Quiet-Sun variability with the solar cycle

In evaluating the contribution of various chromospheric features to total CaII K-emission flux and UV irradiance variability, the quiet-Sun is also one of the essential and important component. We have segregated the disc center intensity in a quiet-network and intranetwork regions, considered as a quiet-Sun component, from a digitized Call K-spectroheliograms of 1992 and 1980, observed at the National Solar Observatory at Sacramento Peak. It is shown, for the first time in the K-line, that the longer-term variation of the quiet-Sun during the solar activity cycle is in phase with the full-disk K-emission flux (the spatial K index), and UV irradiance measured in MgII h and k lines. Thus, it indicates that the quiet-Sun will contribute in the variations of UV irradiance, and its contribution should be taken into account in UV irradiance models. Using a standard FFT time series analysis, our results show, for the first time, the 51-day periodicity in the quiet-Sun variability. This evidence suggests that the 51-day period may be related to a new emerging weak magnetic fields, and the emission flux disappearance continues in the quiet-Sun regions at a slower rate. In addition, the variability of the chromospheric quiet-Sun suggest that it is very dynamic, not quiet, and is crucial for understanding of the chromosphere itself.