A New Method of Calibration of Photographic Plates from Three Historic Data Sets

Abstract

We analyze the synoptic data taken in the Ca II K spectral line with spectroheliographs at Kodaikanal Observatory from 1907 to 1999, at Mount Wilson Observatory from 1915 to 1985, and at the National Solar Observatory at Sacramento Peak from 1963 to 2002. Photographic data were digitized and calibrated following the same set of procedures developed by the authors of this paper. Using calibrated data, we have outlined bright plages and have calculated a plage index defined as the fraction of solar hemisphere occupied by the chromospheric plages and enhanced network. We present a detailed description of our method and provide a brief comparison of Ca II K plage indices derived using data from these three historic data sets.

A Century of Solar Ca ii Measurements and Their Implication for Solar UV Driving of Climate

“…This holds also for the NSO Sun-as-a-star time series for the cycles 21 -23 covered by those measurements. This agreement is consistent with the high correlation found (e.g., Tapping, 1987) between sunspot number and the F10.7 microwave index, which measures plage and network area variation somewhat higher in the chromosphere and transition region than the K-line indices.…”

“…Specifically, the major maximum in plage indices between the early 1950s and 1960s corresponds to the broad local minimum of T g spanning roughly the early 1940s through 1970s. The ocean temperature record for the mid20th century may be influenced by a recently identified change in measurement technique (Thompson et al, 2008). This correction cannot, however, explain why the dominant peak seen in the plage indices is absent in T g .…”

Solar change and climate: an update in the light of the current exceptional solar minimum

“…Foukal & Bernasconi (2008) and Foukal et al (2009) have studied historic images of the chromospheric features responsible for much of the UV variation and argue that there is no longterm change. However, the images are far from a homogeneous dataset, and this conclusion does not agree with the studies by Tlatov et al (2009) and Ermolli et al (2009). Hence, long-term change of the solar UV remains likely, if poorly understood.…”

Analysis of full disc Ca II K spectroheliograms

“…Ermolli et al (2009b) applied the method presented by Mickaelian et al (2007) for calibration of photographic plates of star surveys, which is based on information stored in the unexposed and darker parts of the plate. Tlatov et al (2009) suggested to calibrate SHG by using a linear relation between a standardized centre-to-limb variation (CLV, hereafter) profile published by Pierce & Slaughter (1977, de-rived with a second degree polynomial fit and corresponding to 390.928 nm) and the values on the analysed image at two positions.…”

“…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.…”