Abstract: The Solar Stellar Irradiance Comparison Experiment (SOLSTICE) is a small spectrometer obtaining solar irradiance measurements in the ultraviolet, 120 to 420 nm. This radiation, especially at wavelengths below 300 nm, is absorbed by the earth's middle and upper atmosphere providing the dominant energy input. Even small changes in this radiation field will have an impact on the atmosphere's composition, temperature, and dynamics. The challenge is to measure solar irradiance with better than 1% relative accuracy over arbitrarily long time periods. The SOLSTICE has the unique capability of observing both the Sun and early type stars using the same optics and detectors. Only apertures and integration times are altered to cover a dynamic range of nine orders of magnitude. Individually these 'standard' stars should vary by only small fractions of a percent over time periods of thousands of years, and the ensemble average flux from twenty or thirty of these stars is an even more stable reference. We describe the SOLSTICE technique and results from the Upper Atmosphere Research Satellite (UARS) SOLSTICE that has made daily observations since October 1991. A second generation SOLSTICE is now being designed for the Earth Observing System (EOS), to be flown within the next seven years, hopefully overlapping the UARS mission. !9
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