Status of UARS solar UV irradiance data

Accurate measurement of solar ultraviolet (UV) irradiance variations is important for understanding both solar processes and energetic input to the Earth's atmosphere. Satellite instruments are capable of providing such data, but must correct for significant spectral and temporal response changes during the observing lifetime of the instrument. The Upper Atmospheric Research Satellite (UARS) carries two instruments dedicated to monitoring long-term solar UV irradiance: the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) and the Solar Stellar Intercomparison Experiment (SOLSTICE). Both instruments include comprehensive on-board calibration systems designed to characterize and remove response changes from the irradiance data. This paper presents the status of the SUSIM and SOLSTICE data sets. We find that both instruments produce meaningful direct estimates of solar cycle UV irradiance variations in the wavelength range 120-250 nm. Between 250 and 300 nm, the reduced magnitude of solar variability becomes comparable to the long-term calibration uncertainty. Longward of 300 nm, solar cycle irradiance variations cannot be detected. The SUSIM and SOLSTICE irradiance data sets represent the first fully calibrated solar UV irradiance data sets to cover a complete solar cycle. (C) 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.