Long-Term Variation of the Number of Sunspots per Group

Abstract

Long-term reconstructions of sunspot number (SSN) and group number (GN) often tacitly assume that the basic characteristics of solar activity remain unchanged even over long times, e.g., that the number of sunspots and the number of sunspot groups now and, say, 100 or 500 years ago have the same relation. However, this assumption needs examination, especially as the long-term homogeneity between sunspots and several other solar activity parameters has recently been challenged (Mursula et al. 2024). Here we use long series of sunspot observations to study if and how the number of sunspots per group varies at different time scales. We use observations from the Kislovodsk Mountain Astronomical Station (KMAS) and the Solar Observing Optical Network (SOON) to create a reference series for the overlapping period of 1982 – 2016. We then scale other historical data to this reference series in order to have a unified time series for 1749 – 2024. We find that the yearly mean number of sunspots per group varies between about 2 and 10 (average of 6.8), closely in phase with the solar cycle over the whole 270-year time interval. We also find that the number of sunspots per group depicts a very similar secular variation as the sunspot number.