbra ratio is similar to other epochs. For the Dalton minimum, newly recovered historical observations confirm a higher activity level than in a true Grand Minimum.

The first-ever revision of the sunspot number was released in 2015 by the World Data Center (WDC) SILSO. We describe the main diagnosed corrections to the sunspot and group number series, and also review newly published alternate reconstructions. We show the convergence of the determinations of the 1947 scale jump in the sunspot number around a value of 1.18 for cycle maxima. We also assess new proposed reconstructions of the group number, like the “backbone” and “active-day fraction” methods. No agreement was reached yet for this series.We highlight the main impacts of those recent upgrades on different scientific applications. As this first revision also marks a transition towards a dynamical series open to future improvements, we finally introduce the ongoing collaborative process for preparing the next upgrade (Version 3). From now on, our scientific users must be prepared for a flexible integration of an evolving sunspot number series.

We create a continuous series of daily and monthly hemispheric sunspot numbers (HSNs) from 1874 to 2020, which will be continuously expanded in the future with the HSNs provided by SILSO. Based on the available daily measurements of hemispheric sunspot areas from 1874 to 2016 from Greenwich Royal Observatory and NOAA, we derive the relative fractions of the northern and southern activity. These fractions are applied to the international sunspot number (ISN) to derive the HSNs. This method and obtained data are validated against published HSNs for the period 1945--2020. We provide a continuous data series and catalogue of daily, monthly mean, and 13-month smoothed monthly mean HSNs for the time range 1874--2020 that are consistent with the newly calibrated ISN. Validation of the reconstructed HSNs against the direct data available since 1945 reveals a high level of consistency, with a correlation of r=0.94 (0.97) for the daily (monthly) data. The cumulative hemispheric asymmetries for cycles 12-24 give a mean value of 16%, with no obvious pattern in north-south predominance over the cycle evolution. The strongest asymmetry occurs for cycle no. 19, in which the northern hemisphere shows a cumulated predominance of 42%. The phase shift between the peaks of solar activity in the two hemispheres may be up to 28 months, with a mean absolute value of 16.4 months. The phase shifts reveal an overall asymmetry of the northern hemisphere reaching its cycle maximum earlier (in 10 out of 13 cases). Relating the ISN and HSN peak growth rates during the cycle rise phase with the cycle amplitude reveals higher correlations when considering the two hemispheres individually, with r = 0.9. Our findings demonstrate that empirical solar cycle prediction methods can be improved by investigating the solar cycle dynamics in terms of the hemispheric sunspot numbers., Comment: Accepted by Astron. Astrophys. 12 pages

The sunspot record is the only observational tracer of solar activity that provides a fundamental, multi-century reference. Its homogeneity has been largely maintained with a succession of long-duration visual observers. In this paper, we examine observations of one of the primary reference sunspot observers, Hisako Koyama. By consulting original archives of the National Museum of Nature and Science of Japan (hereafter, NMNS), we retrace the main steps of her solar-observing career, from 1945 to 1996. We also present the reconstruction of a full digital database of her sunspot observations at the NMNS, with her original drawings and logbooks. Here, we extend the availability of her observational data from 1947-1984 to 1945-1996. Comparisons with the international sunspot number (version 2) and with the group sunspot number series show a good global stability of Koyama's observations, with only temporary fluctuations over the main interval 1947-1982. Identifying drawings made by alternate observers throughout the series, we find that a single downward baseline shift in the record coincides with the partial contribution of replacement observers mostly after 1983. We determine the correction factor to bring the second part (1983-1996) to the same scale with Koyama's main interval (1947-1982). We find a downward jump by 9% after 1983, which then remains stable until 1996. Overall, the high quality of Koyama's observations with her life-long dedication leaves a lasting legacy of this exceptional personal achievement. With this comprehensive recovery, we now make the totality of this legacy directly accessible and exploitable for future research., Main text 31 pages, references 6 pages, 13 figures, 3 tabes, accepted for publication in Monthly Notices of the Royal Astronomical Society, 2020

This topical issue contains articles on the effort to recalibrate the sunspot number (SN) that was initiated by the Sunspot Number Workshops. These workshops led to a revision of the Wolf sunspot number (WSN) and a new construction of the group sunspot number (GSN), both published herein. In addition, this topical issue includes three independently proposed alternative SN time series (two Wolf and one group), as well as articles providing historical context, critical assessments, correlative analyses, and observational data, both historical and modern, pertaining to the sunspot-number time series. The ongoing effort to understand and reconcile the differences between the various new sunspot number series is briefly discussed.

