Determining the Variations of Ca-K index and Features using a Century Long Equal Contrast Images from Kodaikanal Observatory
In the earlier analysis of Ca-K spectroheliograms obtained at Kodaikanal Observatory, the "Good" images were used to investigate variations in the chromosphere. Still, the contrast of the images varied on a day-to-day basis. We developed a new methodology to generate images to form a uniform time series. We adjusted each image's contrast until the FWHM of the normalized intensity distribution attained a value between 0.10 and 0.11. This methodology of the "Equal Contrast technique" is expected to compensate for the change of emulsion, development, contrast of the images due to centering of Ca-K line on the exit slit and sky transparency. Besides, this procedure will correct variations in density-to-intensity conversion for different images. We find that the correlation between sunspot and Ca-K line data improves by a large amount. For example, correlation coefficient (CC) between monthly averaged sunspots and Ca-K plage areas for the equal contrast data improves to 0.9 compared to 0.75 for the "Good" data with unequal contrast. The CC for equal contrast images improves to $\sim$0.78 from $\sim$0.46 for the "Okay" data with unequal contrast. Even the CC between the plage area and the daily sunspot number is 0.85 for 100 years of data. This methodology also permits us to study the variations in Enhanced, Active, and Quiet networks with time along with good accuracy for about a century, for the first time. Further, this procedure can be used to combine data from different observatories to make a long time series.