Evidence for Radiative Recombination of O+ Ions as a Significant Source of O 844.6 nm Emission Excitation

Photoelectron (PE) impact on ground-state O(P-3) atoms is well known as a major source of twilight 844.6nm emission in the midlatitude thermosphere. Knowledge of the PE flux can be used to infer thermospheric oxygen density, [O], from photometric measurements of 844.6nm airglow, provided that PE impact is the dominant process generating the observed emission. During several spring observational campaigns at Arecibo Observatory, however, we have observed significant 844.6nm emission throughout the night, which is unlikely to arise from PE impact excitation which requires solar illumination of either the local or geomagnetically conjugate thermosphere. Here we show that radiative recombination (RR) of O+ ions is likely responsible for the observed nighttime emission, based on model predictions of electron and O+ ion density and temperature by the Incoherent Scatter Radar Ionosphere Model. The calculated emission brightness produced by O + RR exhibits good agreement with the airglow data, in that both decay approximately monotonically throughout the night at similar rates. We conclude that the conventional assumption of a pure PE impact source is most likely to be invalid during dusk twilight, when RR-generated emission is most significant. Estimation of [O] from measurements of 844.6nm emission demands isolation of the PE impact source via coincident estimation of the RR source, and the effective cross section for RR-generated emission is found here to be consistent with optically thin conditions. Plain Language Summary Atoms and molecules comprising gaseous atmospheres undergo numerous interactions, which cause them to emit light in characteristic spectral patterns. Remote sensing of the spectral distribution of these so-called airglow emissions is a common means of estimating atmospheric conditions such as winds and temperatures at high altitudes. Estimation of atmospheric parameters from measurements of airglow emission brightness is more challenging, since it not only requires complete knowledge of the photochemical processes responsible for generating the observed emission but also that the processes themselves are sensitive to changes in the parameter under investigation. Few airglow emission lines meet these requirements. One exception has long been considered to be atomic oxygen emission at 844.6nm. In this work, we present experimental evidence that the conventional formulation of the 844.6nm emission model is incomplete, in that it neglects a significant mechanism for emission generation that was thought to be negligible at midlatitudes. This finding implies that accurate interpretation of this airglow emission in terms of unknown atmospheric parameters is more challenging than previously believed.