Noctilucent clouds and thermospheric dust: Their diffusion and height distribution

Abstract

AbstractRocket sampling in latitude 66°N has shown that at heights around that of the mesopause, dust nuclei are far more abundant there when noctilucent clouds (hereinafter abbreviated to NLC) are present than when they are absent. The dust particles appear to be nickel‐based micrometeorites, and many of them, when NLC are present, appear to be ice‐coated. Hence the occurrence of NLC seems to require an adequate density of both water vapour and dust. The water vapour is supposed to rise to the mesopause by convection (in summer in latitudes above 45°); thence it diffuses into the thermosphere, where its upward penetration is limited by dissociation by ultra‐violet light, so that its effective scale height is estimated to be less than 2 km. Assuming that the air just above the mesopause is non‐turbulent, the scale height of the uncoated dust passing steadily downwards there is the same as that of the air, about 4.5 km. The estimated scale height of the NLC particles is about 1.5 km. Eddy diffusion below the mesopause causes the number density of both NLC and uncoated dust particles to decrease sharply just below the mesopause. Thus both the thinness and the location of NLC are explained.The diffusion problems considered refer to both the dust and the water vapour; the recent study of dust diffusion by Yu and Klein (1964) is extended. Also the influence of upward dust convection is considered. Conclusions are drawn as to the form of the NLC particles from their estimated rate of descent.The appearance of NLC is ascribed to the simultaneous occurrence (mainly, if not exclusively in the higher latitudes in summer) of a descent of the turbopause to the mesopause, a low mesopause temperature, and humid air below the mesopause.