EVOLUTION OF CHROMOSPHERIC STRUCTURES DERIVED FROM CA-II-K SPECTROHELIOGRAMS - IMPLICATIONS FOR SOLAR ULTRAVIOLET IRRADIANCE VARIABILITY

Over 1400 National Solar Observatory Ca II K spectroheliograms are ana lyzed for solar structure evolution for the years 1980, 1985, 1987, 19 88, 1989, and 1992 through 1996 July with about a 50% time coverage. T his 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 Angstrom and about 8 '' 5, respectively. The plage, enhanced-network, active-network, and quiet-Sun features are id entified on each image with an algorithm that uses criteria of intensi ty, size, filling factor, and continuity. This decomposition leads to time series of the plage and network areas, and their spatial distribu tion on the solar disk, which describe the solar cycle evolution of th ese structures. Thus, either the resultant structure masks or structur e time series can be used for modeling the solar irradiance at other w avelengths. 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 o nly the brighter active-network structures. However, the active networ k we identify on the Ca II K image can be used as a proxy for active-n etwork evolution over the solar cycle. Many results that are useful fo r modeling solar irradiance variability are also presented. We find th at the intensity contrasts (ratio of structure intensity to quiet Sun intensity) of the plage, enhanced network, and active network, average d over the solar disk, are 1.95, 1.51, and 1.33, respectively. These c ontrasts remain essentially constant over the solar cycle. As expected , we find that the plage and the enhanced-network time series show a s trong 27 day rotational modulation, but we also find that the active n etwork can have a weak rotational modulation despite its wider longitu dinal dispersion. The plage and enhanced network typically cover about 13% and 10%, respectively, of the solar disk during solar-maximum tim e periods. During solar moderate and minimum activity levels, the tota l plage and enhanced-network areas can reach zero, but the active netw ork can still cover a large portion of the solar disk.