Stability of the mesopause region: Influence of dissipating gravity waves on the transport of heat, momentum and constituents

More than 200 hours of Na density, wind and temperature observations in the mesopause region (80–105 km) were obtained at the Starfire Optical Range (35°N, 106.5°W), NM during 94–95 period and 98–99 period. The structure and seasonal variations of static and dynamic instabilities are examined using horizontal wind and temperature data. The mesopause region is more stable in summer than in winter. Tides can establish the environment for instability to develop. As gravity waves propagate through the layer of reduced static stability, instabilities are induced. With the presence of tides, the statically unstable conditions are generally preceded by dynamically unstable conditions. The vertical dynamical transport of Na associated with dissipating gravity waves is studied using wind and Na density data. The vertical constituent flux can be related in a simple way to the vertical heat flux. Breaking gravity waves also contribute to eddy transport by generating turbulence, which transports constituent along the concentration gradient via diffusion. The observational results are consistent with theoretical predictions and show that dynamical transport often exceeds the vertical transport associated with eddy diffusion. The theoretical models are used to predict the dynamical and eddy fluxes of atomic oxygen and show that for this constituent, dynamical transport is the dominant transport mechanism. Seasonal variations of gravity wave characters and vertical flux profiles of horizontal momentum, heat and Na are studied using wind, temperature and Na density data. Strong dissipation below 95 km causes the local minimum of gravity wave energy just above 95 km. Gravity wave energy exhibits semiannual variations with maximum in winter and minimum in fall. The vertical fluxes of momentum, heat and Na exhibit annual variation with maximum in winter except for zonal momentum flux. Downward heat transport below 95 km causes strong cooling effects at 90 km. The deposit of momentum accelerates the (eastward for zonal wind, northward for meridional wind) mean flow for most of the time except for winter in zonal direction. Predicted dynamical Na are in good agreement with the observed dynamical Na flux and both are larger than the eddy transport of Na.