Solar EUV Energy Deposition Rate Calculations for General Circulation Models

With new measurements of solar irradiance, such as by the TIMED solar extreme-ultraviolet experiment, now available in the extreme ultraviolet (EUV) and soft X-ray (XUV) regions of the spectrum from ~1 to ~103 nm, it is important to re-examine our quantitative representations of their effect on the Earth's atmosphere. EUV and XUV radiation photoionizes the upper atmosphere and creates the ionosphere, and also causes a variety of ancillary processes, including dissociation, excitation, and generation of photoelectrons. The solar spectrum is highly structured in parts of this region, so precise calculations of these effects should be made using high spectral resolution. This may be appropriate for detailed models of thermosphere/ionosphere processes, but global models require greater computational efficiency. Representation of photoelectron ionization is a particular problem for global models, because the ratio of photoelectron to photon ionization is so variable with altitude. We have developed a new method for parameterizing solar energy deposition and partitioning in the thermosphere that can be employed at fairly low spectral resolution with accurate results. This method is described, and its application to modeled and measured solar spectra demonstrated.