Solar Spectral Irradiance Variability in the Visible and Infrared During the SORCE Mission

The Spectral Irradiance Monitor (SIM) is a dual prism spectrometer onboard the SORCE (Solar Irradiance and Climate Experiment) satellite that was launched in January 2003. SIM covers the wavelength region 200- 2700 nm with a spectral resolution varying from 0.25 to 34 nm over this range. The primary detector for this instrument is an electrical substitution radiometer (ESR), and three additional photodiode detectors complement the ESR measurement and span the spectral range from 308 to 1600 nm; these photodiodes provide the bulk of the data used to study solar variability. The ESR calibrates the radiant sensitivities of the photodiodes in flight, and pre-flight measurements of the prism transmission and the spectral response function give the absolute calibration of the instrument. The SIM solar spectrum is in good agreement with other standard solar spectra such as the SOLSPEC spectrum (Thuillier et al. Metrologia, 35, 689, 1998) and the Davos World Radiation Center Reference Spectrum (Wehrli, C., World Radiation Center (WRC) Publication No. 615, Davos-Dorf, Switzerland, July 1985). SIM is able to detect short-term spectral irradiance variability of about 0.1% in the of 27-day solar rotation period induced by the appearance and varying intensity of solar structural features (such as sun spots and plage) relative to the quiet sun. A comparative study of the SIM solar spectrum relative to other standard spectra and an analysis of short-term solar variability as measured by SIM will be presented.