VALIDATION OF THE UARS SOLAR ULTRAVIOLET IRRADIANCES - COMPARISON WITH THE ATLAS-1 AND ATLAS-2 MEASUREMENTS

The measurements of the solar ultraviolet spectral irradiance made by the two Upper Atmosphere Research Satellite (UARS) solar instruments, Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) and SOLar STella r Irradiance Comparison Experiment (SOLSTICE), are compared with same- day measurements by two solar instruments on the shuttle ATmospheric L aboratory for Applications and Science (ATLAS) missions, ATLAS SUSIM a nd Shuttle Solar Backscatter UltraViolet (SSBUV) experiment. These mea surements from the four instruments agree to within the 2 sigma uncert ainty of any one instrument, which is 5 to 10% for all wavelengths abo ve 160 nm and for strong emission features below 160 nm. Additionally, the long-term relative accuracy of the two UARS data sets is better t han the original 2% goal, especially at wavelengths greater than 160 n m. This level of agreement is credited to accurate preflight calibrati ons coupled with comprehensive inflight calibrations to track instrume nt degradation. Two solar irradiance spectra, 119 to 410 nm, are prese nted; the first combines observations from UARS SUSIM and UARS SOLSTIC E taken on March 29, 1992, during the ATLAS 1 mission, and the second combines spectra for April 15, 1993, during the ATLAS 2 mission. The A TLAS 1 mission coincided with the initial decline from the maximum of solar cycle 22 when solar activity was relatively high. The ATLAS 2 mi ssion occurred somewhat later during the declining phase of the solar cycle 22 when solar activity was more moderate.