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 STellar Irradiance Comparison Experiment (SOLSTICE), are compared with same‐day measurements by two solar instruments on the shuttle ATmospheric Laboratory for Applications and Science (ATLAS) missions, ATLAS SUSIM and Shuttle Solar Backscatter UltraViolet (SSBUV) experiment. These measurements from the four instruments agree to within the 2σ uncertainty of any one instrument, which is 5 to 10% for all wavelengths above 160 nm and for strong emission features below 160 nm. Additionally, the long‐term relative accuracy of the two UARS data sets is better than the original 2% goal, especially at wavelengths greater than 160 nm. This level of agreement is credited to accurate preflight calibrations coupled with comprehensive inflight calibrations to track instrument degradation. Two solar irradiance spectra, 119 to 410 nm, are presented; the first combines observations from UARS SUSIM and UARS SOLSTICE 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 ATLAS 1 mission coincided with the initial decline from the maximum of solar cycle 22 when solar activity was relatively high. The ATLAS 2 mission occurred somewhat later during the declining phase of the solar cycle 22 when solar activity was more moderate.

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A detailed radiative transfer calculation has been carried out to estimate the effects of rotational Raman scattering (RRS) on satellite measurements of backscattered ultraviolet radiation. Raman-scattered light is shifted in frequency from the incident light, which causes filling in of solar Fraunhofer lines in the observed backscattered spectrum (also known as the Ring effect). The magnitude of the rotational Raman scattering filling in is a function of wavelength, solar zenith angle, surface reflectance, surface pressure, and instrument spectral resolution. The filling in predicted by our model is found to be in agreement with observations from the Shuttle Solar Backscatter Ultraviolet Radiometer and the Nimbus-7 Solar Backscatter Ultraviolet Radiometer.

[1] Characterization of long-term solar ultraviolet (UV) irradiance variations is important for understanding the radiative forcing of Earth's atmosphere. Extending such a characterization to multidecadal timescales requires the merging of multiple satellite data sets. Currently available irradiance data sets show both absolute offsets and timedependent differences that vary between spectral ranges. We have created a composite spectral UV irradiance data set with daily spectra covering the wavelength range 120-400 nm for the time period November 1978 to August 2005. A reference irradiance spectrum is used for normalization to remove absolute biases between instruments. We selected the best irradiance data set for each spectral and temporal interval based on intercomparisons between overlapping data sets and comparisons with proxy model predictions. The long-term behavior of the merged composite irradiance data set is consistent with expectations based on other solar activity indicators. There is no difference between the minimum values for solar cycles 21 and 22 within the uncertainty of the data, and the maximum irradiance values for cycle 23 are slightly lower than those observed for cycles 21 and 22. We also compare our composite data set with published synthetic irradiance data. These two data sets generally agree within ±2% over most spectral and temporal ranges. We hope to extend this data set in the future using measurements from the SORCE satellite.Citation: DeLand, M. T., and R. P. Cebula (2008), Creation of a composite solar ultraviolet irradiance data set,

change in solar physics. Our analysis of short-tenn solar variability is much less sensitive to residual instrument response changes than the observations of long-term variations. The SORCE long-term UV results can be explained by undercorrection of instrument response changes during the first few years of measurements, rather than requiring an unexpected change in the physical behavior of the Sun.