Seasonal variation in the solar diurnal tide and its possible influence on the atmospheric sodium layer

In a paper published 12 years ago, we showed that the height of the atmospheric sodium layer at our location is about 1 km lower in November than at any other time of the year. We also showed that the decrease in height of the sodium layer was accompanied by an increase in the intensity of the OI 557.7 nm and OH(9,4) band airglow emissions. At that time we suggested that this behavior could be the result of large scale convective transport in the MLT region. We have now had the opportunity to compare the diurnal variations of the sodium layer and airglow emissions with the tidal winds measured by meteor radar over the past 5 years. We find that the amplitude of the diurnal tide is much smaller in November than at other times of the year. Since most of the sodium measurements and all of the airglow observations are for night-time conditions only, a change in the amplitude of the 24-h tide could strongly influence the average measured sodium and airglow parameters. It is shown that the observed changes in the tidal winds are qualitatively consistent with the sodium measurements, but the amplitude of the observed height change is much greater than would be expected from the tidal winds.