Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsa54a..08q&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SA54A-08
Other
0340 Middle Atmosphere: Composition And Chemistry, 0341 Middle Atmosphere: Constituent Transport And Chemistry (3334), 0355 Thermosphere: Composition And Chemistry, 3334 Middle Atmosphere Dynamics (0341, 0342), 3379 Turbulence (4490)
Scientific paper
Thermospheric neutral density and composition exhibit a strong seasonal variation, with maxima near the equinoxes, a primary minimum during northern hemisphere summer, and a secondary minimum during southern hemisphere summer. This pattern of variation is described by thermospheric empirical models. However, the mechanisms are not well understood. The annual insolation variation due to the Sun-Earth distance can cause an annual variation; large-scale inter-hemispheric circulation can cause a global semiannual variation; and geomagnetic activity can also have a small contribution to the semiannual amplitude. However, simulations by the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) indicate that these seasonal effects do not fully account for the observed annual/semiannual amplitude, primarily due to the lack of a minimum during northern hemisphere summer. A candidate for causing this variation is change in composition, driven by eddy mixing in the mesopause region. Other observations and model studies suggest that eddy diffusion in the mesopause region has a strong seasonal variation, with eddy diffusion larger during solstices than equinoxes, and stronger turbulence in summer than in winter. A seasonal variation of eddy diffusion compatible with this description is obtained. Simulations show that when this function is imposed at the lower boundary of the TIE-GCM, neutral density variation consistent with satellite drag data, and O/N2 consistent with measurements by TIMED/GUVI, are obtained. These model-data comparisons and analyses indicate that turbulent mixing in the mesopause region may contribute to seasonal variation in the thermosphere, particularly the asymmetry between solstices that cannot be explained by other mechanisms.
Kane Timothy J.
Qian Liwen
Solomon Stanley C.
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