An analysis of historical Sun–Earth connection events in the context of the most extreme space weather events of the last $\sim150$ years is presented. To identify the key factors leading to these extreme events, a sample of the most important geomagnetic storms was selected based mainly on the well-known aa index and on geomagnetic parameters described in the accompanying paper (Vennerstrom et al., Solar Phys. in this issue, 2016, hereafter Paper I). This part of the analysis focuses on associating and characterizing the active regions (sunspot groups) that are most likely linked to these major geomagnetic storms. For this purpose, we used detailed sunspot catalogs as well as solar images and drawings from 1868 to 2010. We have systematically collected the most pertinent sunspot parameters back to 1868, gathering and digitizing solar drawings from different sources such as the Greenwich archives, and extracting the missing sunspot parameters. We present a detailed statistical analysis of the active region parameters (sunspots, flares) relative to the geomagnetic parameters developed in Paper I. In accordance with previous studies, but focusing on a much larger statistical sample, we find that the level of the geomagnetic storm is highly correlated to the size of the active regions at the time of the flare and correlated with the size of the flare itself. We also show that the origin at the Sun is most often a complex active region that is also most of the time close to the central meridian when the event is identified at the Sun. Because we are dealing with extremely severe storms, and not the usual severe storm sample, there is also a strong correlation between the size of the linked active region, the estimated transit speed, and the level of the geomagnetic event. In addition, we confirm that the geomagnetic events studied here and the associated events at the Sun present a low probability of occurring at low sunspot number value and are associated mainly with the maximum and descending part of the solar cycle.

We investigate periodicities in mean heliographic latitudes of sunspot groups, called active latitudes, for the last six complete solar cycles (1945-2008). For this purpose, the Multi Taper Method and Morlet Wavelet analysis methods were used. We found the following: 1) Solar rotation periodicities (26-38 days) are present in active latitudes of both hemispheres for all the investigated cycles (18 to 23). 2) Both in the northern and southern hemispheres, active latitudes drifted towards the equator starting from the beginning to the end of each cycle by following an oscillating path. These motions are well described by a second order polynomial. 3) There are no meaningful periods between 55 and about 300 days in either hemisphere for all cycles. 4) A 300 to 370 day periodicity appears in both hemispheres for Cycle 23, in the northern hemisphere for Cycle 20, and in the southern hemisphere for Cycle 18., Comment: Accepted for publication by Solar Physics

In this paper we develop a modern approach to solar cycle forecasting, based on the mathematical theory of nonlinear dynamics. We start from the design of a static curve fitting model for the experimental yearly sunspot number series, over a time scale of 306 years, starting from year 1700 and we establish a least-squares optimal pulse shape of a solar cycle. The cycle-to-cycle evolution of the parameters of the cycle shape displays different patterns, such as a Gleissberg cycle and a strong anomaly in the cycle evolution during the Dalton minimum. In a second step, we extract a chaotic mapping for the successive values of one of the key model parameters – the rate of the exponential growth-decrease of the solar activity during the n-th cycle. We examine piece-wise linear techniques for the approximation of the derived mapping and we provide its probabilistic analysis: calculation of the invariant distribution and autocorrelation function. We find analytical relationships for the sunspot maxima and minima, as well as their occurrence times, as functions of chaotic values of the above parameter. Based on a Lyapunov spectrum analysis of the embedded mapping, we finally establish a horizon of predictability for the method, which allows us to give the most probable forecasting of the upcoming solar cycle 24, with an expected peak height of 93±21 occurring in 2011/2012.

Short-term funding strategies present serious problems for programs like solar activity studies, where observations and analysis span decades or longer.

Our knowledge of the long-term evolution of solar activity and of its primary modulation, the 11-year cycle, largely depends on a single direct observational record: the visual sunspot counts that retrace the last 4 centuries, since the invention of the astronomical telescope. Currently, this activity index is available in two main forms: the International Sunspot Number initiated by R. Wolf in 1849 and the Group Number constructed more recently by Hoyt and Schatten (1998a,b). Unfortunately, those two series do not match by various aspects, inducing confusions and contradictions when used in crucial contemporary studies of the solar dynamo or of the solar forcing on the Earth climate. Recently, new efforts have been undertaken to diagnose and correct flaws and biases affecting both sunspot series, in the framework of a series of dedicated Sunspot Number Workshops. Here, we present a global overview of our current understanding of the sunspot number calibration. While the early part of the sunspot record before 1800 is still characterized by large uncertainties due to poorly observed periods, the more recent sunspot numbers are mainly affected by three main inhomogeneities: in 1880-1915 for the Group Number and in 1947 and 1980-2014 for the Sunspot Number. The newly corrected series clearly indicates a progressive decline of solar activity before the onset of the Maunder Minimum, while the slowly rising trend of the activity after the Maunder Minimum is strongly reduced, suggesting that by the mid 18th century, solar activity had already returned to the level of those observed in recent solar cycles in the 20th century. We finally conclude with future prospects opened by this epochal revision of the Sunspot Number, the first one since Wolf himself, and its reconciliation with the Group Number, a long-awaited modernization that will feed solar cycle research into the 21st century.

