The advent of the laser-radar technique allowed sodium densities and sodium emissions to be monitored simultaneously, and great scientific progress has been achieved since. We describe these results in a broad sense, keeping a balance between theory and experiment, addressing the following subjects: evolution of the photochemical theory, the model for diurnal variations, the sodium source, tidal effects, and problems at the top of the layer.
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V. W. J. H. Kirchhoff (1986) studied this question.