Recent Progress in Understanding the Sodium Nightglow D2/D1 Ratio, and Future Prospects for Measuring the Mesopause [O]/[O2] Ratio
Abstract:The mechanism for sodium nightglow was proposed by Chapman in 1939, in which the emitting sodium atoms in the Na(2P) state were produced by the reaction of NaO and O in the mesopause region. Laboratory experiments in the early 1990s have shown that the NaO produced by the reaction Na + O3 are in the low-lying NaO(A2S+) excited electronic state, rather than the NaO(X2P) ground state, suggesting that the intensity ratio RD between the D2 line and the D1 line should be 2.0, in accord with their statistical spin-orbit weights. However, observed high resolution airglow spectra during the past decade clearly showed, surprisingly, that RD is quite variable. This led Slanger et al. (2005) to propose a modified Champman model, based on the fact that the ground state, NaO(X2P), resulting from quenching of NaO(A2S+) by atmospheric O2, provides a competitive pathway for the production of Na(2P). The paper also published a laboratory measured relationship between RD and [O]/[O2].
Harrell et al. (2010) then noted that the Na(2P) atoms produced from NaO(A2S+) and NaO(X2P) will have different kinetic energies and hence emission linewidths, so that the D line ratio arising from each state, RA and RX respectively, should be separately measurable. They developed a Faraday-filter-based sodium spectrometer which in addition to RD measures RA and RX: values of RA = 2.22 ± 0.72 and RX = 1.44 ± 0.48 were reported from 5 nights of data. They also showed that the ratio R2 (the ratio of the D2 emission intensity from NaO(A2S+) to that from NaO(X2P)), and similarly R1 for the D1 emission ratio, are directly proportional to [O]/[O2]. Plane et al. (2012) then showed by considering correlation of the electronic potential energy surfaces connecting the reactants NaO(A2S+)+O and NaO(X2P) +O with the products Na+O2 through the NaO2 intermediate that the theoretical values of RA and RX should be 2.0 and 1.5, respectively, in good agreement with the observations. In this paper, we will provide a connected overview of these recent theoretical and experimental advances, and discuss a modification of the Faraday-filter-based sodium spectrometer that will increase the nightglow signal by two orders of magnitude, thus opening the possibility of a direct validation of the current theoretical model and an accurate, real-time method for measuring atmospheric [O]/[O2].