Hello,
I described previously the terrestric origin of Na-airglow visible in some spectra. In the meantime I also looked at the literature (with the help of Peter Schlatter) and found some supporting literature. Very profound and giving all the relevant details is the following review by D.M. Hunten:
http://adsabs.harvard.edu/abs/1967SSRv....6..493H
From this the geometry of the Na layer illumination can be seen in more detail:
together with some equations for the calculation of the shadow effect. It confirms my basic assumptions. To be more precise I should have talked about resonance scattering instead of fluoroscence.
Much more about the phenomenon can be found in the article. Nowadays the observation of the Na airglow is not so important any more, more precise measurements of the height and density of the layer can be made with LIDAR especially also for the Na layer as a guide star for adaptive optics.
Since the effect varies quite fast during twilight, it is important to subtract the background taken under similar conditions (same sun elevation at the same sky elevation).
Regards, Martin