Evolution of Chromospheric Structures Derived from CaiiK Spectroheliograms: Implications for Solar Ultraviolet Irradiance Variability

Abstract

Over 1400 National Solar Observatory Ca II K spectroheliograms are analyzed for solar structure evolution for the years 1980, 1985, 1987, 1988, 1989, and 1992 through 1996 July with about a 50% time coverage. This time period includes the maximum of solar cycle 21 and almost all phases of solar cycle 22. The spectral bandpass and spatial resolution of these images are 0.5 Å and about 85, respectively. The plage, enhanced-network, active-network, and quiet-Sun features are identified on each image with an algorithm that uses criteria of intensity, size, filling factor, and continuity. This decomposition leads to time series of the plage and network areas, and their spatial distribution on the solar disk, which describe the solar cycle evolution of these structures. Thus, either the resultant structure masks or structure time series can be used for modeling the solar irradiance at other wavelengths. We cannot accurately identify all of the active network on the solar disk in Ca II K because the active-network intensities are not much greater than those of the quiet Sun. Therefore, we identify only the brighter active-network structures. However, the active network we identify on the Ca II K image can be used as a proxy for active-network evolution over the solar cycle. Many results that are useful for modeling solar irradiance variability are also presented. We find that the intensity contrasts (ratio of structure intensity to quiet Sun intensity) of the plage, enhanced network, and active network, averaged over the solar disk, are 1.95, 1.51, and 1.33, respectively. These contrasts remain essentially constant over the solar cycle. As expected, we find that the plage and the enhanced-network time series show a strong 27 day rotational modulation, but we also find that the active network can have a weak rotational modulation despite its wider longitudinal dispersion. The plage and enhanced network typically cover about 13% and 10%, respectively, of the solar disk during solar-maximum time periods. During solar moderate and minimum activity levels, the total plage and enhanced-network areas can reach zero, but the active network can still cover a large portion of the solar disk.