Discrepancy between the nighttime molecular ion composition given by the International Reference Ionosphere model and airglow measurements at low latitudes

Abstract

The 630‐nm nighttime airglow is radiated by O(1D) atoms, which are produced by the dissociative recombination of O2+ ions. The typical approach used to calculate the red line emission rate at night is based on the assumption that O2+ is mainly produced by the reaction of O+ with molecular oxygen. In the case that the O2+ density is much smaller than the O+ density, [O+] = ne in the F2 region. Good agreement between measured nighttime integrated emission rates and the emission rates calculated by this typical approach, using both electron densities measured by incoherent scatter radars and given by the International Reference Ionosphere (IRI) model, has been shown. However, the O2+ densities given by the IRI model are much higher than the densities produced by the reaction of O+ with O2, and these densities do not correspond to the condition [O+] = ne. In this case, the typical approach cannot be applied and molecular ions must be included in the emission rate calculations. The integrated emission rates calculated including the molecular ion density given by the IRI model have been found to be much higher than the measured 630.0‐nm emission rates. This discrepancy takes place at latitudes below about ±30° in the western longitude sector, mainly for the period from March to November, and the disagreement is higher than 1 order of magnitude at the equator. In addition, we model the F2 region green line O(1S) emission at 557.7 nm resulting from the dissociative recombination of O2+. Using measurements of this volume emission rate made by the Wind Imaging Interferometer (WINDII) satellite, we are able to show that IRI overestimates the O2+ density (and ion fraction) on the bottomside of the F2 region. A revision of the ion composition in the IRI model on the bottomside seems to be needed on the basis of these results. Airglow measurements may be useful in constraining such a revision. A revision could utilize the formulae for the relationship between the molecular ion densities and neutral densities derived here, using the Mass Spectrometer Incoherent Scatter (MSIS) neutral densities and the IRI electron density. These calculations are based on the assumption that O2+ and NO+ are only produced through ion‐molecular reactions. Such a revision would correct the magnitude and altitudinal dependence of the molecular ion fraction in the IRI model.