We describe a revised collection of the number of sunspot groups from 1610 to the present. This new collection is based on the work of Hoyt and Schatten (Solar Phys. 179, 189, 1998). The main changes are the elimination of a considerable number of observations during the Maunder Minimum (hereafter, MM) and the inclusion of several long series of observations. Numerous minor changes are also described. Moreover, we have calculated the active-day percentage during the MM from this new collection as a reliable index of the solar activity. Thus, the level of solar activity obtained in this work is greater than the level obtained using the original Hoyt and Schatten data, although it remains compatible with a grand minimum of solar activity. The new collection is available in digital format., 27 pages, 6 figures, accepted for publication in Solar Physics

Sunspot number series are subject to various uncertainties, which are still poorly known. The need for their better understanding was recently highlighted by the major makeover of the international Sunspot Number [Clette et al., Space Science Reviews, 2014]. We present the first thorough estimation of these uncertainties, which behave as Poisson-like random variables with a multiplicative coefficient that is time- and observatory-dependent. We provide a simple expression for these uncertainties, and reveal how their evolution in time coincides with changes in the observations, and processing of the data. Knowing their value is essential for properly building composites out of multiple observations, and for preserving the stability of the composites in time., Comment: accepted in Solar Physics (2016), 24 pages

In view of the construction of new sunspot-based activity indices and proxies, we conducted a comprehensive survey of all existing catalogs providing detailed parameters of photospheric features over long time intervals. Although there are a fair number of such catalogs, a global evaluation showed that they suffer from multiple limitations: finite or fragmented time coverage, limited temporal overlap between catalogs, and, more importantly, a mismatch in contents and conventions. Starting from the existing material, we demonstrate how the information from parallel catalogs can be merged to form a much more comprehensive record of sunspots and sunspot groups. To do this, we use the uniquely detailed Debrecen Photoheliographic Data (DPD), which is already a composite of several ground-based observatories and of SOHO data, and the USAF/Mount Wilson catalog from the Solar Observing Optical Network (SOON). We also outline our cross-identification method, which was needed to match the non-overlapping solar active-region nomenclature. This proved to be the most critical and subtle step when working with multiple catalogs. This effort, focused here first on the last two solar cycles, should lead to a better central database that collects all available sunspot group parameters to address future solar-cycle studies beyond the traditional sunspot-index time series [R i].

Sunspot drawings and images are the base material at our disposal to derive more detailed information about the long-term evolution of the solar cycle, beyond the classical total sunspot index R i . In the context of the new SoTerIA project, the SIDC World Data Center “Sunspot” will pursue two complementary goals. We will develop tools to digitize and encode the largely unexploited information recorded in sunspot drawing collections, starting with the Uccle station. Turning towards the future, we will define and evaluate possible global activity indices based on full-disk sunspot CCD imagery. As a support, we will also provide such CCD images from the Uccle solar telescopes. Here, we describe the main steps of this work and the available data sets. We also outline the prospects and resulting products in the 3-year framework of the SoTerIA project.

When consulting detailed photospheric catalogs for solar activity studies spanning long time intervals, solar physicists face multiple limitations in the existing catalogs: finite or fragmented time coverage, limited time overlap between catalogs and even more importantly, a mismatch in contents and conventions. In view of a study of new sunspot-based activity indices, we have conducted a comprehensive survey of existing catalogs.In a first approach, we illustrate how the information from parallel catalogs can be merged to form a much more comprehensive record of sunspot groups. For this, we use the unique Debrecen Photoheliographic Data (DPD), which is already a composite of several ground observatories and SOHO data, and the USAF/Mount Wilson catalog from the Solar Optical Observing Network (SOON). We also describe our semi-interactive cross-identification method, which was needed to match the non-overlapping solar active region nomenclature, the most critical and subtle step when working with multiple catalogs. This effort, focused here first on the last two solar cycles, should lead to a better central database collecting all available sunspot group parameters to address future solar cycle studies beyond the traditional sunspot index time series Ri.

