Sunspot-Cycle Evolution of Major Periodicities of Solar Activity

Solar activity is generally periodic in nature, having a main periodicity of ≈ 11 years, known as the solar cycle. Sunspots are one of the visible indicators of this cycle, and its different characteristics have been known for many years. Due to the presence of irregularities – long and short-term trends, stochasticity, etc. – it is difficult to predict solar activity. In sunspot cycles, amplitude and duration are highly varying parameters, making their prediction challenging. In this study, wavelet analysis of sunspot numbers for the period 1819 – 2019 is carried out to find the long- and short-term periodic variations in different cycles and changes in cycle duration. The assumptions of the flux-transport dynamo model of the sunspot cycle are used as the basis for the interpretation of the results. The results bring out 10.1-, 10.8-, and 11.6-year periods as the basic periodicities of the sunspot cycle and the ones having the highest strength in a sunspot cycle. Further, their amplitudes have been found to depend on whether the solar cycle belongs to a grand episode or a regular one. Solar-cycle duration is mainly determined by the length of the basic periodicities of the sunspot cycle, and major irregularities in the duration of Solar Cycles 9, 20, and 23 are caused by modulation of periodic trends. Since all the types of variations shown by the duration of the sunspot cycle appear related to the meridional-flow circulation, the results of the study explain the characteristic changes of this circulation postulated by the flux-transport dynamo model.