Atomic oxygen photoionization rates computed with high resolution cross sections and solar fluxes

Abstract

Accurate knowledge of photoionization rates is fundamental for the study and understanding of gases in the solar system. Past calculations of the photoionization rates of atmospheric gases lack the spectral resolution to accommodate highly structured autoionization features in the photoionization cross section. A new theoretical model of the atomic oxygen photoionization cross section combined with a new solar minimum spectral irradiance model enables calculations at very high spectral resolution (0.001 nm). Our analysis of unattenuated photoionization rates reveals no strong coincidences among myriad bright solar emission lines and autoionization lines in the cross section. However, deeper in the terrestrial atmosphere, opacity effects are significant and the need for high spectral accuracy becomes increasingly important. In our solar minimum example, factor of 3 errors occur when the lower thermospheric photoionization rate of atomic oxygen is computed at 1 nm spectral resolution for both the cross section and solar flux. Copyright 2007 by the American Geophysical Union.