Temporal evolutions and quasi-periodic variations present in the sunspot number and group sunspot area data measured at Kodaikanal Observatory for solar cycles 14 to 24

Belur Ravindra, Partha Chowdhury, Pratap Chandra Ray, Kumaravel Pichamani

Submitted on 3 November 2022


The Kodaikanal Observatory has made synoptic observations of the Sun in white light since 1904, and these images are sketched on the Stonyhurst grids called sun charts. These continuous hand-drawn data sets are used for long-term studies of the Sun. This article investigates temporal and periodic variations of the monthly hemispheric sunspot number and sunspot group area for 1905--2016, covering solar cycles 14 to 24. We find that the temporal variations of the sunspot number and group area are different in each hemisphere and peak at different times of the solar cycle in the opposite hemisphere. For both the data sets, Cycle 19 shows maximum amplitude. For the sunspot number time series, Cycle 24 was the weakest, and Cycle 15 for the group area. The existence of double peaks and violation of the ``odd-even rule'' was found in both data sets. We have studied the periodic and quasi-periodic variations in both the time series by wavelet technique. We noticed that along with the fundamental mode of the 11~year cycle and polarity reversal period of 22~years, the sunspot activity data also exhibited several mid-term periodicities in the opposite hemispheres, specifically the Riger group and quasi-biennial periodicities. The temporal evolution of these detected quasi-periodicities also differs in the northern and southern hemispheres. We analyzed the data set statistically to understand the bulk properties and coupling between the opposite hemispheres. The study indicates that the two hemispheric data sets differ, but some dependency could be present.


Comment: 31 pages, 19 Figures, 7 Tables, Accepted for publication

Subject: Astrophysics - Solar and Stellar Astrophysics