Using High-Resolution Photoionization Cross Sections and Solar Irradiance to Resolve E-region Data/Model Discrepancies

Accurate photoionization production rates are crucial for our understanding of planetary ionospheres and thus it is essential for modeled properties to reproduce observations. However, ionospheric E-region photochemical models tend to underestimate electron densities when compared with observations. Various ad hoc approaches to mitigate this deficiency including increasing the soft solar x-ray flux, scaling model inputs and parameters, and adjusting photochemistry have not resolved the issue because they do not address the source of the limitation, which is the rudimentary characterization of solar photoionization rates. Model calculations of terrestrial photoionization rates lack the spectral resolution necessary to account for highly structured cross sections and solar spectral irradiances. To alleviate this limitation, our model calculations utilize new high-resolution (0.01 Å) O and N2 photoionization and N2 photoabsorption cross sections, which preserve autoionization lines and rotational structure, and high-resolution (0.01 Å) solar spectral irradiances. We present new high-resolution N2 photoabsorption cross sections and solar spectra, and photoionization rate calculations using the Atmospheric Ultraviolet Radiance Integrated Code [AURIC; Strickland et al., 1999]. We find that AURIC calculations utilizing new high-resolution cross sections and solar spectra produce enhanced volume production rates (VPRs) for both O and O2 at E-region altitudes (90-140 km) with an increase of over 200% in VPR at 115 km and 130 km, respectively, in comparison to the low-resolution (~1 Å) cross sections currently used in AURIC. The total N2 VPR ratios on the other hand illustrate a decrease in the VPR by as much as 17% below 140 km when using the high-resolution cross sections as inputs in comparison to low-resolution cross section model results. AURIC results based on the new high-resolution cross sections are in good agreement with VPR ratios produced by the photoionization code presented in the work by Meier et al. [2007]. These results demonstrate that high-resolution cross sections and solar spectra allow photons to penetrate deeper into the Earth's atmosphere, producing larger total ionization rates and electron densities.