Calculation of solar irradiances. I. Synthesis of the solar spectrum

Variations in the total radiative output of the Sun as well as the detailed spectral irradiance are of interest to terrestrial and solar-stellar atmos phere studies. Recent observations provide measurements of spectral irradia nce variations at wavelengths in the range 1100-8650 Angstrom with improved accuracy, and correlative studies give procedures for estimating the spect ral irradiance changes from solar activity records using indicators such as those derived from Ca II K and Mg II indices. Here we describe our approac h to physical modeling of irradiance variations using seven semiempirical m odels to represent sunspots, plage, network, and quiet atmosphere. This pap er gives methods and details, and some preliminary results of our synthesis of the variations of the entire irradiance spectrum. Our calculation uses object-oriented programming techniques that are very efficient and flexible . We compute at high spectral resolution the intensity as a function of wav elength and position on the disk for each of the structure types correspond ing to our models. These calculations include three different approximation s for the line source function: one suited for the very strong resonance li nes where partial redistribution (PRD) is important, another for the most i mportant nonresonance lines, and another approximation for the many narrow lines that are provided in Kurucz's listings. The image analysis and calcul ations of the irradiance variation as a function of time will be described in a later paper. This work provides an understanding of the sources of var iability arising from solar-activity surface structures. We compute the Ly alpha irradiance to within 3% of the observed values. The difference betwee n our computations and the Neckel & Labs data is 3% or less in the near-TR wavelengths at 8650 Angstrom, and less than 1% in the red at 6080 Angstrom. Near 4100 Angstrom we overestimate the irradiance by 9%-19% because of opa city sources missing in our calculations. We also compute a solar cycle var iability of 49% in the Ly alpha irradiance, which is very close to observed values. At wavelengths between 4100 Angstrom and 1.6 mu m, we obtain spect ral irradiance variations ranging from -0.06% to 0.46% in the visible-the h igher values correspond to the presence of strong lines. The variability in the IR between 1.3 and 2.2 mu m is similar to-0.15%.