Investigation of the Long-term Variation of Solar Ca ii K Intensity. I. Density-to-intensity Calibration Formula for Historical Photographic Plates

Abstract

Abstract This paper presents a novel formula to convert photographic density into intensity for historical photographic plates of the Sun. Photographic photometry is of great importance as the very first step of analyzing century-long archived photographic data to study the long-term variation. The calibration formula is newly developed by using the intensity fluctuation originating in the network pattern in the quiet region, taking into consideration the nonlinearity between the density and the intensity. The formula is useful especially when experimentally obtained characteristic curves are insufficient for homogeneous analysis of a whole long-term data set. An example procedure of the calibration using the formula is demonstrated. Resultant intensity images calibrated from the photographic data can reproduce both the fluctuation and the center-to-limb variation of modern reference data. The calibration procedure is also tested with a whole twentieth century data set and shows good performance, while the preliminary result of the long-term intensity variation seems to be partially affected by the quality of the original plates. The resultant solar Ca II K intensity variation will be updated and discussed in a subsequent paper with the comparison to the solar UV variation related to the terrestrial environment.