The new Sunspot Number: assembling all corrections

The Sunspot Number, created by R.Wolf in 1849, provides a direct long-term record of solar activity from 1700 to the present. In spite of its central role in multiple studies of the solar dynamo and of the past Sun-Earth relations, it was never submitted to a global critical revision. However, various discrepancies with other solar indices recently motivated a full re-calibration of this series. Based on various diagnostics and corrections established in the framework of several Sunspot Number Workshops and described in Clette et al. 2014, we assembled all corrections in order to produce a new standard version of this reference time series. In this paper, we explain the three main corrections and the criteria used to choose a final optimal version of each correction factor or function, given the available information and published analyses. We then discuss the good agreement obtained with the Group sunspot Number derived from a recent reconstruction. Among the implications emerging from this re-calibrated series, we also discuss the absence of a rising secular trend in the newly-determined solar cycle amplitudes, also in relation with contradictory indications derived from cosmogenic radionuclides. As conclusion, we introduce the new version management scheme now implemented at the World Data Center - SILSO, which reflects a major conceptual transition: beyond the re-scaled numbers, this first revision of the Sunspot Number also transforms the former locked data archive into a living observational series open to future improvements

The revised Brussels-Locarno Sunspot Number (1981-2015)

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 table

Revisiting the Sunspot Number

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

Nonparametric monitoring of sunspot number observations: a case study

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 many kinds of errors. The data from some stations experience therefore severe and various deviations over time. In this paper, we propose the first systematic and thorough statistical approach for monitoring these complex and important series. It consists of three steps essential for successful treatment of the data: smoothing on multiple timescales, 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.Comment: 27 pages (without appendices), 6 figure

Sunspot Observations by Hisako Koyama: 1945-1996

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.Comment: Main text 31 pages, references 6 pages, 13 figures, 3 tabes, accepted for publication in Monthly Notices of the Royal Astronomical Society, 202

EIT Observations of the Extreme Ultraviolet Sun

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 x 45 arc min field of view in four emission line groups: Feix, x, Fexii, Fexv, and Heii. 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

Is the

The F10.7cm 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. Moreover, a temporal drift was recently found in the relative scale of those two solar indices. 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 over the whole range of solar activity. We also study the role of temporal averaging on the regression, and we investigate the issue of the all-quiet F10.7 background flux. Finally, we check for any change in the F10.7–Sunspot Number relation over the entire period 1947–2015. We find that, with a 4th-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 F10.7 background is not absolute and actually depends on the duration of the spotless periods. Finally, we find that the F10.7cm time series is inhomogeneous, with an abrupt 10.5% upward jump occurring between 1980 and 1981, and splitting the series in two stable intervals. Our new proxy relations bring a strong improvement and show the importance of temporal scale for choosing the appropriate proxy and the F10.7 quiet-Sun background level. From historical evidence, we conclude that the 1981 jump is most likely due to a unique change in the F10.7 scientific team and the data processing, and that the newly re-calibrated sunspot number (version 2) will probably provide the only possible reference to correct this inhomogeneity

Application de la photométrie solaire depuis le sol à l'observation des modes acoustiques globaux en intensité

Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

Are the sunspots really vanishing?

Context: The elapsed solar cycle (23) ended with an exceptionally long period of low activity and with unprecedented low levels for various series of solar irradiance and particle flux measurements. This unpredicted evolution of solar activity raised multiple questions about a future decline of the solar cycles and launched a quest for precursor signs of this possible deep solar transition over the last decade. Aim: We present here a review and overall interpretation of most current diagnostics of solar cycle 23, including the recent disagreements that appeared among solar reference indices and standard solar-based geo-indices, the indication of a changed pattern of internal torsional waves (helioseismology) or the announced fading and magnetic weakening of sunspots. Methods: Based on a statistical analysis of detailed sunspot properties over the last 24 years, we complete the picture with new evidence of a strong global deficit of the smallest sunspots starting around 2000, in order to answer the question: are all sunspots about to disappear? Results: This global scale-dependent change in sunspot properties is confirmed to be real and not due to uncontrolled biases in some of the indices. It can also explain the recent discrepancies between solar indices by their different sensitivities to small and weak magnetic elements (small spots). The International Sunspot Index Ri, based on unweighted sunspot counts, proved to be particularly sensitive to this particular small-scale solar evolution. Conclusions: Our results and interpretation show the necessity to look backwards in time, more than 80 years ago. Indeed, the Sun seems to be actually returning to a past and hardly explored activity regime ending before the 1955–1995 Grand Maximum, which probably biased our current space-age view of solar activity