By encompassing four centuries of solar evolution, the sunspot number provides the longest available record of solar activity. Nowadays, it is widely used as the main reference solar index on which hundreds of published studies are based, in various fields of science. In this review, we will retrace the history of this crucial solar index, from its roots at the Zurich Observatory up to the current multiple indices established and distributed by the Solar Influences Data Analysis Center (SIDC), World Data Center for the International Sunspot Index, which was founded in 1981, exactly 25 years ago. We describe the principles now in use for the statistical processing of input data coming from the worldwide observing network (∼80 stations). Among the various SIDC data products and innovations, we highlight some recent ones, including the daily Estimated International Sunspot Number. Taking a wider perspective, we show how the sunspot index stands the test of time versus more recent quantitative indices, but we also consider the prospects and possible options for a future transition from the visual sunspot index heritage towards an equivalent global activity index. Based on past historical flaws, we conclude on the key requirements involved in the maintenance of any robust long-term solar activity index.

In 1981, the production of the international Sunspot Number moved from the Z\"{u}rich Observatory to the Royal Observatory of Belgium, marking a very important transition in the history of the Sunspot Number. Those recent decades are particularly important for linking recent modern solar indices and fluxes and the past Sunspot Number series. However, large variations have been recently identified in the scale of the Sunspot Number between 1981 and the present. Here, we reconstruct a new average Sunspot Number series $S_N$ using long-duration stations between 1981 and 2015. We also extend this reconstruction using long-time series from 35 stations over 1945-2015, which includes the 1981 transition. In both reconstructions, we also derive a parallel Group Number series $G_N$. Our results confirm the variable trends of the Locarno pilot station. We also verify the scale of the resulting 1981-2015 correction factor relative to the preceding period 1945--1980. By comparing the new $S_N$ and $G_N$ series, we find that a constant quadratic relation exists between those two indices. This proxy relation leads to a fully constant and cycle-independent $S_N/G_N$ ratio over cycles 19 to 23, with the exception of cycle 24. We find a very good agreement between our reconstructed $G_N$ and the new "backbone" Group Number but inhomogeneities in the original Group Number as well as the $F_{10.7}$ radio flux and the American sunspot number $R_a$. This analysis opens the way to the implementation of a more advanced method for producing the Sunspot Number in the future. In particular, we identify the existence of distinct subsets of observing stations sharing very similar personal k factors, which may be a key element for building a future multi-station reference in place of the past single pilot station., Comment: 33 pages, 23 figures, 2 tables

Our knowledge of the long-term evolution of solar activity and of its primary modulation, the 11-year cycle, largely depends on a single direct observational record: the visual sunspot counts that retrace the last 4 centuries, since the invention of the astronomical telescope. Currently, this activity index is available in two main forms: the International Sunspot Number initiated by R. Wolf in 1849 and the Group Number constructed more recently by Hoyt and Schatten (1998a,b). Unfortunately, those two series do not match by various aspects, inducing confusions and contradictions when used in crucial contemporary studies of the solar dynamo or of the solar forcing on the Earth climate. Recently, new efforts have been undertaken to diagnose and correct flaws and biases affecting both sunspot series, in the framework of a series of dedicated Sunspot Number Workshops. Here, we present a global overview of our current understanding of the sunspot number calibration. While the early part of the sunspot record before 1800 is still characterized by large uncertainties due to poorly observed periods, the more recent sunspot numbers are mainly affected by three main inhomogeneities: in 1880-1915 for the Group Number and in 1947 and 1980-2014 for the Sunspot Number. The newly corrected series clearly indicates a progressive decline of solar activity before the onset of the Maunder Minimum, while the slowly rising trend of the activity after the Maunder Minimum is strongly reduced, suggesting that by the mid 18th century, solar activity had already returned to the level of those observed in recent solar cycles in the 20th century. We finally conclude with future prospects opened by this epochal revision of the Sunspot Number, the first one since Wolf himself, and its reconciliation with the Group Number, a long-awaited modernization that will feed solar cycle research into the 21st century.

Full-disc full-resolution (FDFR) solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) were used to analyse the centre-to-limb function and latitudinal distribution of coronal bright points. The results obtained with the interactive and the automatic method, as well as for three subtypes of coronal bright points for the time period 4 June 1998 to 22 May 1999 are presented and compared. An indication of a two-component latitudinal distribution of coronal bright points was found. The central latitude of coronal bright points traced with the interactive method lies between 10∘ and 20∘. This is closer to the equator than the average latitude of sunspots in the same period. Possible implications for the interpretation of the solar differential rotation are discussed. In the appendix, possible differences between the two solar hemispheres are analysed. More coronal bright points were present in the southern solar hemisphere than in the northern one. This asymmetry is statistically significant for the interactive method and not for the automatic method. The visibility function is symmetrical around the central meridian.

