COMPARISON AMONG Ca II K SPECTROHELIOGRAM TIME SERIES WITH AN APPLICATION TO SOLAR ACTIVITY STUDIES

Abstract

Various observatories around the globe started regular full-disk imaging of the solar atmosphere in the Ca ii K line in the early decades of the 20th century. The archives made by these observations have the potential of providing far more detailed information on solar magnetism than just the sunspot number and area records to which most studies of solar activity and irradiance changes are restricted. We evaluate the image quality and contents of three Ca ii K spectroheliogram time series, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives, in order to estimate their value for studies focusing on timescales longer than the solar cycle. We analyze the quality of these data and compare the results obtained with those achieved for similar present-day observations taken with the Meudon spectroheliograph and with the Rome-PSPT. We also investigate whether image-segmentation techniques, such as those developed for identification of plage regions on present-day Ca ii K observations, can be used to process historic series. We show that historic data suffer from stronger geometrical distortions and photometric uncertainties than similar presentday observations. The latter uncertainties mostly originate from the photographic calibration of the original data and from stray-light effects. We also show that the image contents of the three analyzed series vary in time. These variations are probably due to instrument changes and aging of the spectrographs used, as well as changes of the observing programs. The segmentation technique tested in this study gives reasonably consistent results for the three analyzed series after application of a simple photographic calibration. Although the plage areas measured from the three analyzed series differ somewhat, the difference to previously published results is larger.

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

“…The plot shows that there is a slight variation in the ellipticity, which could be due to the quality of the image, or to a slight variation in fitting the elliptical to the detected limb points. The ellipticity value is also very low and is close to the ellipticity value of the recent data taken with the CCD cameras (Ermolli et al 2009). We would like to point out here that we did not measure the ellipticity that can arise from the atmospheric refraction.…”

“…This procedure is similar to the one adopted by Ermolli et al (2009) to calibrate the Ca II K images obtained from various observatories around the world. …”

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.…”

Recent variability of the solar spectral irradiance and its impact on climate modelling

“…Reconstructions going back to the early 20th century can be derived from semi-empirical models based on historical ground-based solar observations (e.g. Ermolli et al, 2009). …”