NONTHERMAL IONOSPHERIC PLASMA STUDIES USING THE INCOHERENT-SCATTER TECHNIQUE

Non-thermal phenomena are a common occurrence in the auroral ionospher e. The incoherent scatter technique is now frequently used to study se veral of the features associated with the departure of the plasma from a thermal equilibrium configuration. In this review we limit our disc ussion to E and F region phenomena. The first observation that we disc uss is that of a large increase in the auroral E region electron tempe rature during periods of strong convection electric fields. We illustr ate these observations with a few examples and summarize the current s tate of the theoretical debate on electron heating by turbulent ion-ac oustic waves. Strongly asymmetric incoherent scatter spectra offer a s econd example of the effect of ion-acoustic turbulence on the wave spe ctra. We describe the observations and review three different possible physical mechanisms which have been proposed in the literature. The f inal part of this review deals with the signature of non-Maxwellian io n velocity distributions on incoherent scatter spectra. These non-Maxw ellian 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 paramet ers of the incoherent scatter spectra during the disturbed periods tha t lead to the non-Maxwellian signatures. We emphasize recent advances in the data analysis and discuss the remaining limitations while evalu ating the direction of future work on this subject. (C) 1996 Elsevier Science Ltd.