The Contribution of Large Recurrent Sunspot Groups to Solar Activity: Empirical Evidence

Abstract

We identify large sunspot nestlets (SN) mostly containing recurrent sunspot groups and investigate the indices of solar activity defined as the 11- or 22-year moving average of the daily areas of the SN. These nestlets, 667 in total, are constructed from the daily 1874–2020 RGO/SOON catalogue, which contains 41,394 groups according to their IDs, with a machine-learning technique. Within solar cycles 15–19, the index contributed disproportionately strongly to the overall solar activity: the index is normalized to a quasi-constant shape by a power function of the activity, where the exponent is approximately 1.35. Large SN contribute to solar activity even more in cycle 22, underlying the second largest peak of solar activity within the last Gleissberg cycle in ∼1985. Introducing another composite, moderate SN normalized by the overall activity, we observe its quasi-constant shape in cycles 15–19 and a general anti-correlation with the first normalized composite. The constructed sunspot nestlets constitute a modified catalogue of solar activity. We define the average lifetime per day in 22-year windows for the modified catalogue, in line with Henwood et al. (SoPhys 262, 299, 2010), and reproduce the dynamics of this quantity they revealed for 1900–1965. The average lifetime derived from the moderate SN is found to form a wave with minima at the beginning of the 20th and 21st centuries, resembling the Gleissberg cycle with long minima. The average lifetime characterizing large SN exhibited a deeper minimum at the beginning of the 20th century than 100 years later.