Long-term Variations in the Intensity of Plages and Networks as Observed in Kodaikanal Ca-K Digitized Data

Abstract

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.

21 pages, 11 Figures

References in corpus (2)

• Sun-as-a-Star Spectrum Variations 1974-2006
• The Mount Wilson Ca II K index

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• Determining the Variations of Ca-K index and Features using a Century Long Equal Contrast Images from Kodaikanal Observatory
• Variation of Chromospheric Features as a Function of Latitude and Time using Ca-K Spectroheliograms for Solar Cycles 15-23: Implications for Meridional Flow
• Variation of small scale magnetic fields over a century using Ca-K images as proxy
• An Image Processing approach to identify solar plages observed at 393.37 nm by the Kodaikanal Solar Observatory