Semiannual Variation at the Base of the Thermosphere

Abstract

In the 80- to 105-km height region, zonal winds and their vertical shears, as well as ionospheric drifts, show a semiannual variation that is in phase with a similar oscillation in temperature and density at satellite heights (190?1130 km). At higher latitudes, the effect is purely semiannual, but at 35° latitude a 12-mo term seems to be superimposed. Similar variations must exist in the latitudinal temperature gradient that is associated with vertical shears. Temperature data were not available for this study, requiring a chain of assumptions in order to draw some inferences regarding this parameter. Under these assumptions, the mean temperature of the 80- to 105-km layer could possess temporal variations nearly parallel to changes observed in shears. This would suggest, for midlatitudes, a temperature variation composed of a 12-mo and a 6-mo term of roughly equal magnitudes. Such variations in temperature and in the latitudinal temperature gradient could be explained by systematic changes in the vertical thermal structure of the 80- to 105-km layer. At least in the parameters of motion, a latitudinal effect appears to be present up to a height of 160 km, whereas seasonal changes attenuate rapidly above 100 km. It is thus possible that the semiannual variation in motion parameters is exposed, as a primary factor, at heights not far removed from 120 km. Whether an analogous situation exists in temperature is at present a matter of conjecture.