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

Abstract

Context. Long-term sunspot observations are key to understanding and predicting the solar activities and its effects on space weather. Consistent observations, which are crucial for long-term variations studies, are generally not available due to upgradation/modification of observatories over the course of time. We present data for a period of 90 yr acquired from persistent observation at the Kodaikanal observatory in India.

Aims. We aim to build a uniform sunspot area time series along with their positions for a 90-yr period between 1921 and 2011, as obtained from the newly digitized and calibrated white-light images from the Kodaikanal observatory. Our aim is to compare this new time series with known sources and confirm some of the earlier reported results with additional new aspects.

Methods. We use an advanced semi-automated algorithm to detect the sunspots form each calibrated white-light image. Area, longitude and latitude of each of the detected sunspots are derived. Implementation of a semi-automated method is extremely necessary in such studies as it minimizes the human bias in the detection procedure.

Results. Daily, monthly, and yearly sunspot area variations, obtained from the Kodaikanal, compared well with the Greenwich sunspot area data. We find an exponentially decaying distribution for the individual sunspot area for each of the solar cycles. Analyzing the histograms of the latitudinal distribution of the detected sunspots, we find Gaussian distributions, in both the hemispheres, with centers at ~15° latitude. The height of the Gaussian distributions are different for the two hemispheres for a particular cycle. Using our data, we show clear presence of Waldmeier effect, which correlates the rise time with the cycle amplitude. Using the wavelet analysis, we explored different periodicities on different time scales present in the sunspot area times series.