Intercomparison of SCIAMACHY and SIM vis-IR irradiance over several solar rotational timescales

Abstract

The two satellite spectrometers SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) aboard ENVISAT (Environmental Satellite), and SIM (Spectral Irradiance Monitor) aboard SORCE (Solar Radiation and Climate Experiment) observe since 2002 and 2003, respectively, daily solar spectral irradiance (SSI) not only in UV but extending to the visible and near- infrared (vis-NIR) regions. In this work, we intercompare (1) spectra and (2) timeseries of SSI measurements from SCIAMACHY and SIM. In (1) same-day (April 21, 2004) SSI measurements from these two instruments are compared to reference spectra from ground (new Kurucz), high-altitude (Hall and Anderson, Neckel and Labs, and Wehrli composite), and space (SOLSPEC/ATLAS 3, and SUSIM/UARS). In (2) timeseries of measurements (July 3 to August 21, 2004) covering several solar rotations in 2004 are compared to VIRGO sunphotometers (SPM) aboard SOHO. In general, SCIAMACHY and SIM are in agreement to within 4% over the common spectral domain and with respect to the other reference data. Apart from SSI and its variability, we integrate SSI over selected wavelength intervals and compare qualitatively to total solar irradiance (TSI) variability from PMOD/WRC and TIM/SORCE. Timeseries of integrated SSI in the vis (400–700 nm), NIR (700–1600 nm), and UV-vis-NIR (240–1600 nm) bands are compared. The overall rise and fall of integrated SCIAMACHY and SIM irradiances over several solar rotations are in good agreement and agree in most cases qualitatively with TSI variations in the visible and near IR. The application of White Light Source (WLS) corrections brings SCIAMACHY irradiances in closer agreement with SIM. Since WLS is also degrading with time, the WLS lamp ratios cannot be used for SSI degradation corrections after 2004.