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.
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.
Long-term synoptic observations in the resonance line of Ca ii K constitute a fundamental database for a variety of retrospective analyses of the state of the solar magnetism. Synoptic Ca ii K observations began in late 1904 at the Kodaikanal Observatory, in India. In early 1970s, the National Solar Observatory (NSO) at Sacramento Peak (USA) started a new program of daily Sun-as-a-star observations in the Ca ii K line. Today the NSO is continuing these observations through its Synoptic Optical Long-term Investigations of the Sun (SOLIS) facility. These different data sets can be combined into a single disk-integrated Ca ii K index time series that describes the average properties of the chromospheric emission over several solar cycles. We present such a Ca ii K composite and discuss its correlation with the new entirely revised sunspot number data series. For this preliminary investigation, the scaling factor between pairs of time series was determined assuming a simple linear model for the relationship between the monthly mean values during the duration of overlapping observations.
Spectroheliograms and disk-integrated flux monitoring in the strong resonance line of Ca II (K line) provide the longest record of chromospheric magnetic plages. We compare recent reductions of the Ca II K spectroheliograms obtained since 1907 at the Kodaikanal, Mt. Wilson, and US National Solar Observatories. Certain differences between the individual plage indices appear to be caused mainly by differences in the spectral passbands used. Our main finding is that the indices show remarkably consistent behavior on the multidecadal time scales of greatest interest to global warming studies. solar ultraviolet flux variation from these indices differs significantly from the 20th-century global temperature record. This difference is consistent with other findings that, although solar UV irradiance variation may affect climate through influence on precipitation and storm tracks, its significance in global temperature remains elusive.
Calcium network diameters are shown to be smaller by 5 % at solar maximum than at minimum. The average cell size at minimum is 22 115 + 99 km. The average size at solar maximum is 20 920 • 112 km, though individual maxima perform differently from each other depending probably on the dispersed remnant magnetic fields. The change in size of the network is interpreted in terms of changes in the size of the supergranular convective cell.
In our previous article (Priyal et al., S olar Phys., 289, 127) we have discussed the details of observations and methodology adopted to analyze the Ca-K spectroheliograms obtained at Kodaikanal Observatory (KO) to study the variation of Ca-K plage areas, enhanced network (EN) and active network (AN) for the three solar cycles, namely 19, 20, and 21. Now, we have derived the areas of chromospheric features using KO Ca-K spectroheliograms to study the long term variations of solar cycles between 14 and 21. The comparison of the derived plage areas from the data obtained at KO observatory for the period 1906 -1985 with that of MWO, NSO for the period 1965 -2002, earlier measurements made by Tlatov, Pevtsov, and Singh (2009, S olar Phys., 255, 239) for KO data and the SIDC sunspot numbers shows a good correlation. Uniformity of the data obtained with the same instrument remaining with the same specifications provided a unique opportunity to study long term intensity variations in plages and network regions. Therefore, we have investigated the variation of intensity contrast of these features with time at a temporal resolution of 6-months assuming the quiet background chromosphere remains unchanged during the period of 1906 -2005 and found that average intensity of AN, representing the changes in small scale activity over solar surface, varies with solar cycle being less during the minimum phase. In addition, the average intensity of plages and EN varies with a very long period having a maximum value during the Solar Cycle number 19 which is the strongest solar cycle of 20th century.
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