Since January 1981, the Royal Observatory of Belgium (ROB) has operated the Sunspot Index Data Center (SIDC), the World Data Center for the Sunspot Index. From 2000, the SIDC obtained the status of Regional Warning Center (RWC) of the International Space Environment Service (ISES) and became the “Solar Influences Data analysis Center”. As a data analysis service of the Federation of Astronomical and Geophysical data analysis Services (FAGS), the SIDC collects monthly observations from worldwide stations in order to calculate the International Sunspot Number, R i . The center broadcasts the daily, monthly, yearly sunspot numbers, with middle-range predictions (up to 12 months). Since August 1992, hemispheric sunspot numbers are also provided.

Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) are used to analyse solar differential rotation by tracing coronal bright points for the period June 4, 1998 to May 22, 1999. A method for the simultaneous determination of the true solar synodic rotation velocity and the height of the tracers is applied to data sets analysed with interactive and automatic methods. The calculated height of coronal bright points is on average 8000-12000 km above the photosphere. Corrected rotation velocities are transformed into sidereal ones and compared with results from the literature, obtained with various methods and tracers. The differential rotation profile determined by coronal bright points with the interactive method corresponds roughly to the profile obtained by correlating photospheric magnetic fields and the profile obtained from the automatic method corresponds roughly to the rotation of sunspot groups. This result is interpreted in terms of the differences obtained in the latitudinal distribution of coronal bright points using the two methods.

Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) were used to analyse solar dierential rotation by tracing coronal bright points. The results obtained with the interactive and the automatic method for the time period June 4, 1998 to May 22, 1999 are presented and compared. A possible north-south rotational asymmetry and dierences in the rotation velocity curves for various subtypes of tracers are investigated.

Since 1981, the Royal Observatory of Belgium has operated the Sunspot Index Data Centre, the World Data Centre for the Sunspot Index. Recently, the Space Weather Forecast Centre of Paris–Meudon was transferred and added to the activities of the SIDC. Moreover, a complete archive of all images of the SOHO instrument EIT has become available at the SIDC. Given all these extensions, the new style SIDC has become a ‘Solar Influences Data Centre’ that analyses solar activity and provides services on three different time scales: 1. Fast warnings and real time monitoring . As the Regional Warning Centre (RWC) for Western Europe of the International Space Environment Service (ISES), the SIDC collects and redistributes solar, geomagnetic, and ionospheric data in Western Europe. Short-term predictions (3 days) and alerts are produced on a daily basis. 2. Forecasts and middle term analysis . The SIDC takes care of the calculation of a sunspot index, called the International Sunspot Number. We compute and broadcast the daily, monthly, yearly international sunspot numbers, with middle range predictions (up to 12 months). 3. Post-event analysis and long-term solar cycle analysis . Since the launch of SOHO, EIT offers a global view of the EUV corona over the whole rising phase of the solar activity cycle. Such a long-duration data series is unprecedented and allows the study of the evolution over the solar cycle of objects classes such as active regions, coronal holes, coronal mass ejections or flares.

© S.M. Stankov et al., Published by EDP Sciences 2017. A total solar eclipse occurred on 20 March 2015, with a totality path passing mostly above the North Atlantic Ocean, which resulted in a partial solar eclipse over Belgium and large parts of Europe. In anticipation of this event, a dedicated observational campaign was set up at the Belgian Solar-Terrestrial Centre of Excellence (STCE). The objective was to perform high-quality observations of the eclipse and the associated effects on the geospace environment by utilising the advanced space- and ground-based instrumentation available to the STCE in order to further our understanding of these effects, particularly on the ionosphere. The study highlights the crucial importance of taking into account the eclipse geometry when analysing the ionospheric behaviour during eclipses and interpreting the eclipse effects. A detailed review of the eclipse geometry proves that considering the actual obscuration level and solar zenith angle at ionospheric heights is much more important for the analysis than at the commonly referenced Earth's surface or at the plasmaspheric heights. The eclipse occurred during the recovery phase of a strong geomagnetic storm which certainly had an impact on (some of) the ionospheric characteristics and perhaps caused the omission of some "low-profile" effects. However, the analysis of the ionosonde measurements, carried out at unprecedented high rates during the eclipse, suggests the occurrence of travelling ionospheric disturbances (TIDs). Also, the high temporal and spatial resolution measurements proved very important in revealing and estimating some finer details of the delay in the ionospheric reaction and the ionospheric disturbances. ispartof: JOURNAL OF SPACE WEATHER AND SPACE CLIMATE vol:7 status: published

Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) were used to analyse solar dierential rotation determined by tracing coronal bright points. Two dierent procedures were developed and compared: an interactive and an automatic method. The interactive method is based on the visual tracing of coronal bright points in consecutive images using computer programs written in the Interactive Data Language (IDL). The automatic method relies on the IDL procedure \Regions Of Interest (ROI) segmentation" which is used to detect and follow bright points in triplets of consecutive images. The test-results obtained applying both methods by dierent persons who performed tracing are presented and compared. The advantages and disadvantages of the two methods are discussed.

