New O Partial Photoionization Cross Sections Resolve Ionospheric EUV Remote Sensing Issues

• Published in: Journal of geophysical research. Space physics 2023-10, Vol.128 (10), p.n/a
• Main Authors: Meier, R. R., Soto, Emmaris, Evans, J. Scott, Stephan, Andrew W., Tashiro, Motomichi
• Format: Article
• Language: English
• Subjects: Algorithms; Cross-sections; Electron density; Electron density measurement; Emission analysis; Emission measurements; EUV dayglow; Excitation; Ionization; ionospheric remote sensing; Mathematical analysis; new atomic oxygen partial photoionization cross sections; Photoionization; Remote observing; Remote sensing; Solar minimum; Spectroscopy
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2023JA031533

The ionospheric O+ number density can be measured remotely during the day by observing its optically thick 83.4 nm radiance. Some ambiguity is present in the process of retrieving the density due to uncertainties in the initial excitation rate. This can be removed by observing a companion optically thin emission at 61.7 nm originating from the O+(3s 2P) state, providing that the ratio of the initial excitation rates is known. Analyses of ICON EUV data using an 83.4/61.7 emission ratio of order 10 result in O+ densities lower by ∼2 than other measurements. Key to relating the two emissions is accurate knowledge of the partial photoionization cross sections and the spectroscopy of O+—the topic of this paper. Up to now, no independent evaluation of the ratio of the 83.4/61.6 emission ratio exists. The recent availability of state‐of‐the‐art calculations of O partial photoionization cross sections into a variety of O+ states presents an opportunity to evaluate the O+(2p4 4P)/O+(3s 2P) ionization rate ratio. We calculate excitation of these parent states of the emissions including both direct and cascade excitation from higher lying O+ energy states. The resulting theoretical prediction gives ratios that range from 13.5 to 12 from solar minimum to maximum, larger than the value of 10 used by the ICON 83.4 and 61.7 nm algorithm. The higher theoretical values for the ratio reconcile the ∼2 discrepancy between simultaneous ICON and other electron density measurements. Key Points OII 83.4 and 61.7 nm emission rates are being used for remote sensing of O+ in the daytime ionosphere New O partial photoionization cross sections predict higher than currently used values for the 83.4/61.7 nm volume emission rate ratio The higher theoretical values of the 83.4/61.7 nm ratio reconcile discrepancies between ICON and other simultaneous observations