Non-thermal ionospheric plasma studies using the incoherent scatter technique

Non-thermal phenomena are a common occurrence in the aurora] ionosphere. The incoherent scatter technique is now frequently used to study several of the features associated with the departure of the plasma from a thermal equilibrium configuration. In this review we limit our discussion to E and F region phenomena. The first observation that we discuss is that of a large increase in the auroral E region electron temperature during periods of strong convection electric fields. We illustrate these observations with a few examples and summarize the current state of the theoretical debate on electron heating by turbulent ion-acoustic waves. Strongly asymmetric incoherent scatter spectra offer a second example of the effect of ion-acoustic turbulence on the wave spectra. We describe the observations and review three different possible physical mechanisms which have been proposed in the literature. The final part of this review deals with the signature of non-Maxwellian ion velocity distributions on incoherent scatter spectra. These non-Maxwellian distributions appear in the presence of a strong electric field. The first theories dealing with the creation of these distributions and with its effect on the incoherent scatter spectrum were developed in the mid-1970s and evolved in the 1980s. The most important problem, currently, is with the interpretation in terms of ionospheric parameters of the incoherent scatter spectra during the disturbed periods that lead to the non-Maxwellian signatures. We emphasize recent advances in the data analysis and discuss the remaining limitations while evaluating the direction of future work on this subject.