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David C. Fritts, Robert C. Blanchard, and Lawrence Coy

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.

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David C. Fritts, Ding-Yi Wang, and Robert C. Blanchard

Abstract

This study presents an analysis of density measurements made using high-resolution accelerometers aboard several space shuttles at altitudes from 60 to 140 km during reentry into the earth's atmosphere. The observed density fluctuations are interpreted in terms of gravity waves and tides and provide evidence of the importance of such motions well into the thermosphere. Height profiles of fractional density variance reveal that wave amplitudes increase at a rate consistent with observations at lower levels up to ∼90 km. The rate of amplitude growth decreases at greater heights, however, and appears to cease above ∼110 km. Wave amplitudes are nevertheless large at these heights and suggest that gravity waves may play an important role in forcing of the lower thermosphere.

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