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The Ratio Between the Number of Sunspot and the Number of Sunspot Groups1 ∑

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The Ratio Between the Number of Sunspot and the Number of Sunspot Groups1 ∑ ISSN 0016-7932, Geomagnetism and Aeronomy, 2017, Vol. 57, No. 7, pp. 1–7. © Pleiades Publishing, Ltd., 2017. The Ratio Between the Number of Sunspot and the Number of Sunspot Groups1 K. Georgievaa, A. Kilçikb, Yu. Nagovitsync, and B. Kirova aSRTI-BAS, Sofia, Bulgaria bAkdeniz University, Antalya, Turkey cCentral Astronomical Observatory at Pulkovo, St. Petersburg, Russia Received March 9, 2017; in final form, March 27, 2017 Abstract⎯Data from three solar observatories (Learmonth, Holloman, and San Vito) are used to study the variations in the average number of sunspots per sunspot group. It is found that the different types of sunspot groups and the number of sunspots in these groups have different solar cycle and cycle to cycle variations. The varying ratio between the average number of sunspots and the number of sunspot groups is shown to be a real feature and not a result of changing observational instruments, observers’ experience, calculation schemes, etc., and is a result of variations in the solar magnetic fields. Therefore, the attempts to minimize the discrep- ancies between the sunspot number and sunspot group series are not justified, and lead to the loss of import- ant information about the variability of the solar dynamo. DOI: 10.1134/S001679321707009X 1. INTRODUCTION reflect the magnetic activity of the Sun and are related to geoeffective solar events, so they have been widely Sun is the main source of energy for the Earth’s used to evaluate the long-term evolution of solar activ- system, providing orders of magnitude more energy ity, and its effects on the terrestrial system. than all other extraterrestrial sources taken together The original “relative sunspot number”, known (irradiance from distant stars, gamma ray bursts, also as “Wolf number” or “Zurich international sun- galactic cosmic rays, etc.). Sun emits a continuous but spot number”, R , was defined by Wolf as variable flow of matter (the solar wind – the ever Z expanding solar corona), recurrent high speed solar RZ = k(10g + n), (1) wind streams from long-living solar coronal holes where n is the number of individual sunspots, g – the which persist for several solar rotations and bathe the number of sunspot groups, and k is the correction fac- Earth each time their source region is in a geoeffective tor for each observer accounting for the differences in position, outbursts of plasma with embedded mag- instruments, measurement techniques, viewing con- netic fields from the corona (coronal mass ejections), ditions, observers’ experience, etc. (Waldmeier, 1961). solar energetic particles associated with strong impul- The yearly/monthly values of the RZ series cover the sive solar events, and electromagnetic radiation across period from 1700/1749, respectively, to June 2015. virtually the entire range of wavelengths (the total solar irradiance), with sporadic sudden, rapid, and intense The group sunspot number RG was introduced by variations in brightness (solar flares). All variations in Hoyt and Schatten (1998). It is based on the parameter the energy output and the associated outlook of the g in equation (1) – the number of sunspot groups Sun are defined as “solar activity”. They are all mani- which is more reliably determined and allows the festations of the solar magnetic field which is in turn inclusion of earlier observations, so it expands the the result of the action of the solar dynamo. record back to the earliest telescopic observations in 1610. RG is defined as There are different indicators of solar activity, reflecting different solar processes and using different RkG= 12.08∑ ' , (2) direct or indirect measurable or proxy parameters. The GiiN number of sunspots and the number of sunspot groups where G is the number of sunspot groups observed by are the longest instrumental data records of solar i ' activity. Sunspots – dark spots on the solar surface – the i-th observer, ki is the i-th observer’s correction do not themselves in any way affect the Earth, but they factor, N is the number of observations used to calcu- late RG, and 12.08 is a normalization number chosen to 1 The article is published in the original. make the mean RG’s identical with the mean RZ’s for 1 2 GEORGIEVA et al. 18 2.0 16 1.8 G Z / 14 Z R 1.6 / R N 12 S Ratio Ratio 1.4 Ratio Ratio 10 1.2 8 1870 1890 1910 1930 1950 1970 1.0 Year 1700 1750 1800 1850 1950 2000 Fig. 1. Variations in the ratio between the International Zurich Fig. 2. The corrections applied to RZ to transform it into sunspot number RZ and the number of sunspot groups. the new series SN. the period 1874 to 1976 when the Royal Greenwich director of the Swiss Federal Institute of Technology Observatory actively made sunspot