New reconstruction of the sunspot group numbers since 1739 using direct calibration and “backbone” methods

Abstract

Context. The group sunspot number (GSN) series constitute the longest instrumental astronomical database providing information on solar activity. This database is a compilation of observations by many individual observers, and their inter-calibration has usually been performed using linear rescaling. There are multiple published series that show different long-term trends for solar activity. Aims. We aim at producing a GSN series, with a non-linear non-parametric calibration. The only underlying assumptions are that the differences between the various series are due to different acuity thresholds of the observers, and that the threshold of each observer remains constant throughout the observing period. Methods. We used a daisy chain process with backbone (BB) observers and calibrated all overlapping observers to them. We performed the calibration of each individual observer with a probability distribution function (PDF) matrix constructed considering all daily values for the overlapping period with the BB. The calibration of the BBs was carried out in a similar manner. The final series was constructed by merging different BB series. We modelled the propagation of errors straightforwardly with Monte Carlo simulations. A potential bias due to the selection of BBs was investigated and the effect was shown to lie within the 1σ interval of the produced series. The exact selection of the reference period was shown to have a rather small effect on our calibration as well. Results. The final series extends back to 1739 and includes data from 314 observers. This series suggests moderate activity during the 18th and 19th century, which is significantly lower than the high level of solar activity predicted by other recent reconstructions applying linear regressions. Conclusions. The new series provides a robust reconstruction, based on modern and non-parametric methods, of sunspot group numbers since 1739, and it confirms the existence of the modern grand maximum of solar activity in the second half of the 20th century.

Reconstruction of the Sunspot Group Number: The Backbone Method

“…An application of the 'correction matrix' method has recently been published by Chatzistergos et al [2017]. Unfortunately, the article is marred by the usual misrepresentations.…”

“…As we noted in Section 9, one should not invent group numbers when there is no activity. The Chatzistergos et al [2017] reconstruction has the usual problem shared with Usoskin et al [2016] of being too high by ~0.3 groups at sunspot minimum, otherwise the relationship with the Svalgaard & Schatten [2016] reconstruction shows close to perfect proportionality (R 2 = 0.997), belying their claim that "such methods assume that counts by two observers are proportional to each other, which is generally not correct". Downscaling the annual Chatzistergos et al [2017] values by the linear fit y = 0.956 x -0.311 to put them on the Wolfer Backbone scale established by Svalgaard & Schatten [2016] removes the solar minimum anomaly and shows that the two methods (when the data are good) agree extremely well, Figure 38, regardless of the persistent claim that the Svalgaard & Schatten [2016] backbone method is generally invalid and unsound compared to the "modern and non-parametric" methods advocated by Chatzistergos et al [2017] and Usoskin et al [2016].…”

Sunspot Observations Made by Hallaschka During the Dalton Minimum

“…Carrasco et al 2 (Clette et al, 2014). Thus, new series related with the sunspot number have been recently published in order to solve the problems detected (Clette and Lefèvre, 2016;Lockwood et al, 2016;Svalgaard and Schatten, 2016;Usoskin et al, 2016;Chatzistergos et al, 2017).…”

Sunspot observations by Hisako Koyama: 1945–1996

“…This disagreement leads to incompatible interpretations of the evolution of solar activity before 1885 (Cliver, 2016(Cliver, , 2017Cliver and Ling, 2016;Usoskin, 2017). Therefore, a thorough revision of the calibration of both series is currently ongoing on the basis of original observations and using new modern methodologies (Clette et al, 2014Vaquero et al, 2016;Svalgaard and Schatten, 2016;Usoskin et al, 2016;Cliver and Ling, 2016;Chatzistergos et al, 2017;Muñoz-Jaramillo and Vaquero, 2019). While observations were conducted over many decades in various institutional observatories such as Greenwich (Willis et al, 2013(Willis et al, , 2016a(Willis et al, , 2016b, Debrecen (Baranyi et al, 2016;Győri et al, 2017), or Mount Wilson (Lefèvre et al, 2005Pevtsov et al, 2019), long-term observations at smaller observatories by individual astronomers -frequently skilled amateur observers -are of particular importance in terms of stability.…”