A BUTTERFLY DIAGRAM AND CARRINGTON MAPS FOR CENTURY-LONG Ca ii K SPECTROHELIOGRAMS FROM THE KODAIKANAL OBSERVATORY

Abstract

The century-long Ca II K spectroheliograms from the Kodaikanal Solar Observatory (KSO) are calibrated, processed, and analyzed to follow the evolution of the bright on-disc structures called plages, possible representatives of magnetic activity on the Sun. This is the longest data set studied in Ca II K to date, covering about 9.5 cycles of 11 yr periods. Plages are segmented with area 1 arcmin 2 using global thresholds for individual full disc images and subsequent application of a morphological closing operation. The plage index is calculated and is seen to have a close positive correlation with the fractional disc area covered by plages. The newly generated plage area cycle (from KSO) was compared with the same from the Mount Wilson Observatory (correlation 95.6%) for the overlapping years, i.e., 1915-2000. This study illustrates the time-latitude distribution of plage centroids by rendering a butterfly diagram (as observed for sunspots). The 3D visualization of the diagram shows one-to-one mapping between plage location, time, and area. This work further delineates the positional correlation between magnetic patches and plage regions through the comparison of synoptic maps derived from both KSO Ca II K images and space-based full disc line-of-sight magnetograms. Regular synoptic magnetograms from ground-based observatories are available only after 1970s. Thus the long term Ca II K data from KSO can be used as a proxy for estimating magnetic activity locations and their strengths at earlier times.

Analysis of full disc Ca II K spectroheliograms

“…value of R/6. This finding is in contrast to that of Chatterjee et al (2016) who used a window width of ∼R/13. Figure 11 shows one example of testing different window widths on an image from subset 1 (shown in Fig.…”

“…Some examples are computations that result in radially symmetric backgrounds (Brandt & Steinegger 1998;Walton et al 1998;Johannesson et al 1998;Denker et al 1999;Zharkova et al 2003), application of a median filter (Lefebvre et al 2005;Bertello et al 2010;Chatterjee et al 2016), 2D polynomial fittings to the entire image (Worden et al 1998b;Caccin et al 1998), or interpolation of the mode pixel values within radial and azimuthal disc sectors (Tlatov et al 2009). However, all of these methods are unable to thoroughly account for the artefacts affecting historical data.…”

“…The long list of references above show that many methods have been employed for processing and calibrating historical (Priyal et al 2013;Chatterjee et al 2016). (b) The two values correspond to the period before and after 02/03/1960, when the images were stored on photographic plates and on 24 × 35 mm sheet film, respectively.…”

Variation of Supergranule Parameters with Solar Cycles: Results from Century-long Kodaikanal Digitized Ca ii K Data

“…The primary data used in this study are taken from the digitized archive of the Kodaikanal Ca II K observations 1 (Priyalet al 2014;Chatterjee et al 2016). Apart from that we have also used data from Precision Solar Photometric Telescope (PSPT) (Ermolli et al 1998).…”

“…To identify the locations of the ARs from the Ca II K images, we have used the plage locations as proxies for the magnetic field (Sheeley et al 2011). All the full disc limb darkening corrected KSO Ca II K images were used to detect plages with a fully-automated method as described in Chatterjee et al (2016). Next we used a rectangular mask, around each of the detected plage, with sides 3 times the maximum distances of plage structure coordinates from centroid along X and Y.…”