Digitized archive of the Kodaikanal images: Representative results of solar cycle variation from sunspot area determination

Abstract

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.

Kodaikanal digitized white-light data archive (1921–2011): Analysis of various solar cycle features

“…For a better use of the scientific data, a new program for the high resolution digitization and calibration of the solar images recorded in the photographic plates and films, was initiated a few years back at Kodaikanal. The first representative results (for three solar cycles), from this digitization, was presented in Ravindra et al (2013). The raw images have been digitized by a 4k×4k scientific grade CCD camera using a uniform source sphere digitizer unit.…”

“…The raw images have been digitized by a 4k×4k scientific grade CCD camera using a uniform source sphere digitizer unit. Calibration of the digitized data includes flat-fielding, conversion from emulsion density to relative density, identification of solar radius and the Sun center along with rotation correction (see Ravindra et al (2013) for details of each process). Calibrated white-light data covers the period from 1920 to 2011.…”

“…Here we must emphasize the fact that though the optics was unchanged for the past 90 years, but the image quality may have gone thorough several changes over this period due various reasons such as variable seeing, change of storage device from photographic plates to films and so on. These aspects of the time series for the Kodaikanal white-light digitized images have been studied thoroughly by Ravindra et al (2013).…”

“…This modified version of the code uses two intensity thresholds, in contrast to a single thresholding used in the original code. The working principle of this code is explained in details in Ravindra et al (2013). Although this procedure detects the sunspots automatically but it occasionally picks up dirt and the east-west line in the image.…”

“…These artifacts have been removed manually by checking every contour drawn on the detected spots and using a cursor to rejects those which does not belong to a sunspot. Also, we have modified the code to get the position information (longitude and latitude from the 'center of gravity' method) of the detected sunspots which was not implemented in the code used in Ravindra et al (2013). Figure 1 shows a representative image of Kodaikanal white-light data where the detected sunspots are shown with the contours overplotted on them.…”