Motivations for Improving Solar Irradiance Spectral and Temporal Characterization

Improvements to spectral and temporal solar irradiances are often based upon increasingly accurate and precise measurements as well as upon better understood physics. This paper reports on one example in an emerging trend for solar irradiance models that can be characterized as hybrid irradiance modeling. Empirical and physics-based modeling of irradiances are combined and take advantage of strengths within both methods to provide a variety of solar irradiance products to science and engineering users. The SOLAR2000 (S2K) version 1.24 model (v1.24) described in this paper has gone through 17 upgrades since it was originally released in 1999 as v0.10 and now incorporates three theoretical continua, 13 rocket spectra, and time series data from five satellites using 17 instruments. S2K currently produces six integrated irradiance proxies for science and engineering applications in addition to spectrally resolved irradiances in three common wavelength formats. Integrated irradiance proxies include the E10.7 integrated EUV energy flux, QEUV total thermospheric EUV heating rate, PEUV hemispheric EUV power, T∞ exospheric temperature, RSN derived sunspot number, and S integrated spectrum. Besides three spectral wavelength and six integrated irradiance formats there are three time frames of historical, nowcast, and forecast irradiance products produced by four model grades. The Research Grade (RG) model is developed for aeronomical and climate change research, the Contract Grade (CG) model is developed for space system engineering applications, the Operational Grade (OP) model is developed for institutional and agency real-time operational space weather applications, and the Commercial Grade (CM) model is developed for commercial operational and production applications. This report describes these model characteristics as well as the current state of operational irradiances which are now in the second release of a first generation forecast methodology. Forecast Generation 1x (FGen 1x) produces a 72-hour forecast of E10.7 at high solar activity with 1-sigma uncertainties at the 8% level. All SOLAR2000 irradiance products from each of the versions, grades, and forecast generations are compliant with the ISO CD 21348 solar irradiance draft standard being developed as a standard method to specify all solar irradiances for use by space systems and materials users.