Abstract
The coupled thermosphere-ionosphere-plasmasphere model CTIP is used to study the global three-dimensional circulation and its effect on neutral composition in the midlatitude F-layer. At equinox, the vertical air motion is basically up by day, down by night, and the atomic oxygen/molecular nitrogen [O/N2] concentration ratio is symmetrical about the equator. At solstice there is a summer-to-winter flow of air, with downwelling at subauroral latitudes in winter that produces regions of large [O/N2] ratio. Because the thermospheric circulation is influenced by the high-latitude energy inputs, which are related to the geometry of the Earth's magnetic field, the latitude of the downwelling regions varies with longitude. The downwelling regions give rise to large F2-layer electron densities when they are sunlit, but not when they are in darkness, with implications for the distribution of seasonal and semiannual variations of the F2-layer. It is also found that the vertical distributions of O and N2 may depart appreciably from diffusive equilibrium at heights up to about 160 km, especially in the summer hemisphere where there is strong upwelling. Atmospheric composition and structure (thermosphere · composition and chemistry) · Ionosphere (ionosphere · atmosphere interactions)
Highlights
It has long been known that the F2-layer shows seasonal and semiannual anomalies that depart from normal solar-controlled behaviour
Suggested that the seasonal anomaly is caused by changes in chemical composition, speci®cally in the ratio of the concentrations of atomic oxygen and molecular nitrogen. Such changes of the O=N2 ratio would account for the anomaly in noon NmF2
The major neutral constituents at F2-layer heights are atomic oxygen and molecular nitrogen, their scale heights being in the ratio 28:16. If these gases are in diusive equilibrium, it is seen that their partial pressures p vary with height in such a way that ln p(O) ÿ 16 ln p N2 const: 9
Summary
It has long been known that the F2-layer shows seasonal and semiannual anomalies that depart from normal solar-controlled behaviour They are described in some detail by Yonezawa and Arima (1959) and by Torr and. Millward et al (1996a) showed that the seasonal composition changes, taken in conjunction with the seasonal changes of solar zenith angle, can broadly account for the existence of both annual and semiannual variations in NmF2. This has been con®rmed by a comprehensive modelling study by Zou et al. The present work describes the ®rst detailed computational study of the vertical circulation and the related variations of the neutral O=N2 ratio. MuÈller-Wodarg: Vertical circulation and thermospheric composition: a modelling study winds and the neutral O=N2 ratio, Sects. 6 and 7 conclude the report
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