Long-term Variations in the Intensity of Plages and Networks as Observed in Kodaikanal Ca-K Digitized Data
In our previous article (Priyal et al. in Solar Phys. 289, 127, 2014) we have discussed the details of observations and methodology adopted to analyze the Ca-K spectroheliograms obtained at the Kodaikanal Observatory (KO) to study the variation of Ca-K plage areas, enhanced network (EN), and active network (AN) for Solar Cycles, namely 19, 20, and 21. Now, we have derived the areas of chromospheric features using KO Ca-K spectroheliograms to study the long-term variations of Solar Cycles 14 to 21. The comparison of the derived plage areas from the data obtained at the KO observatory for the period 1906 – 1985 with that of MWO, NSO for the period 1965 – 2002, earlier measurements made by Tlatov, Pevtsov, and Singh (Solar Phys. 255, 239, 2009) for KO data, and the SIDC sunspot numbers shows a good correlation. The uniformity of the data obtained with the same instrument remaining with the same specifications provided a unique opportunity to study long-term intensity variations in plages and network regions. Therefore, we have investigated the variation of the intensity contrast of these features with time at a temporal resolution of six months assuming that the quiet-background chromosphere remains unchanged during the period 1906 – 2005 and found that the average intensity of the AN, representing the changes in small-scale activity over solar surface, varies with solar cycle being less during the minimum phase. In addition, the average intensity of plages and EN varies with a very long period having a maximum value during Solar Cycle 19, which was the strongest solar cycle of twentieth century.
KeywordsSun Chromosphere Active and quiet Intensity variations Solar cycle
We thank T.G. Priya, K. Amareswari, Nazia, S.M.A. Fathima, S. Kamesh, R. Janani, M. Aysha Banu, and P. Sathish for their help in digitizing the photographic images and running of computer programs to analyze the data. We also thank the large number of observers who have obtained the data and maintained the data in good environment conditions. This project to digitize the photographic solar data at KO was planned, designed, digitized, calibrated the data by Jagdev Singh and a team trained by him. The authors thank the referee for insightful comments on the original version of the manuscript.
Disclosure of Potential Conflicts of interest
The authors declare that they have no conflicts of interest.