Principal Investigator (PI): Martin Snow, University of Colorado Boulder

Solar spectral irradiance (SSI) in the ultraviolet wavelength range (0-400 nm) provides the primary direct energy input to the Earth's atmosphere. This irradiance has been measured by a variety of space-based instruments for the past 30 years. However, the datasets from these individual instruments have different levels of uncertainty in their absolute calibration and corrections for instrumental drifts. The atmospheric and global climate change communities need a single, consistent, long-term dataset that combines the individual instrumental records.

In this proposal, we will compare the ultraviolet SSI measurements from NASA’s Solar Radiation and Climate Experiment (SORCE) to the observations from National Oceanic and Atmospheric Administration Solar Backscatter Ultraviolet (SBUV/2). The SBUV/2 instrument has been included on both current and prior NOAA missions. Additionally, we will compare the SORCE measurements to European Space Agency's Global Ozone Monitoring Experiment and Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY). The SORCE instruments were carefully calibrated before launch and their uncertainties are well understood. The other SSI instruments on other satellites in this study do not have strong preflight calibrations or methods for maintaining their inflight accuracy. Our analysis will transfer the SORCE calibration to these other datasets which overlap SORCE in both wavelength and time.

Previous composite ultraviolet SSI datasets (e.g. DeLand and Cebula, 2008) only use data through 2004. Our work will extend these datasets to the current epoch including the recent solar minimum and the rise of cycle 24. We also propose to extend the multi-satellite composite to include the Extreme Ultraviolet (EUV) wavelength range (0-120 nm). Previous work used only the 120-400 nm range. Since there are long periods without observational coverage in the EUV, we propose to use the Flare Irradiance Spectral Model published by Chamberlin et al. (2007) to fill these gaps. This model is an empirical model based on solar activity proxies.

In addition to the SSI composite, we will produce a composite Mg II index from 1978 to the present. The Mg II index is used as a proxy for ultraviolet solar activity in many climate models, and the currently available composites end before the last solar minimum.