This paper reports on a qualitative study on the weakest flarelike brightenings in active region that can be observed with current coronal imagers. Specicallly, we investigate the correspondence of the \active region transient brightenings" (ARTB) rst observed almost a decade ago with SXT with similar brightenings in the EUV that are now observed by EIT and TRACE. For this goal, exceptionally high cadence image sequences were acquired of a small but rapidly evolving active region (NOAA 8218), simultaneously by SXT, EIT, and TRACE. Within the timeframe of this coordinated campaign, we detected 41 soft X-ray brightenings and 373 EUV brightenings. We nd that the strongest brightenings observed by EIT are indeed the EUV counterparts of the ARTBs seen by SXT. Weaker brightenings seen by EIT often do not have an X-ray counterpart. Among the brightenings detected with SXT we discover a new subpopulation, consisting of events that brighten in soft X-rays only, at a footpoint of a pre-existing SXT loop shortly after an ARTB occurred at the other footpoint. The propagation speed of the perturbation suggests an interpretation in terms of slow mode MHD waves.

Since the start of the SoHO mission, EIT -the Extreme ultraviolet Imaging Telescope- offers a global view of the solar corona over the whole rising phase of the current activity cycle. Such a dataset is unprecedented. We give here the current results of an on-going investigation over the entire EIT archive. In the Fe XV images (2 MK), the on-disk and off-disk intensity distributions have been evaluated, and their evolution is described. Additionally, we developed an image processing technique that extracts the smallest detectable features. The cosmic ray hits are statistically disentangled from the solar point-like phenomena, and the trends in both rates are assessed.

We report on observations of the solar luminosity variations in the Fe xii line (195 A) over the period 1996–1999, which corresponds to the minimum and rising phase of the current 23rd solar cycle. The relatively or rather high temporal cadence and spatial resolution of the Extreme-ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) allowed a nearly continuous measurement of intensity of different structures on the Sun. We find that a significant contribution to the longitudinal asymmetry, and thus to the 27-day variability of the solar EUV radiation, is produced by the numerous intermediate brightness elements that are globally distributed over large areas (up to about \( \sim \frac{2}{3} \) of the whole surface of the Sun). When activity is low, this component even becomes dominant over the contribution from localized active regions and bright points. This suggests that weak magnetic field areas outside active regions constitute an important factor through which solar activity modulates the solar EUV luminosity.

This paper presents the preflight photometric calibration of the Extreme-ultraviolet Imag- ing Telescope (EIT) aboard the Solar and Heliospheric Observatory (SOHO). The EIT consists of a Ritchey - Chretien telescope with multilayer coatings applied to four quadrants of the primary and secondary mirrors, several filters and a backside-thinned CCD detector. The quadrants of the EIT optics were used to observe the Sun in 4 wavelength bands that peak near 171, 195, 284, and 304 A. Before the launch of SOHO, the EIT mirror reflectivities, the filter transmissivities and the CCD quantum efficiency were measured and these values are described here. The instrumental throughput in terms of an effective area is presented for each of the various mirror quadrant and filter wheel combinations. The response to a coronal plasma as a function of temperature is also determined and the expected count rates are compared to the count rates observed in a coronal hole, the quiet Sun and an active region.

On 13 May 1998, the Extreme-Ultraviolet Imaging Telescope (EIT, on board SOHO) has produced a unique image sequence operating in 'shutterless mode' (SOHO JOP 80). In JOP 80, EIT is the lead instrument, followed by several space-born instruments (SXT, TRACE, MDI, CDS, SUMER), as well as two observatories on the ground (in La Palma and Sac Peak). The target of the campaign was a relatively small but rapidly evolving active region (AR 8218). For the EIT contribution, a 15 s cadence was achieved in the Fe xii bandpass at 195 A by leaving EIT's shutter open for 1 h and operating the CCD in frame-transfer mode. In this paper, we start the analysis of the huge data set, by making an inventory of the transients observed in the EIT image sequence. Besides scatter plots of duration, size and radiative output of the detected EUV brightenings, we discuss in full detail the morphology and evolution of several typical events. These transients range from a B3.5 flare producing a large plasma flow along pre-existing loops, to EUV versions of active region transient brightenings as previously observed by SXT on board Yohkoh. In addition, a new class of weaker footpoint brightenings is discussed that produce wave-like disturbances propagating along quasi-open field lines. This new class of propagating disturbances extends the wide variety of transient phenomena that we discovered in the EIT data, and makes the potential for inter-instrumental studies of the JOP 80 data all the more exciting. We stress the necessity of such forthcoming studies to reach an instrument-independent classification of small-scale solar transients.

