THE SOLAR IRRADIANCE BY COMPUTATION Robert L. Kurucz Harvard-Smithsonian Center for Astrophysics 60 Garden St, Cambridge, MA 02138, USA November 25, 1997 This work has been presented at several meetings. As far as I know, this paper has not been published. It was written to be included with the publication of the HITRAN 96 (High Resolution Atmospheric Transmission) papers edited by Laurence Rothman but it was left out of the final publication. The text, the figures, and the data file are on the web site KURUCZ.HARVARD.EDU under IRRADIANCE in the SUN directory. THE SOLAR IRRADIANCE BY COMPUTATION Robert L. Kurucz Harvard-Smithsonian Center for Astrophysics 60 Garden St, Cambridge, MA 02138 ABSTRACT I am now able to compute a purely theoretical model photo- sphere (Kurucz 1992a;b;c) that reproduces the irradiance measure- ments of Neckel and Labs (1984) in the visible for bandpasses of approximately 2 nm. That model, and Avrett's empirical quiet sun model (Fontenla, Avrett, and Loeser 1993) that includes the chromosphere, are used to predict the irradiance out to 200 microns in one wavenumber bins. To get a feel for the scope of the monochromatic irradiance problem I have computed the spectrum from 150 nm to 200 microns at a resolution of 500000 using 58 million lines, both predicted and observed (Kurucz 1992a;b;c). If this spectrum is degraded to the resolution of the model, it looks like the model. At any given wavelength the spectrum is not reliable. But at a resolution of 1000, say, it approaches measurement accuracy. In regions of low transmission it is more reliable than existing measurements. Gail Anderson asked me to degrade the computed spectrum into wavenumber bins for use as irradiance input for atmospheric transmission calculations (Anderson 1996). I compared spectra computed from my theoretical model and from Avrett's empirical model which has a chromosphere. The computed spectra differ in the cores of hydrogen lines where the empirical model is better, and in strong CO fundamental lines where the theoretical model is better. I made a merged spectrum replacing the theoretical hydrogen lines with the Avrett hydrogen lines. Then I binned the spectrum with bins centered on whole wavenumbers from 50 to 50000 cm-1. Figures 1 and 2 show the spectrum plotted twice, the first time very compressed to give an impression of the whole, and the second more spread out to show features. In the visible and ultraviolet individual bins can be highly unreliable. The errors should average out over 10 to 20 bins. The data file is on the HITRAN 1996 CD-ROM (Rothman et al 1998). There is a review of current work on atlases in Kurucz (1995). I am producing atlases of the solar flux, central intensity, and limb spectra taken by James Brault at Kitt Peak. One atlas "Solar Flux Atlas from 294 to 1300 nm" by Kurucz, Furenlid, Brault and Testerman (1984), has been published thus far. Now I am working on a revised version with an improved reduction for atmospheric trans- mission that takes into account structure in ozone and O2 dimer. The replacement for the flux atlas will show the spectrum normalized to a continuum, the state-of-the art computed transmitted spectrum, and line identifications. If I can obtain a color laser printer, I will make a color-coded solar spectrum, transmission spectrum, transmitted spectrum, and the line identifications. I have flux spectra that will continue the atlas out to 5 microns. I also have the central intensity spectra from the "Photometric Atlas of the Solar Spectrum from 1,850 to 10,100 cm-1" by Delbouille, Roland, Brault, and Testerman (1981) also taken at Kitt Peak, and the ATMOS central intensity atlas from 650 to 4800 cm-1 taken by Farmer and Norton (1989) from Spacelab 3. I am also reducing Solar Maximum Mission (SMM) spectra at shorter wavelengths. Atlases will be made for the solar center and limb in collaboration with my colleague Barbara Bell. I will produce CD-ROMS with the spectrum and line data and I will have the files available on my web site. I use these atlases to test the pure calculations of solar spectra and transmission spectra. I identify problems with the line data and I try to make generic corrections that improve hundreds or thousands of lines at a time. If the spectrum calcu- lations look good in the regions of high transmission, I can have some confidence that the regions of low transmission are computed accurately. The main problem has been continuum placement. Ozone and O2 "dimer" features are difficult to determine because the atlases are each made up of a number of sharply peaked FTS scans that are of similar scale to the ozone features. The continuum placement affects the appearance of line wings and the apparent depth of weak features. Parts of the flux atlases directly give the residual irradiance spectrum but much is confused or obscured by terrestrial lines. I can compute the lines away except where the transmission is too low. There I will fill in with the purely theoretical solar spectrum. In this way I will finally produce an atlas showing the solar spectrum above the atmosphere. REFERENCES Anderson, G. 1996. Personal communication. Delbouille, L., Roland, G., Brault, J., and Testerman, L. 1981. Photometric Atlas of the Solar Spectrum from 1850 to 10000 cm-1. (Tucson: Kitt Peak National Observatory), 189 pp. Farmer, C.B. and Norton, R.H. 1989. A High-Resolution Atlas of the Infrared Spectrum of the Sun and Earth Atmosphere from Space. NASA Reference Pub. 1224, in two volumes, 1216 pp. Fontenla, J.M., Avrett, E.H., and Loeser, R. 1993. Energy balance in the solar transition region. III. Helium emission in hydrostatic, constant-abundance models with diffusion. Astrophysical Journal 406, pp. 319-345. Kurucz, R.L. 1992a,b,c Atomic and molecular data for opacity calculations. pp.45-48 "Finding" the "missing" solar ultraviolet opacity. pp.181-186 Remaining line opacity problems for the solar spectrum.187-194. All presented at the Workshop on Astrophysical Opacities, Caracas, 15-19 July 1991. Revista Mexicana de Astronomia y Astrofisica, vol. 23. Kurucz, R.L. 1995. The solar spectrum: atlases and line identifications. In Laboratory and Astronomical High Resolution Spectra, Astron. Soc. of the Pacific Conf. Series 81, (eds. A.J. Sauval, R. Blomme, and N. Grevesse) pp. 17-31. Kurucz, R.L., Furenlid, I., Brault, J., and Testerman, L. 1984. Solar Flux Atlas from 296 to 1300nm. (Sunspot, New Mexico: National Solar Observatory), 240 pp. Neckel, H. and Labs, D. 1984. The solar radiation between 3300 and 12500 A. Solar Physics 90, pp. 205-258. Rothman, L.S. et al 1998. The Hitran molecular spectroscopic database and Hawks (Hitran atmospheric workstation): 1996 edition. Journal of Quantitative Spectroscopy and Radiative Transfer, 60, pp. 665-710. Figure 1. The predicted solar irradiance from 50 to 50000 cm-1. Figure 2. The predicted solar irradiance from 50 to 50000 cm-1.