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

TL;DR

This paper digitized over a century of Kodaikanal solar images, calibrated them, and analyzed sunspot areas to study solar cycle variations, demonstrating the data's consistency and correlation with established records.

Contribution

It provides a digitized, calibrated archive of Kodaikanal solar images and presents preliminary analysis of sunspot area variations over 90 years, enhancing long-term solar activity studies.

Findings

Sunspot areas closely match Greenwich data across four solar cycles.

Images have uniform spatial resolution over 90 years.

High correlation between yearly sunspot areas and Greenwich data.

Abstract

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. 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 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. 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 The Kodaikanal data archive is hosted at http://kso.iiap.res.in. The long time sequence of the Kodaikanal white light 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.