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Solar UV/visible/IR irradiance changes in terrestrial atmospheric bands derived from SUSIM, SCIAMACHY, and SIM satellite observations

Joseph Pagaran(1), Mark Weber(1), Linton Floyd(2), Matthew DeLand(3), Natalie Krivova(4), Sami Solanki(4), Jerald Harder(5) and John Burrows(1)

(1) Institute of Environmental Physics, Otto-Hahn-Allee 1, D-28359 Bremen, Germany
(2) Naval Research Laboratory and Interferometrics Inc, 4555 Overlook Ave., SW, Washington, DC 20375, United States
(3) Science Systems and Applications Inc. (SSAI), 10210 Greenbelt Rd., Suite 400, Lanham, MD 20706, United States
(4) Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Srtasse 2,, Katlenburg-Lindau, D-37191, Germany
(5) Laboratory for Atmospheric and Space Physics, 234 Innovation Drive, Boulder, CO 80303, United States


The Sun is a variable star, whose radiative output (total solar irradiance (TSI) or solar constant) is known to change during the 11-year sunspot cycle. Whether this 11-year variation of TSI has the potential to drive Earth's climate system, requires TSI 11-year change to be resolved into its spectral decomposition (solar spectral irradiance or SSI). Solar variablity estimates in SSI can only be obtained from spaceborne observations. However, no direct measurements of the 11-year SSI variability, particularly in the visible, and IR, have so far been made. Routine spaceborne solar observations covering the entire optical range and near IR have become only recently available with SCIAMACHY (since 2002) and SIM (since 2003). Another challenge is the fact that SSI variations above 400 nm are well below 1% and, therefore, below the radiometric accuracy and long-term stability of space instruments. Nevertheless, in short time periods (several solar rotations) the relative accuracy can be shown to be in the per mill percentage range. By expressing the spectral solar variability in terms of solar proxies (sunspot darkening and faculae brightening), representing the dominant features of solar magnetic activity, the solar changes over an 11-year solar cycle can be estimated. In this paper, we describe how the SSI variations are derived from SCIAMACHY and SIM in the spectral range 200-1700 nm. The solar cycle estimates are compared to SUSIM's direct observations of solar cycle change and the SATIRE empirical model. Based upon SCIAMACHY observations the contribution of the UV spectral region (<400 nm) to TSI variation is about 55%, somewhat lower than SATIRE results (62%), and higher than the 30% estimate from Lean et al. (1997) during solar cycle 22. Results for 11-year spectral changes in terrestrial radiation bands relevant for climate modeling during solar cycle 23 (1996-2007) are presented. The results from SUSIM (120-400 nm) are combined with SCIAMCHY or SIM to provide solar cycle amplitudes in the Lyman alpha, Herzberg, Hartley, Huggins, and Chappuis ozone bands and the NIR (700-1000 nm) an SWIR (1000-1600 nm) bands. Estimates are also derived for solar cycles 21 and 22 and compared with DeLand's SSA UV composite database and with Lean's proxy-based SSI modeling using SBUV and recent SIM data.