Sunspot number recalibration: The ~1840–1920 anomaly in the observer normalization factors of the group sunspot number

Abstract

We analyze the normalization factors (k ′-factors) used to scale secondary observers to the Royal Greenwich Observatory (RGO) reference series of the , ) group sunspot number (GSN). A time series of these k ′-factors exhibits an anomaly from 1841 to 1920, viz. , the average k ′-factor for all observers who began reporting groups from 1841 to 1883 is 1.075 vs. 1.431 for those who began from 1884 to 1920, with a progressive rise, on average, during the latter period. The 1883–1884 break between the two subintervals occurs precisely at the point where Hoyt and Schatten began to use a complex daisy-chaining method to scale observers to RGO. The 1841–1920 anomaly implies, implausibly, that the average sunspot observer who began from 1841 to 1883 was nearly as proficient at counting groups as mid-20th century RGO (for which k ′ = 1.0 by definition) while observers beginning during the 1884–1920 period regressed in group counting capability relative to those from the earlier interval. Instead, as shown elsewhere and substantiated here, RGO group counts increased relative to those of other long-term observers from 1874 to ~1915. This apparent inhomogeneity in the RGO group count series is primarily responsible for the increase in k ′-factors from 1884 to 1920 and the suppression, by 44% on average, of the Hoyt and Schatten GSN relative to the original Wolf sunspot number (WSN) before ~1885. Correcting for the early “learning curve” in the RGO reference series and minimizing the use of daisy-chaining rectifies the anomalous behavior of the k ′-factor series. The resultant GSN time series (designated GSN*) is in reasonable agreement with the revised WSN (S N *; ) and the backbone-based group sunspot number (R GS ; ) but significantly higher than other recent reconstructions (Friedli, personal communication, 2016; , ; ). This result is substantiated by a “correction-factor” (CF) time series defined as the ratio of annual group counts of the , ) series to the average raw (unscaled) group counts of all observers, as well as by a comparison of the GSN and GSN* time series with a recent reconstruction of solar wind B from 1845 to the present. The ~1840–1920 k ′-factor anomaly and its impact on the Hoyt and Schatten GSN are discussed in the context of the ongoing effort to recalibrate the sunspot number time series.