Ground-based determination of the spectral ultraviolet extraterrestrial solar irradiance:: Providing a link between space-based and ground-based solar UV measurements

The extraterrestrial solar spectrum between 295 and 355 nm has been determined from direct irradiance measurements made with a Brewer double spectrophotometer, using the Langley method. The measurements in this study consist of 449 half days of data collected during 1998 at Mauna Loa Observatory, Hawaii. The +/-2.3% accuracy of this extraterrestrial solar spectrum is obtained by a careful investigation of the instrument calibration and the systematic errors that can arise because of atmospheric and instrument instabilities as well as finite slit width effects and is limited by the uncertainty of the absolute irradiance scale transfer between the standard laboratory and the instrument. A comparison between this extraterrestrial solar spectrum measured from the ground with the mean UARS and ATLAS-1 spectrum show an agreement better than 3%. The mean ratios are 1.002 for the mean UARS spectrum, 1.003 for the mean ATLAS-1 spectrum, 1.013 for the SOLSPEC spectrum, and 1.017 for the ATLAS-3 spectrum.