JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D6, PAGES 9541-9569, APRIL 30, 1996 Validation of the UARS solar ultraviolet irradiances: Comparison with the ATLAS I and 2 measurements T. N. Woods, • D. K. Prinz, 2 G. J.Rottman, • J.London, 3 P. C. Crane, 4 R. P. Cebula, 5 E. Hilsenrath, 6 G. E. Brueckner, 2 M. Do Andrews, 5 O. R. White, • M. E. VanHoosier, 2 Lo E. Floyd, 4 L. C. Herring, 4 Bo G. Knapp, • C. K. Pankratz, 1 po A. Reiser 4 Abstract. The measurements of thesolar ultraviolet spectral irradiance made by thetwo 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 bytwo solar instruments on the shuttle ATmospheric Laboratory forApplications and Science (ATLAS) missions, ATLAS SUSIM and Shutfie Solar Backscatter UltraViolet (SSBUV)experiment. These measurements from thefourinstruments agree towithin the 2c• uncertainty of any one instrument, which is5 to 10% forallwavelengths above 160nmand for strong emission features below 160nm. Additionally, thelong-term relative accuracy of the two UARSdata sets isbetter than the original 2% goal, especially at wavelengths greater than 160 nm. This level of agreement iscredited to accurate preflight calibrations coupled with comprehensive inflight calibrations totrack instrument degradation. Twosolar irradiance spectra, 119 to410nm, are presented; the first combines observations from UARS SUSIM andUARS SOLSTICE takenon March 29, 1992, during the ATLAS 1 mission, and the second combines spectra forApril 15, 1993, during the ATLAS 2 mission. TheATLAS 1 mission coincided with the initial decline from themaximum of solar cycle22 whensolar activity was relatively high.The ATLAS 2 mission occurred somewhat later during the declining phase of thesolar cycle 22 when solar activity was more moderate. 1. Introduction The solarultraviolet(UV) irradiance in the spectralinterval of approximately 115 to 420 nm originates in the solar photosphere and chromosphere• and time variationsof this radiation are one measure of solar activity. Absorption of the solar UV radiative energy in the Earth's atmosphereplays a dominant role in the temperaturedistribution, photochem- istry, and overall momentum balance in the stratosphere, mesosphere, and lower thermosphere. This solar-terrestrial interaction was recognized by an ad hoc Scientific Working Group (SWG) established by the Office of Space Science (NASA) when theyproposed a research programwhoseprinci- pal goals "were to includelong-term observations of solar irradiance at ultraviolet wavelengths, the global distribution and time variation of atmospheric trace species, and the dynamic behavior of the upper atmosphere" [NASA/Jet •High Altitude Observatory, National Center for Atmospheric Research, Boulder.Colorado. 2E.O. Hulbert Center for Space Research, Naval Research Laboratory, Washington, D.C. 3Department of Astrophyscial, Planetary, and Atmospheric Sciences, University of Colorado, Boulder, Colorado. 4Interferometrics, Inc., Viena,Virginia. SHuges STX Corporation, Lanham, Maryland.. 6NASA Goddard Space Flight Center, Greenbelt, Maryland. Copyright 1996by the American Geophysical Union. Paper number 96JD00225. 0148-0227/96/96JD-00225 $05.00 Propulsion Laboratory (JPL), 1978]o The SWG was motivated by the needto develop "an evolutionary program centered on spacecraft observations extending over a period of 5 to 10 years." It was strongly recommended that redundant instru- ments be a part of the UARS program• The focusof the pro- gram with regardto solar UV irradiance measurements was to providedaily spectral irradiance valuesat 1 nm resolutionover the wavelength interval 120 to 400 nm with a 2• accuracy of 10% and precision of 6%. These values would be used to determinethe spectral distribution of the solar rotation (27 days) and solar cycle (•-11 years) irradiance variations• The substance of these recommendations was restated about 10 years later by a panel of the National Research Council who proposed"o..a major effort in developmentof stable spectro- photometers, detectors, and standards to attain the precision limits of a few percent over the solar cycle that are required, for example, to study ozone trends" [National Academy of Sciences,1988]. A more recent panel of the National Research Council recommended that "the highest priority and most urgent activity for determining solar influences on global change is to monitor the total and spectral solar irradiance from an uninterrupted, overlapping seriesof spacecraft radio- meters employing inflight sensitivity tracking" [National Academy of Sciences, 1994], and we consider the UARS mission the first in this seriesof spacecraft radiometers. The integrated solar UV irradianceat wavelengths below 300 nm represents lo4% (8% below 400 nm) of the total aver- age radiative energy receivedfrom the Sun at the top of our atmosphere. For clear skies, subject only to atmospheric scat- tering and absorptionby trace gases, approximately 50% of the extraterrestrial irradiance at 400 nm reaches the Earth's 9541