The incoherent scatter radar as a tool for studying the ionospheric D-region

The theory of the D-region incoherent scatter process and the techniques for incoherent scatter radar measurements in the D-region have both reached sufficient maturity that a synopsis of this area for the middle atmosphere community is appropriate. For example, all current UHF incoherent scatter radars can make useful D-region measurements, at least under some conditions, and the Arecibo 430 MHz radar is being used for a regular program of D-region measurements. Atmospheric and ionospheric parameters which are accessible, to varying degrees, as functions of time and height include electron concentration, ion-neutral collision frequency, neutral atmosphere temperature pressure and winds and mean negative and positive ion masses and concentrations. A qualitative view of the collision dominated incoherent scatter process is given as an aid in understanding the capabilities and limitations of these measurements. Included here is a discussion of some of the issues still surrounding the theory of collision dominated incoherent scattering. Also presented is an overview of data gathering, processing and interpretation techniques, with comments on how total power and power spectrum measurements should be combined to give optimum results. Finally, a comprehensive list of outstanding problems in D-region aeronomy is given, along with suggestions of how incoherent scatter radar along with other MAP measurements can address them. Included in this list are the winter absorption anomaly. D-region 'ledge' chemistry and high latitude D-region phenomena.