Tidal oscillations in the atmospheric sodium layer

Abstract

The vertical distribution of atmospheric sodium has been measured at São José dos Campos (23°S, 46°W) over a total of about 20 complete diurnal cycles between April and August 1981. Average time variations of the sodium density show strong oscillations with 12‐ and 24‐hour periods. Both the diurnal and semidiurnal components of the oscillation display large amplitudes and a 180° phase inversion near the layer peak. These features are interpreted in terms of the propagation of tides in the layer, taking into account the interaction between the tide and the minor constituent layer. It is shown that the vertical wind is the most important factor that determines the amplitude of the oscillation in sodium density at a fixed height, thus making it possible to estimate the phase and amplitude of the wind oscillations over a limited height range. Diurnal and semidiurnal vertical wind amplitudes of 2–6 cm s−1 and 5–20 cm s−1, respectively, have been inferred. The 12‐hour component shows vertical phase propagation with a wavelength of ∼50 km, in agreement with recent theories. The 24‐hour component, however, shows characteristics of an evanescent mode instead of the expected S1.1 mode. Maximum upward vertical velocity occurs at about 2100 LT at all heights for the diurnal component and at 0600 and 1800 LT at 85 km for the semidiurnal component. These results appear to be the first reported measurements of tidally induced vertical displacements in the mesopause region.