Some properties of upper atmosphere dynamics

Composition and temperature data from satellite measurements and from radar backscatter observations show that dynamics is a major key in our understanding of the upper atmosphere. Individual gases are tracers of wind fields which in turn are driven by the energy and momentum sources of the complex magnetosphere‐thermosphere‐lower atmosphere system. As is shown with data from Ogo 6, ion drag effects on winds impose a magnetic field control on the thermospheric density structure. Differential heating manifests itself in day‐night and summer‐winter temperature contrasts and drives large‐scale circulation that redistributes the minor gases to account for the observed winter bulges in O and He (a factor of 40 larger in winter than in summer) and the large phase separations between atmospheric species in the diurnal tide. Large‐scale atomic oxygen transport within the thermosphere significantly enhances the annual temperature amplitude (by more than a factor of 2) and, through release of chemical energy, contributes to the mesospheric temperature anomaly. The latter phenomenon and the associated reversals in circulation are largely determined by the annual tide excited near 50 km and possibly by dissipation of gravity and planetary waves in the winter hemisphere. During magnetic storms, Joule heating and viscous dissipation drive winds that effectively deplete O and He in the auroral zones and accumulate them at lower latitudes. Mass transport of O amplifies and confines the temperature enhancement to high latitudes, compatible with the observed global response of mass density. The timing of wind and composition effects significantly contributes to the positive and negative phases in ionospheric storms. Owing to the rotational nature of ion drift momentum sources, their signatures in the atmosphere are comparatively weak and differ substantially from those associated with heat sources.