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.
The focus of this study is to reveal the reason behind a scale problem detected around 1849 in the historical version of the International Sunspot Number Series, i.e. version 1 (Leussu et al. in
...Astron. Astrophys.
559
, A28,
2013
; Friedli in
Solar Phys.
291
, 2505,
2016
). From 1826 to 1848 Heinrich Schwabe’s observations were considered primary by Rudolf Wolf, and a shift of primary observer from Schwabe to Wolf in 1849 seems to have led to an inconsistency in the Sunspot Number series. In this study we benefited from various datasets, the most important being Schwabe’s raw counts from the
Mittheilungen
(Prof. Wolf’s Journals) that have been digitised at the Royal Observatory of Belgium between 2017 and 2019. We provide a robust quantification of the detected problem by using classic algebraic calculations, but also different methods such as a method inspired by Lockwood et al. (
J. Geophys. Res.
119
(7), 5172,
2014
), hence assigning a modern k-factor to Schwabe’s observations before 1849. We also assess the implications of this 1849 inconsistency on the International Sunspot Number series (Versions 1 and 2) before and after 1849.
Context. The variability of the 11-year cycle of solar activity on secular timescales is well established through the sunspot record, but it remains unpredictable. Indeed, the duration of the last ...solar cycle 23 was exceptionally long and took the solar physics community by surprise. The long minimum was marked by particularly low and often unprecedented levels of the international sunspot index Ri and most other solar indices. Earlier in the course of cycle 23, discrepancies appeared between several of those indices, raising a new issue: is there something wrong in the recent index values or is a real physical change occurring inside the Sun? Aims. By exploiting detailed sunspot information, we look for independent evidence of a concrete and significant global change in sunspot properties appearing in the course of cycle 23. Methods. To achieve this goal, we compared existing sunspot indices, such as the international sunspot number (Ri), and exploited the most complete information currently available on individual sunspots, obtained by merging two recent and complementary catalogs. Detailed statistics were obtained according to group type and spot size. Results. We find that the Sun has shown an important deficit in small spots since the last activity maximum around 2000. While the number of large-scale spots remained largely unaffected, the occurrence rate of the smallest sunspots, and among them the ones with the shortest lifetimes, was more than halved during cycle 23. This explains the divergence between indices, weighted in favor of the largest active regions/magnetic structures, and sunspot counts that do not include such a weighting. It also confirms an actual intrinsic transition in the magnetic field generation inside the Sun, arising years before the exceptional activity minimum. Conclusions. Our results thus reveal the potential of such detailed sunspot analyses for understanding and predicting future trends in the solar cycle. The change found here in the small individual sunspots suggests that solar and solar-terrestrial proxies should be redefined for the current state of the Sun, replacing the present ones. This scale-dependent change also provides support to dynamo models involving the coexistence of a deep and a superficial dynamo.
We report progress on the ongoing recalibration of the Wolf sunspot number (SN) and group-sunspot number (GN) following the release of version 2.0 of SN in 2015. This report constitutes both an ...update of the efforts reported in the 2016 Topical Issue of Solar Physics and a summary of work by the International Space Science Institute (ISSI) International Team formed in 2017 to develop optimal SN and GN reconstruction methods while continuing to expand the historical sunspot-number database. Significant progress has been made on the database side while more work is needed to bring the various proposed SN and (primarily) GN reconstruction methods closer to maturity, after which the new reconstructions (or combinations thereof) can be compared with (a) “benchmark” expectations for any normalization scheme (e.g., a general increase in observer normalization factors going back in time), and (b) independent proxy data series such as F10.7 and the daily range of variations of Earth’s undisturbed magnetic field. New versions of the underlying databases for SN and GN will shortly become available for years through 2022 and we anticipate the release of the next versions of these two time series in 2024.
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.
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
).
Sunspot catalogs are very useful for studying the solar activity of the recent past. In this context, a catalog covering more than three solar cycles made by the astronomers of the Madrid ...Astronomical Observatory in Spain (nowadays, the National Astronomical Observatory) from 1952 until 1986 has been recovered. Moreover, a machine-readable version of this catalog has been made available. We have recovered abundant metadata and studied the reliability of this dataset by comparing it with other sunspot catalogs.
We present the first large statistical study of extreme geomagnetic storms based on historical data from the time period 1868 – 2010. This article is the first of two companion papers. Here we ...describe how the storms were selected and focus on their near-Earth characteristics. The second article presents our investigation of the corresponding solar events and their characteristics. The storms were selected based on their intensity in the aa index, which constitutes the longest existing continuous series of geomagnetic activity. They are analyzed statistically in the context of more well-known geomagnetic indices, such as the Kp and Dcx/Dst index. This reveals that neither Kp nor Dcx/Dst provide a comprehensive geomagnetic measure of the extreme storms. We rank the storms by including long series of single magnetic observatory data. The top storms on the rank list are the New York Railroad storm occurring in May 1921 and the Quebec storm from March 1989. We identify key characteristics of the storms by combining several different available data sources, lists of storm sudden commencements (SSCs) signifying occurrence of interplanetary shocks, solar wind
in-situ
measurements, neutron monitor data, and associated identifications of Forbush decreases as well as satellite measurements of energetic proton fluxes in the near-Earth space environment. From this we find, among other results, that the extreme storms are very strongly correlated with the occurrence of interplanetary shocks (91 – 100 %), Forbush decreases (100 %), and energetic solar proton events (70 %). A quantitative comparison of these associations relative to less intense storms is also presented. Most notably, we find that most often the extreme storms are characterized by a complexity that is associated with multiple, often interacting, solar wind disturbances and that they frequently occur when the geomagnetic activity is already elevated. We also investigate the semiannual variation in storm occurrence and confirm previous findings that geomagnetic storms tend to occur less frequently near solstices and that this tendency increases with storm intensity. However, we find that the semiannual variation depends on both the solar wind source and the storm level. Storms associated with weak SSC do not show any semiannual variation, in contrast to weak storms without SSC.
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 Fexii bandpass at 195 Å 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.PUBLICATION ABSTRACT
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