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The vertical profile of mesospheric sodium has been measured by laser radar at São José dos Campos (23°S, 46°W) on 344 nights since 1972. Large day‐to‐day variations are observed, but averaging over a large number of measurements consistently shows an annual variation by a factor of 2 in the sodium abundance, with maximum in local winter. The average nocturnal variation is consistent with photochemical control of the bottomside of the layer and diffusive control of the topside. Variations observed on individual nights appear to be mainly dynamic in origin. Vertical structure with a characteristic wavelength of the order of 10 km is frequently seen to propagate downward through the layer at rates between 1 and 4 km h −1 . Comparison of our results with twilight and daytime absorption abundances suggests a latitudinal asymmetry in the seasonal variation of abundance.
Simonich et al. (1979) studied this question.