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 C aII K-line images are reduced to a new index of integrated excess emis sion, which reproduces both the 27 day rotational modulation and the s olar cycle decrease in Ly alpha irradiance. Our analysis shows that wh ile 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.