SOHO EIT spectroheliograms showing the polar coronal holes during the present sunspot minimum are compared with National Solar Observatory (Kitt Peak) magnetograms taken in Fe I λ8688 and Ca II λ8542. The chromospheric λ8542 magnetograms, obtained on a routine, near-daily basis since 1996 June, reveal the Sun's strong polar fields with remarkable clarity. We find that the Fe IX λ171 polar plumes occur where minority-polarity flux is in contact with flux of the dominant polarity inside each polar hole. Moreover, the locations of "plume haze" coincide approximately with the patterns of brightened He II λ304 network within the coronal hole. The observations appear to be consistent with mechanisms of plume formation involving magnetic reconnection between unipolar flux concentrations and nearby bipoles. The fact that minority-polarity fields constitute only a small fraction of the total magnetic flux within the polar holes suggests that plumes are not the main source of the high-speed polar wind.

The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT observes the Sun over a 45 × 45 arc min field of view in four emission line groups: Fe IX, X, Fe XIIi, Fe xv, and He II. A post-launch determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops.

The SOHO (SOlar and Heliospheric Observatory) satellite was launched on December 2nd 1995. After arriving at the Earth-Sun (L1) Lagrangian point on February 14th 1996, it began to continuously observe the Sun. As one of the instruments onboard SOHO, the EIT (Extreme ultraviolet Imaging Telescope) images the Sun's corona in 4 EUV wavelengths. The He II filter at 304 A images the chromosphere and the base of the transition region at a temperature of 5 − 8 × 104 K; the Fe IX–X filter at 171 A images the corona at a temperature of ∼ 1.3 × 106 K; the Fe XII filter at 195 A images the quiet corona outside coronal holes at a temperature of ∼ 1.6 × 106 K; and the Fe XV filter at 284 A images active regions with a temperature of ∼ 2.0 × 106 K. About 5000 images have been obtained up to the present. In this paper, we describe also some aspects of the telescope and the detector performance for application in the observations. Images and movies of all the wavelengths allow a look at different phenomena present in the Sun's corona, and in particular, magnetic field reconnection.

The Extreme-ultraviolet Imaging Telescope (EIT) will provide wide-field images of the corona and transition region on the solar disc and up to 1.5 R⊙ above the solar limb. Its normal incidence multilayer-coated optics will select spectral emission lines from Fe IX (171 A), Fe XII (195 A), Fe XV (284 A), and He II (304 A) to provide sensitive temperature diagnostics in the range from 6 × 104 K to 3 × 10 6 K. The telescope has a 45×45 arcmin field of view and 2.6 arcsec pixels which will provide approximately 5-arcsec spatial resolution. The EIT will probe the coronal plasma on a global scale, as well as the underlying cooler and turbulent atmosphere, providing the basis for comparative analyses with observations from both the ground and other SOHO instruments. This paper presents details of the EIT instrumentation, its performance and operating modes.

Using intensity and polarization maps computed from white-light observations of the July 11, 1991 solar eclipse, we present axisymmetrical models of the large-scale electron density distribution in the corona. These models are based on an expansion in Legendre polynomials, and are flexible enough to fit individual features, like streamers and holes. Furthermore, as the symmetry axis of our models can take any orientation, we consider two plausible configurations, aligned on the rotation axis or the mean bipolar magnetic field axis. Their respective abilities to reproduce a strongly non-spherical global magnetic structure are then compared.

The existence of “spikes” with very short time scales (1 to 10 ms) in the decimetric radio spectrum emphasizes the necessity of building radio instruments with very high time resolution. Therefore, we are planning to build a multi-frequency decimetric radioheliograph at the radioastronomical station of the Observatoire Royal de Belgique (Humain, Belgium). The existing array primarily devoted to observe at 408 MHz will constitute the starting structure of the project.

