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T.G. Slanger, D.L. Huestis, P.C. Cosby (SRI International)
It is believed that the metal layers in the terrestrial mesosphere originate with incoming meteors. Two of these metals, sodium and potassium, produce nightglow emission by chemi-luminescence, and it is known that at least sodium emission is prominent in meteor trails. In the nightglow, the emission from sodium at 5889/5896 Åis relatively intense, typically 50 rayleighs. The excitation arises from interactions between Na and NaO with ozone and oxygen atoms. The two emission lines come from the Na(32P3/2,1/2) levels, separated by 17.2 cm-1, and the value expected for the sodium D2/D1 intensity ratio from statistical equilibrium at high temperature is 2.0. However, the Na(2P) is formed via chemi-excitation and has a very short radiative lifetime, so there is little reason to except a statistical ratio.
Using Keck/HIRES sky spectra we have investigated this ratio, in a preliminary sample of seventy-seven 50-minute spectra. We find that the distribution is narrowly clustered around a ratio of 1.6 for October/November observations, with a range of 1.3-1.8. The ratios have no correlation with observation elevation angles between 35 and 75 degrees. There is possibly a seasonal variation, with summer months exhibiting smaller ratios, but there is no apparent effect of time of night, and none of the data are taken under twilight glow conditions. Similarly, because of the site location and the strength of the emission, no contribution from street lighting is expected. It has been suggested [J. M. Plane, private communication, 2000] that the variability of the ratio could be due to different chemical pathways creating the two excited Na levels, with different temperature sensitivities. This idea requires further investigation, and can be explored both in the laboratory and by correlating the ratios obtained from the Keck sky spectra with local temperatures measured by OH rotational distributions - the OH Meinel bands and the Na emission are produced at similar altitudes.
This work has been supported by the NSF Aeronomy program.