Determining the variations of Ca–K index and features using century-long equal-contrast images from Kodaikanal observatory

Abstract

In an earlier analysis of Ca–K spectroheliograms obtained at the Kodaikanal Observatory, the "Good" images were used to investigate variations in the chromosphere. However, the contrast of the images varied on a day-to-day basis. We have 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 "equal-contrast technique" is expected to compensate for the change of emulsion, development, and contrast of the images due to centering of the Ca–K line on the exit slit and sky transparency. In addition, 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 to a large extent. For example, the correlation coefficient (CC) between monthly averaged sunspots and Ca–K plage areas for the equal-contrast data reaches 0.9 compared to 0.75 for the "Good" data with unequal contrast. The CC for equal-contrast images improves to ~0.78 from ~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, for the first time, to study the variations in enhanced, active, and quiet networks with time with high accuracy over about a century. Further, this procedure can be used to combine data from different observatories to make a long time series.