Solar spectral irradiance variability comparisons of the SORCE SIM instrument with monitors of solar activity and spectral synthesis

Abstract

The SORCE (Solar Radiation and Climate Experiment) SIM (Spectral Irradiance Monitor) instrument is a satellite-borne prism spectrometer that measures the solar spectrum from 200-2700 nm with a cadence of at least 2 spectra per day with a resolution of 1-33 nm. The nearly 800-day long data set provides the temporal evolution of solar irradiance throughout ultraviolet, visible and infrared spectral regions. At some wavelengths in the ultraviolet, the SIM measurements exhibit variations similar to Mg II index, and in the visible and IR they show similarities to Total Solar Irradiance (TSI) record, but with differing amplitudes, phases, and shapes relative to the monitors. The TSI can be explained as a complex mix of the various wavelength components observed by SIM. Further insight into the solar variability observed by SIM can be gained from comparing an analysis of the distribution of solar features as measured by PSPT (Precision Solar Photometric Telescope) in conjunction with spectral synthesis. Most of the observed behavior appears to be qualitatively explained by the observed solar surface features that directly relate to the magnetic activity but some IR variations are not readily explained as was noted by Fontenla et al. (2004).