The Discontinuity Circa 1885 in the Group Sunspot Number

Abstract

On average, the international sunspot number ( $R_{\mathrm{I}}$ ) is 44 % higher than the group sunspot number ( $R_{\mathrm{G}}$ ) from 1885 to the beginning of the $R_{\mathrm{I}}$ series in 1700. This is the principal difference between $R_{\mathrm{I}}$ and $R_{\mathrm{G}}$ . Here we show that this difference is primarily due to an inhomogeneity in the Royal Greenwich Observatory (RGO) record of sunspot groups (1874 – 1976) used to derive observer normalization factors (called $k$ -factors) for $R_{\mathrm{G}}$ . Specifically, annual RGO group counts increase relative to those of Wolfer and other long-term observers from 1876 – 1915. A secondary contributing cause is that the $k$ -factors for observers who began observing before 1884 and overlapped with RGO for any years during 1874 – 1883 were not based on direct comparison with RGO but were calculated using one or more intermediary or additional observers. We introduce $R_{\mathrm{GC}}$ by rectifying the RGO group counts from 1874 – 1915 and basing $k$ -factors on direct comparison with RGO across the 1885 discontinuity, which brings the $R_{\mathrm{G}}$ and $R_{\mathrm{I}}$ series into reasonable agreement for the 1841 – 1885 interval (after correcting $R_{\mathrm{I}}$ for an inhomogeneity from 1849 – 1867 (to give $R_{\mathrm{IC}}$ )). Comparison with an independently derived backbone-based reconstruction of $R_{\mathrm{G}}$ ( $R_{\mathrm{BB}}$ ) indicates that $R_{\mathrm{GC}}$ over-corrects $R_{\mathrm{BB}}$ by 4 % on average from 1841 – 1925. Our analysis suggests that the maxima of Cycles 10 (in 1860), 12 (1883/1884), and 13 (1893) in the $R_{\mathrm{IC}}$ series are too low by ≈ 10 %.