A Missing Piece of the E-Region Puzzle: High-Resolution Photoionization Cross Sections and Solar Irradiances in Models

Most ionospheric models cannot sufficiently reproduce the observed electron density profiles in the E-region ionosphere, since they usually underestimate electron densities and do not match the profile shape. Mitigation of these issues is often addressed by increasing the solar soft X-ray flux which is ineffective for resolving data-model discrepancies. We show that low-resolution cross sections and solar spectral irradiances fail to preserve structure within the data, which considerably impacts radiative processes in the E-region, and are largely responsible for the discrepancies between observations and simulations. To resolve data-model inconsistencies, we utilize new high-resolution (0.001 nm) atomic oxygen (O) and molecular nitrogen (N2) cross sections and solar spectral irradiances, which contain autoionization and narrow rotational lines that allow solar photons to reach lower altitudes and increase the photoelectron flux. This work improves upon Meier et al. (2007, ) by additionally incorporating high-resolution N2 photoionization and photoabsorption cross sections in model calculations. Model results with the new inputs show increased O+ production rates of over 500%, larger than those of Meier et al. (2007, ) and total ion production rates of over 125%, while N2+ ${\mathrm{N}}_{2}<^>{+}$ production rates decrease by similar to 15% in the E-region in comparison to the results obtained using the cross section compilation from Conway (1988, ). Low-resolution molecular oxygen (O2) cross sections from the Conway compilation are utilized for all input cases and indicate that O2+ ${\mathrm{O}}_{2}<^>{+}$ is a dominant contributor to the total ion production rate in the E-region. Specifically, the photoionization contributed from longer wavelengths is a main contributor at similar to 120 km. Most ionospheric models cannot sufficiently reproduce the observed electron density profiles in the E-region ionosphere and often attempt to mitigate the issue via unrealistic ad hoc modifications of the solar radiation flux. Here we address the low-resolution cross sections and solar spectral irradiances used as model inputs that are largely responsible for the discrepancy between models and observations. Low spectral resolution cross sections fail to accurately preserve structural features which allow solar photons to reach lower altitudes. We provide new high-resolution atomic oxygen (O) and molecular nitrogen (N2) cross sections and solar spectral irradiances which preserve important structural features that allow photons to leak through and increase the photoionization rate and photoelectron flux. Model results show increased O+ and total ion production rates of over 125% while a small decrease in the N2+ ${\mathrm{N}}_{2}<^>{+}$ production rate over calculations with historical data. Model outputs also indicate that O2+ ${\mathrm{O}}_{2}<^>{+}$ plays an important role in the total ion production rate, particularly at similar to 120 km. These findings demonstrate that it is crucial to update ionosphere models with high-resolution photoionization and photoabsorption cross sections and high-resolution spectral irradiances. We present new high-resolution theoretical cross sections of atomic oxygen and molecular nitrogen for implementation in radiative transfer/photoionization rate models High-resolution cross sections and solar irradiances enable ionizing radiation to reach below 140 km, increasing the total photoionization Models show this radiation increases E-region electron densities; thus, high-resolution cross sections are needed to resolve discrepancies