Recent Progress in Understanding the Sodium Nightglow D2/D1 Ratio, and Future Prospects for Measuring the Mesopause [O]/[O2] Ratio

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 R_D 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 R_D 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 R_D 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, R_A and R_X respectively, should be separately measurable. They developed a Faraday-filter-based sodium spectrometer which in addition to R_D measures R_A and R_X: values of R_A = 2.22 ± 0.72 and R_X = 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 R_A and R_X 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].