Jerald Harder

Laboratory for Atmospheric and Space Physics, University of Colorado Boulder

The Spectral Irradiance Monitor (SIM) measures solar spectral irradiance (SSI) variability in the 200-2400 nm range, accounting for about 97% of the total solar irradiance (TSI). This instrument monitored the descending phase of solar cycle 23 and is now continuing observations in the rising phase of cycle 24. The SIM observations show rotational modulation of spectral irradiance due to the passage of active regions, but also indicate slower evolutionary trends in SSI over longer time periods that are both in and out of phase with the TSI. Because SSI variability can be significantly smaller than the rate of change of instrument sensitivity due to degradation, particular care is needed to make these corrections. The associated measurement equations and inherent challenges for performing these corrections will be discussed. The SIM SSI trends can be interpreted in terms of the Solar Radiation Physical Modeling (SRPM) program and solar images from the Precision Solar Photometric Telescope (PSPT) that provides the areas of solar active regions as function of time to generate a modeled SSI time series. To estimate the atmospheric response to SSI variability implied by SIM, quiet sun and active solar reference spectra were created as input into the Whole Atmosphere Community Climate Model (WACCM). The WACCM output suggests a very different response in ozone than from atmospheric forcing predicted by semi-empirical models of SSI. The model predicts a reduction in the lower mesosphere at higher solar activity and a large increase in mid to upper stratosphere. This atmospheric structure seen in the model is commensurate with contemporaneous observations of ozone from AURA-MLS and SABER. Looking to the future, advancements in spectral radiometry based on the SORCE SIM and applied to the upcoming TSIS SIM will be briefly discussed. A description of a concept for a radiometric imager that advances the science value of PSPT will also be described.