The exospheric plasma during the international years of the quiet sun

Although a great deal of information is now available about the distribution of electrons in the topside ionosphere, much of the published data refer to a single altitude, namely that of the vehicle carrying the experimental payload. It is well known that topside soundings provide accurate routine measurements of the electron density over a wide range of altitudes simultaneously. It is the purpose of this paper to provide information about a number of surveys made as an extensive programme of analysis carried out in the Space Sciences Division of the NASA Ames Research Center, the detailed results of which, presented in a series of NASA Special Publications, provide data on the overall morphology of the topside ionosphere during, and close to, the International Years of the Quiet Sun. The importance is emphasized of the diurnal and latitudinal variations of plasma scale height for the interpretation of the data in terms of the physics of the topside, and as an indication of geomagnetic control of the upper F-region. Attention is mainly concentrated on data derived for the highest altitudes possible (near 1000 km), away from the widely-discussed problems of the F-region peak (Rishbeth, 1968). The scale height distribution can be understood in terms of diffusive equilibrium theory, using which the mean ionic mass of the plasma can be derived.