observations J.O. Stenflo to terminate the 130-year-long Zürich (Hoyt and Schatten, 1998). sunspot number observational program initiated by However, even in this limited period, the ratio R. Wolf (Clette et al., 2007). Figure 2 illustrates the corrections applied to R to between RZ and RG is not constant, and displays long- Z term quasi-cyclic variations (Fig. 1). transform it into the new series SN. As a result of this transformation, the overall level of solar activity as On longer time-scales, both R and R have upward Z G measured by S was significantly increased as com- trends since the Maunder minimum, but the trend is N pared to R (the ratio S /R is always greater than 1), larger for R than for R . This means that using R Z N Z G Z G and the “Modern Grand Maximum” in the second would imply larger long-term solar variability, and half of the 20th century (Usoskin et al., 2007) was consequently more important impact of the Sun on reduced to an ordinary secular maximum of solar the terrestrial variability, for example climate changes, activity, not significantly different from the secular than using . RZ maxima in previous centuries (the ratio since 1950 is The different trends of the two sunspot indices have much lower than the ratio from 1700 to 1950). Conse- inspired efforts to recalibrate the two time series with quently, the Sunspot Group Number data series RG the aim to “rectify discrepancy between Group and was also reconstructed, and a new series GN was pro- International sunspot number series”, and to publish duced (Svalgaard and Schatten, 2016), which matches “a vetted and agreed upon single sunspot number time the new Sunspot Number series SN very closely, and series” (Cliver et al., 2013; Clette et al., 2014), because both indices have practically no long-term trends, “given the importance of the reconstructed time unlike the original RZ and RG. This has important series, the co-existance of two conflicting series is a implications for the evaluation of solar activity effects on highly unsatisfactory solution which should be actively terrestrial processes. For example, at a press briefing addressed” (Cliver et al., 2013). during the IAU XXIX General Assembly in 2015 it was In September 2011, a “Sunspot Number (SSN) announced that the corrected sunspot history suggests Workshop” was held in Sunspot, New Mexico, USA that “rising global temperatures since the industrial to address this question, followed by several other sim- revolution cannot be attributed to increased solar ilar workshops during 2012–2014 (http://ssnwork- activity” (https://www.iau.org/news/pressreleases/ shop.wikia.com/wiki/Home). The net result of this detail/iau1508/). activity was that since July 1st 2015, the Sunspot Index In this way, the goal to “rectify discrepancy Data Center in Brussels terminated the more than between Group and International sunspot number 400 year long data series of the International relative sun- series” was fulfilled. But the other stated goal – to spot number RZ, and replaced it by “a new entirely revised publish an “agreed upon single sunspot number time data series” SN (http://www.sidc.be/silso/datafiles). The series” was not achieved, neither for the sunspot num- Sunspot Index Data Center was created in 1980 in the ber nor for the sunspot group number. On the con- Royal Observatory of Belgium as a World Data Center trary, the presentation of the two new series was met by with the task to continue the International relative vigorous criticism, and led to the ongoing creation of sunspot number record after the decision of the new more and more still newer alternative series by scien- GEOMAGNETISM AND AERONOMY Vol. 57 No. 7 2017 THE RATIO BETWEEN THE NUMBER OF SUNSPOT 3 tists not only outside the Sunspot Workshops initiative from observatories with continuous and homogeneous group, but even by members of this very group (though much shorter) data records could give a clue [http://www.spaceclimate.fi/SC6/presentations/ses- whether the possible variations in the ratio between sion2b/Frederic_Clette_SC6.pdf]. the International Sunspot Number and the Group Clette et al. (2014) summarized the possible flaws Sunspot Number are a real feature or a result of the above reasons. in the RZ and RG series, and justified some of the cor- rections made to transform RZ into SN, and Svalgaard The data we use are from the National Geophysical and Schatten (2016) presented the “backbone Data Center (ftp://ftp.ngdc.noaa.gov/STP/SOLAR_ method” used to construct GN. In the present study we DATA) which provides various active region parame- are not dealing with the sources of uncertainties in the ters including the sunspot group (SG) classification original RZ and RG series, neither with the applied cor- and sunspot counts – the number of sunspots on the rections to remove the possible flaws, nor with the solar disk (SSC) determined for each day.
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