The ionosphere poses an interesting challenge for many radio systems that make use of signal transmission through all or some portion of that medium. Being a magnetoionic medium imbedded in a background neutral atmosphere, it exhibits very interesting refractive properties, including anisotropy, dispersion, and dissipation. The laminar ionosphere introduces an array of effects, which are related to the ionospheric component of radio refractivity. These include ray path bending, phase path increase, group path delay, absorption, Doppler shift, pulse dispersion, Faraday rotation, and magnetoionic path splitting. Inhomogeneities in the ionosphere give rise to temporal and spatial variations in the effects just cited. An understanding of the ionospheric personality provides information about a wide range of solar-terrestrial interactions, and it has significant space-weather implications. Space weather is a new discipline that includes a wide range of exoatmospheric phenomena of major importance to space systems and their operational effectiveness.

The main features of the ionosphere are well known, although details are subjects of continuing research. There are many excellent sources of information about the ionosphere, from both a theoretical and an exper­imental perspective. The books by Davies (1,2,3), Ratcliffe (4), and Giraud and Petit (5) should be consulted. Theoretical and plasma-physics aspects of the ionosphere have been discussed in a book by Kelley (6). A read­able account of the basic physics of the ionosphere has been given by Rishbeth (7). Other useful references, which place the ionosphere within a larger context of the geospace weather system, include the Air Force Handbook of Geophysics and the Space Environment (8), and an Introduction to the Space Environment by Tascione (9). Various techniques for probing the ionosphere have been described in a monograph by Hunsucker (10). From a practical perspective, Goodman (11), Johnson et al. (12), and McNamara (13) have published ex­positions on the ionosphere in connection with radio system applications. There are also proceedings of topical conferences and workshops. The Ionospheric Effects Symposia (14) have chronicled ionospheric research ac­tivities and applications since 1975; and the Commission of the European Communities has published reports dealing with ionospheric prediction and modeling (15,16).

The purpose of this article is to provide a general understanding of the ionosphere. The emphasis is on those ionospheric processes and phenomena that are encountered by users of radio propagation systems. More complete descriptions of the underlying physical processes may be found in various references cited in the text. A final section on the current status of ionospheric research is provided as an aid to specialists and graduate students.

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