Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsa52a..01w&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SA52A-01
Other
2435 Ionospheric Disturbances, 3384 Acoustic-Gravity Waves
Scientific paper
Ducted gravity waves are trapped internal gravity waves that can be sustained with minimal diminishment in the absence of sustained forcing. They are near-resonant waves trapped by evanescence, or rapid vertical variations in the background state. There is accumulating evidence of a large population of ducted gravity waves in the upper mesosphere and thermosphere. Ducted waves may account for much, if not most, of the coherent wave forms seen in the upper atmosphere. Persistence and long distance propagation is favored by some degree of ducting. Wave amplification in the upper atmosphere related to ducting provides a means for waves to propagate to great heights with significant amplitude where they may interact with the ionosphere. Ducts may be caused by thermal stratification (peaks in the Brunt-Väisälä frequency) or by variations in the intrinsic frequency due to wind shear. A thermal duct is always present in the lower thermosphere and another is often present in the upper mesosphere. The latter is due to an inversion below the mesopause and collocated or nearly collocated with the OH emission layer. When the winds are favorable these ducts can support very strong ducting events. I present the theory of wave ducting, show calculations of favored regions in phase-speed, wavenumber space for wave ducting as a function of the background wind and thermal structure and discuss observational evidence for ducting including recent observations of strong bore-like ducting events and standing waves in ionospheric ion-density perturbations.
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