SWAP (Sun Watcher using Active Pixel system detector and image processing) is a solar imager in the extreme ultraviolet (EUV) that has been selected to fly in 2007 on the PROBA 2 technological platform, an ESA program. SWAP will use an off-axis Ritchey Chretien telescope equipped with an EUV enhanced active pixel sensor detector (coated APS). This type of detector has advantages that promise to be very profitable for solar EUV imaging. SWAP will provide solar coronal images at a 1-min cadence in a bandpass centered on 17.5 nm. Observations with this specific wavelength allow detecting phenomena, such as solar flares or EIT-waves, associated with the early phase of coronal mass ejections. Image processing software will be developed that automatically detects these phenomena and sends out space weather warnings. Together with its sister instrument LYRA, also onboard PROBA 2, SWAP will serve as a high performance solar monitoring tool to be used in operational space weather forecasting. The SWAP data will complement the solar observations provided by instruments like SOHO-EIT, and STEREO-SECCHI.

LYRA is the solar UV radiometer that will embark in 2006 onboard Proba2, a technologically oriented ESA micro-mission. LYRA is designed and manufactured by a Belgian–Swiss–German consortium (ROB, PMOD/WRC, IMOMEC, CSL, MPS and BISA) with additional international collaborations. It will monitor the solar irradiance in four UV passbands. They have been chosen for their relevance to Solar Physics, Aeronomy and Space Weather: (1) the 115–125 nm Lyman-α channel, (2) the 200–220 nm Herzberg continuum range, (3) the Aluminium filter channel (17–70 nm) including He II at 30.4 nm and (4) the Zirconium filter channel (1–20 nm). The radiometric calibration will be traceable to synchrotron source standards (PTB and NIST). The stability will be monitored by onboard calibration sources (LEDs), which allow to distinguish between potential degradations of the detectors and filters. Additionally, a redundancy strategy maximizes the accuracy and the stability of the measurements. LYRA will benefit from wide bandgap detectors based on diamond: it will be the first space assessment of a pioneering UV detectors program. Diamond sensors make the instruments radiation-hard and solar-blind: their high bandgap energy makes them insensitive to visible light and, therefore, make dispensable visible light blocking filters, which seriously attenuate the desired ultraviolet signal. Their elimination augments the effective area and hence the signal-to-noise, therefore increasing the precision and the cadence. The SWAP EUV imaging telescope will operate next to LYRA on Proba2. Together, they will establish a high performance solar monitor for operational space weather nowcasting and research. LYRA demonstrates technologies important for future missions such as the ESA Solar Orbiter.

Space Weather nowcasting and forecasting require solar observations because geoeffective disturbances can arise from three types of solar phenomena: coronal mass ejections (CMEs), flares and coronal holes. For each, we discuss their definition and review their precursors in terms of remote sensing and in-situ observations. The objectives of Space Weather require some specific instrumental features, which we list using the experience gained from the daily operations of the Solar Influences Data analysis Centre (SIDC) at the Royal Observatory of Belgium. Nowcasting requires real-time monitoring to assess quickly and reliably the severity of any potentially geoeffective solar event. Both research and forecasting could incorporate more observations in order to feed case studies and data assimilation respectively. Numerical models will result in better predictions of geomagnetic storms and solar energetic particle (SEP) events. We review the data types available to monitor solar activity and interplanetary conditions. They come from space missions and ground observatories and range from sequences of dopplergrams, magnetograms, white-light, chromospheric, coronal, coronagraphic and radio images, to irradiance and in-situ time-series. Their role is summarized together with indications about current and future solar monitoring instruments.

The Solar Influences Data analysis Center (SIDC) is the World Data Center for the production and the distribution of the International Sunspot Index, coordinating a network of about 80 stations worldwide. From this core activity, the SIDC has grown in recent years to a European center for nowcasting and forecasting of solar activity on all timescales. This paper reviews the services (data, forecasts, alerts, software) that the SIDC currently offers to the scientific community. The SIDC operates instruments both on the ground and in space. The USET telescope in Brussels produces daily white light and Hα images. Several members of the SIDC are co-investigators of the EIT instrument onboard SOHO and are involved in the development of the next generation of Europe's solar weather monitoring capabilities. While the SIDC is staffed only during day-time (7 days/week), the monitoring service is a 24 h activity thanks to the implementation of autonomous software for data handling and analysis and the sending of automated alerts. We will give an overview of recently developed techniques for visualization and automated analysis of solar images and detection of events significant for space weather (e.g. CMEs or EIT waves). As part of the involvement of the SIDC in the ESA Pilot Project for Space Weather Applications we have developed services dedicated to the users of the Global Positioning System (GPS). As a Regional Warning Center (RWC) of the International Space Environment Service (ISES), the SIDC produces daily forecasts of flaring probability, geomagnetic activity and 10.7 cm radio flux. The accuracy of these forecasts will be investigated through an in-depth quality analysis.