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In our previous article (Priyal et al., S olar Phys., 289, 127) we have discussed the details of observations and methodology adopted to analyze the Ca-K spectroheliograms obtained at Kodaikanal Observatory (KO) to study the variation of Ca-K plage areas, enhanced network (EN) and active network (AN) for the three 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 between 14 and 21. The comparison of the derived plage areas from the data obtained at 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 (2009, S olar Phys., 255, 239) for KO data and the SIDC sunspot numbers shows a good correlation. 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 intensity contrast of these features with time at a temporal resolution of 6-months assuming the quiet background chromosphere remains unchanged during the period of 1906 -2005 and found that average intensity of 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 the Solar Cycle number 19 which is the strongest solar cycle of 20th century.

The Sun has a polar magnetic field which oscillates with the 11-year sunspot cycle. This polar magnetic field is an important component of the dynamo process which is operating in the solar convection zone and produces the sunspot cycle. We have systematic direct measurements of the Sun's polar magnetic field only from about mid-1970s. There are, however, indirect proxies which give us information about this field at earlier times. The Ca-K spectroheliograms taken in Kodaikanal Solar Observatory during 1904 -2007 have now been digitized with the 4k × 4k CCD and have higher resolution (∼ 0.86 arcsec) than the other available historical datasets. From these Ca-K spectroheliograms, we have developed a completely new proxy (Polar Network Index: PNI) for the Sun's polar magnetic field. We calculate the PNI from the digitized images using an automated algorithm and calibrate our measured PNI against the polar field as measured by the Wilcox Solar Observatory for the period of 1976-1990. This calibration allows us to estimate polar fields for the earlier period up to 1904.The dynamo calculations done with this proxy as input data reproduce the Sun's magnetic behavior for the past century reasonably well.

Context. Sunspots have been observed since Galileo Galilei invented the telescope. Later, sunspot drawings have been upgraded to image storage using photographic plate in the second half of nineteenth century. These photographic images are valuable data resources for studying long-term changes in the solar magnetic field and its influence on the Earth's climate and weather. Aims. Digitized photographic plates cannot be used directly for the scientific analysis. It requires certain steps of calibration and processing before using them for extracting any useful information. The final data can be used to study solar cycle variations over several cycles. Methods. We digitized more than 100 years of white-light images stored in photographic plates and films that are available at Kodaikanal observatory starting from 1904. The images were digitized using a 4k × 4k format CCD-camera-based digitizer unit.The digitized images were calibrated for relative plate density and aligned in such a way that the solar north is in upward direction. A semi-automated sunspot detection technique was used to identify the sunspots on the digitized images.Results. In addition to describing the calibration procedure and availability of the data, we here present preliminary results on the sunspot area measurements and their variation with time. The results show that the white-light images have a uniform spatial resolution throughout the 90 years of observations. However, the contrast of the images decreases from 1968 onwards. The images are circular and do not show any major geometrical distortions. The measured monthly averaged sunspot areas closely match the Greenwich sunspot area over the four solar cycles studied here. The yearly averaged sunspot area shows a high degree of correlation with the Greenwich sunspot area. Though the monthly averaged sunspot number shows a good correlation with the monthly averaged sunspot areas, there is a slight anti-correlation between the two during solar maximum. Conclusions. The Kodaikanal data archive is hosted at http://kso.iiap.res.in. The long time sequence of the Kodaikanal whitelight images provides a consistent data set for sunspot areas and other proxies. Many studies can be performed using Kodaikanal data alone without requiring intercalibration between different data sources.

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