Abstract:
The Kodaikanal Observatory has provided long-term synoptic observations of chromospheric activities in the Ca ii K line (393.34 nm) since 1907. This article investigates temporal and periodic variations of the hemispheric Ca–K-index time series in the low-latitude zone (±40°), utilizing the recently digitized photographic plates of Ca–K images from the Kodaikanal Observatory for the period of 1907–1980. We find that the temporal evolution of the Ca–K index differs from one hemisphere to another, with the solar cycle peaking at different times in the opposite hemisphere, except for cycles 14, 15, and 21, when the phase difference between the two hemispheres was not significant. The monthly averaged data show a higher activity in the northern hemisphere during solar cycles 15, 16, 18, 19, and 20, and in the southern hemisphere during cycles 14, 17, and 21. We notice an exponentially decaying distribution for each hemisphere's Ca–K index and the whole solar disk. We explored different midterm periodicities of the measured Ca–K index using the wavelet technique, including Rieger-type and quasi-biennial oscillations on different timescales present in the time series. We find a clear manifestation of the Waldmeier effect (stronger cycles rise faster than the weaker ones) in both the hemispheres separately and the whole disk in the data. Finally, we have found the presence of the Gnevyshev gap (time interval between two cycle maxmima) in both the hemispheric data during cycles 15 to 20. Possible interpretations of our findings are discussed with the help of existing theoretical models and observations.
