Abstract
Density fluctuations obtained along seven space shuttle reentry tracks are used in this paper to examine the horizontal structure and the vertical distribution of density variance in the mesosphere and lower thermosphere. The tracks lie primarily over open ocean at middle and low latitudes and represent the only measurements of horizontal atmospheric structure at these heights available to date. The density fluctuations are interpreted in terms of gravity wave motions and reveal significant density (and velocity) variance at horizontal scales ranging from ∼10 to 1000 km. Fluctuation amplitudes are used to infer corresponding velocity perturbations and characteristic vertical scales and frequencies of the wave spectrum. Results suggest that the mean velocity variance is smaller over the Pacific ocean than observed over major land masses and that the variance increases with height in a manner consistent with that expected in the presence of wave saturation processes.
