Seasonal variations in mesospheric sodium tidal activity

Diurnal and nocturnal laser radar measurements of mesospheric sodium density at São José dos Campos, Brazil (23°S, 46°W), show strong oscillations with 12 and 24 hour periods. Data obtained in 1981 showed that atmospheric tides, mainly the vertical wind field, are the major causes of the density variations at a fixed height, thus making it possible to infer vertical wind parameters over a limited height range. In this work, 1984 and 1985 data are added to the earlier 1981 data and the 12‐ and 24‐hour components are determined for different seasons in order to study the seasonal variations of the tides. It is shown that the amplitudes of the oscillations in sodium density and consequently the vertical wind amplitudes, in general, are larger in winter than in other seasons. Above 97 km, the semidiurnal amplitude is larger than the diurnal in winter, of the same order of magnitude at the equinoxes and smaller in summer. The phases of the inferred semidiurnal vertical wind component, above 97 km, show regular propagating characteristics with little differences between winter and equinoxes, but a confused pattern in summer. Below 93 km the wavelengths decrease from winter to summer. In all seasons, the phase of the inferred diurnal vertical wind component stays at around 19–23 h below 85 km and above 93 km. The behavior of the phase of the diurnal oscillation in sodium density between 85 and 90 km is quite different in the three seasons. This is explained by seasonal differences in the relative phases of the oscillations in vertical wind and density.