On the sodium D line emission in the terrestrial nightglow

Abstract

Emission from atomic Na, consisting of a doublet of lines at 589.0 and 589.6 nm, is a prominent feature of the earth’s nightglow. A large data-base of measurements of the relative intensities of the D lines ( R D) was gathered at three locations: the ALOMAR observatory, Andenes (Norway, 69°N), Kuujjuarapik (Canada, 55°N) and the Danum Valley (Borneo, 8°N). R D varies between 1.5 and 2.0, with an average value of 1.67. These results were interpreted using a theoretical model of the Na nightglow which involves initial production of electronically excited NaO(A 2Σ) from the reaction between Na and O 3, followed either by reaction with O to generate Na( 2P J ) with a branching ratio of 1/6 and a J=3/2 to 1/2 propensity of 2.0, or quenching of NaO(A) to NaO(X 2Π) by O 2. The resulting NaO(X) then reacts with O to generate Na( 2P J ) with a branching ratio of 1/6 and a J=3/2 to 1/2 propensity of 1.5. These branching ratios and spin-orbit propensities are derived from statistical correlation of the electronic potential energy surfaces connecting the reactants NaO(A)+O and NaO(X)+O with the products Na+O 2, through the Na +O 2 − ion-pair intermediate. A fit of this statistical model to the results of an earlier laboratory study ( Slanger et al., 2005), where R D was measured as a function of the ratio [O]/[O 2], indicates that the rate coefficient for the quenching of NaO(A) by O 2 is around 1×10 −11 cm 3 molecule −1 s −1. The statistical model is also in good accord with recent high resolution observations of the Na D line widths ( Harrell et al., 2010). An atmospheric model is then used to show that gravity wave-driven perturbations to the Na layer can account for the observed variability of R D.