REPRODUCTION OF THE LYMAN-ALPHA IRRADIANCE VARIABILITY FROM ANALYSIS OF FULL-DISK IMAGES IN THE CAII K-LINE

We have compared three years of daily CaII K-line images from the Big Bear Solar Observatory (BBSO) with HI Lyman alpha irradiance data from the Upper Atmosphere Research Satellite (UARS). The daily full-disk CaII K-line images are reduced to a new index of integrated excess emission, which reproduces both the 27 day rotational modulation and the solar cycle decrease in Ly alpha irradiance. Our analysis shows that while plages reproduce the 27-day variation quite well, the total K-line emission excess above the quiet background is needed to reproduce the secular solar cycle trend in the Ly alpha irradiance. The resulting K-line index exhibits a high degree of correlation (0.9) with the time series of measured Ly alpha flux.