Computer Science – Sound
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsa31a0353p&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #SA31A-0353
Computer Science
Sound
3367 Theoretical Modeling, 3369 Thermospheric Dynamics (0358), 3384 Acoustic-Gravity Waves
Scientific paper
The Transfer Function Model (TFM) provides a unique semi-analytic view of gravity wave propagation. Globally propagating, ducted, impulsive gravity wave modes, generated by auroral activity, are the particular focus of the results presented. These ducted modes exhibit a very sharp peak in transfer function frequency spectrum, which indicates near dissipationless propagation. Unlike other spectral features, these sharp peaks are seen to extend all the way down to the ground. We show that ducted waves associated with these peaks: (1) can be generated by auroral heating in the lower thermosphere, (2) propagate downward into the lower atmosphere, (3) are reflected from the earth's surface, and (4) are ultimately ducted by reflection from the large temperature gradient of the lower thermosphere and the reflection boundary at the earth's surface. These waves have a propagation velocity of approximately 250 m/s, characteristic of the sound speed of the lower atmosphere, which is much slower than the approximate 700 m/s propagation velocity of Lamb waves in the thermosphere. The ducted modes then have also shorter horizontal wavelengths than the Lamb waves, and waves with the ducted mode characteristics have been observed in several studies. Given the near dissipationless characteristics, we find that auroral excitation processes produce pronounced geometrical focusing effects near the poles. The near dissipationless propagation of these waves through the lower atmosphere also can produce powerful resonances in the near source region, where reflections from the surface can constructively or destructively interfere with the upper atmospheric motions. Computer animations of the ducted wave propagation will demonstrate the various effects mentioned. Our study suggests that surface generated gravity waves, which have the correct ducted mode characteristics, can themselves produce observable effects in the thermosphere.
Mayr Hans G.
Porter Hayden S.
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