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., 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.

Delving into the Historical Ca ii K Archive from the Kodaikanal Observatory: The Potential of the Most Recent Digitized Series

“…Our results for the plage areas from DS2 data show worse agreement with the series by Ermolli et al (2009b) and Tlatov, Pevtsov, and Singh (2009) from analysis of DS1 data than our results from DS1 data. However, we also notice a worse agreement with our results from DS2 data and those by Priyal et al (2017) and Singh et al (2018) from analysis of DS2 data than our results from DS1 data.…”

“…The number of common days is given within the brackets. The abbreviations CEA16, EEA09, PEA17, and SEA18 refer to the Chatterjee, Banerjee, and Ravindra (2016), Ermolli et al (2009b), Priyal et al (2017), and Singh et al (2018) The error bars denote the RMS error in the derived plage areas due to the disc ellipticity and the processing to photometrically calibrate the images (the latter is applicable only to the Kodaikanal data) as found by Chatzistergos et al (2019b). Bottom: Difference of plage areas derived from the 757 images common to DS2 and Rome/PSPT series plotted against the number of the image.…”

“…Figure 15 shows scatter plots between the plage areas derived by us from the DS2 data and the various published series obtained from Kodaikanal observations. In particular we consider the series by Kuriyan, Muralidharan, and Sampath (1983) derived from the physical photographs, the series by Ermolli et al (2009b) and Tlatov, Pevtsov, and Singh (2009) from the DS1 data, and the series by Chatterjee, Banerjee, and Ravindra (2016), Priyal et al (2017), and Singh et al (2018) from the DS2 data. Note that the series by Kuriyan, Muralidharan, and Sampath (1983) and Tlatov, Pevtsov, and Singh (2009) are only available as annual values and most likely include different selections of observations.…”

“…This information was not stored, but rather was used to crop and center the images. These cropped images were then used by Chatterjee, Banerjee, and Ravindra (2016), Priyal et al (2017), andSingh et al (2018) to derive their plage areas. Ermolli et al (2009b) and Tlatov, Pevtsov, and Singh (2009) defined the radius and center coordinates for DS1 independently.…”

“…The availability of the Ca II K series as digital data has initiated their extensive exploitation for studies of the long-term variation of the chromospheric magnetic field and for a variety of retrospective analyses. For example, Foukal (1996), Ermolli et al (2009b), Tlatov, Pevtsov, and Singh (2009), Chatterjee, Banerjee, and Ravindra (2016), and Priyal et al (2017) presented plage-area time series derived from the analysis of different archives with distinct image-processing methods. Harvey (1992), Ermolli et al (2009a), and Chatterjee, Banerjee, and Ravindra (2016) produced butterfly diagrams of plage regions from the Mt.…”