Active experimentation with the ionospheric plasma

Considerable effort in recent years has been directed towards experiments that actively perturb the ionospheric plasma, producing a measure of control over its properties (locally and ephemerally) and also exciting a variety of plasma instabilities. Space vehicles have been used for the release in the ionosphere of exotic chemicals and for the operation in the ionosphere of charged particle accelerators. Ground based radio transmitters at VHF perturb the magnetosphere, while transmitters at HF and higher frequencies perturb the ionosphere. This paper discusses the last technique, often called ionospheric heating, which uses powerful transmitters to change electron densities and temperatures and to excite a variety of parametric instabilities in the ionospheric plasma. Recent work in this area is cited and a qualitative description is given of the physical mechanisms that underlie many of the observed phenomena. Bibliographies of recent work are included for the other three techniques of active experimentation.