Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa21b..08l&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SA21B-08
Physics
[0355] Atmospheric Composition And Structure / Thermosphere: Composition And Chemistry, [0399] Atmospheric Composition And Structure / General Or Miscellaneous, [2437] Ionosphere / Ionospheric Dynamics
Scientific paper
A striking feature of chemical release trails in the lower thermosphere is the transition from turbulent flow to apparently laminar flow at the nominal turbopause height. The transition is very sharp, occurring over a vertical distance of a few hundred meters or less and is easily recognizable in the images. Detailed quantitative information about turbulence properties in the critical transition region is difficult to obtain. The chemical trails are one of the few sources of such information, although extracting information about turbulent energy dissipation rates and turbulent diffusion coefficients requires images with good time and angular resolution and is a time intensive analysis process. The number of such analyses is therefore limited, but the results consistently show direct evidence of anomalous diffusion over an altitude range that extends 10-15 km above the nominal turbopause, i.e., in an altitude range where the flow qualitatively appears to be laminar. Anomalous diffusion, in this case, refers to tracer expansion that is more rapid than expected from molecular diffusion alone. The region where the anomalous trail expansion is found is also characterized by large wind speeds that tend to maximize within that altitude range. The maximum winds generally exceed 80 m/s and are often in the 100 to 150 m/s range, as shown by a large database of more than 500 chemical release wind measurements obtained over an extended period at a variety of locations and local times. In addition, rapid transport of Space Shuttle exhaust products from low to high latitudes is also known to occur in the same altitude range, suggesting that the transport is effective over large scales. The observational evidence thus indicates that the lower thermosphere, in the altitude range between 100 and 115 km, is a region of enhanced anomalous transport that operates over a broad range of spatial scales, from a kilometer or less to the planetary scale. Evidence for two-dimensional turbulence as an important process in the lower thermosphere, including Hall drag effects at larger scales, will be presented, and the role of such turbulent processes as an explanation for the observed transport properties will be discussed.
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