This review summarizes some of the recent results on the disturbed upper atmosphere obtained by satellite‐borne gas analyzers. According to these measurements, magnetospheric activity leads to the development of two different disturbance zones. The high‐ and middle‐latitude region is characterized by an increase in the heavier constituents Ar, N2, and O2, by a height‐dependent behavior of O, and by a significant decrease of He. The reaction time of the atmosphere is much smaller than one orbital period. At lower latitudes a moderate increase of all constituents is observed. A comparison between atmospheric and ionospheric data demonstrates that, in contrast to positive effects, negative ionospheric storms are closely coupled to changes in the neutral composition. In addition, model calculations fully support a causal relation between both phenomena. Given this correlation, atmospheric and negative ionospheric disturbance effects have certain variations in common. These include systematic changes with the magnetic storm intensity, with magnetic position, with local time, and with season. Whereas the presently available empirical and theoretical models are quite capable of reproducing the basic properties of the observed atmospheric perturbations, these algorithms are not sophisticated enough for a more detailed description of the data.

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Cover picture: Dayglow and aurora imaged from the DEI satellite at an altitude of about 20,000 km. Further information may be found in Sections 3.3.8 and 7.4 (L.A. Frank, University ofIowa).

The upper atmosphere constitutes the outer region of the terrestrial gas envelope above about 100 km altitude. The energy budget of this outer gas layer is partly controlled by the dissipation of solar wind energy. Since this energy input is largely irregular, the resulting density changes are considered as perturbations. The properties and physics of such density perturbations are reviewed here. Besides being an important link in the complex chain of solar-terrestrial relations, such disturbances are also of practical interest because they affect the orbits of satellites and space stations and are responsible for ionospheric disturbance effects.Keywords Thermosphere (density and composition) Á Thermosphere (disturbances and storms) Á Solar-terrestrial relations (space weather effects)

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