Abstract:
We analyze historical Ca II K images from the Kodaikanal Observatory (KO) spanning 1907 to 1996, encompassing Solar Cycles 14 through 22. These digitized images were processed using the Equal Contrast Technique (ECT) to ensure uniform data quality for studying long and short-term variations. From these standardized images, we identify and compute the areas of both plages and network regions in both solar hemispheres in every image. We then utilizy this revised, uniform Ca II K plage area time series for Solar Cycles 14 to 22. Our primary objective is to investigate the presence of short, Rieger-type periods and quasi-biennial oscillations (QBOs), specifically those near ≈ 1.3 years. To achieve this, we employ both Lomb-Scargle periodograms and Morlet wavelet maps. Our power spectrum analysis consistently shows that Rieger-type periods are significant across all solar cycles, in both the northern and southern hemispheres and in the whole disk data. However, the wavelet analysis reveals that both Rieger-type and QBO periodicities are intermittent, exhibiting varying periods in different cycles and hemispheres. This indicates that plages and network areas demonstrate asymmetric behavior between the two hemispheres. We have also discussed the potential reasons behind these observed periodicities.
