“Generative” Indices of Sunspot Solar Activity: 145-Year Composite Series

Abstract

Traditional indices, which we call apparent indices, describe the current state of solar activity and are very important for understanding the influence of the Sun on Earth. To study the productivity of the solar dynamo, however, we need a different index that would describe each magnetic region once – at the moment of its maximum area. We refer to such indices as generative ones. The aim of this work is to produce such a generative series improving the procedure by Nagovitsyn et al. (Geomagn. Aeron. 58, 1170, 2018). We use sunspot group data from two archives – Royal Greenwich Observatory (1875 – 1976) and Kislovodsk Mountain Astronomical Station (1955 – 2019). We produce generative indices with annual values of sunspot group areas $A$ , and counts of sunspot groups $C$ . Unified 145-year series of these indices were compiled. We study the characteristics of the indices $A$ and $C$ , and indices derived from them for two sunspot group populations – large long-living groups and small short-living groups according to Nagovitsyn and Pevtsov (Astrophys. J. 833, 94, 2016) – for the northern and southern hemispheres separately. We find a 50-year variation in several indices on the 145-year interval, and detect a similar phase variation in the north-south asymmetry taken with the opposite sign. Perhaps this indicates the existence of another secular cycle along with the Gleissberg cycle on the Sun. The obtained series of indices $A$ and $C$ can be useful for studying the long-term properties of the solar dynamo.