Although we are aware that the theories reviewed in Chapter 5 neither address nor explain all aspects of currently available observations, the authors hope nevertheless that the Chapter has described the state of the art in this field of investigation. We have outlined, in a historical perspective, the successive steps in modelling the plasmasphere and its outer boundary, the plasmapause. As has happened in many other fields of investigation, the first model is the best known of all, but several successive generations of models were proposed later on to improve or replace the initial picture. The successive improvements on the preceding work have been outlined, as well as the limitations of each of the successive models. The authors will be rewarded for their efforts in writing this last Chapter and the whole book if it succeeds in setting the stage for the generation of future theories, if it stimulates new ideas and if it produces a revival of interest for the plasmasphere and plasmapause region.

The history of the discovery of the plasmapause by Gringauz and Carpenter, respectively, in the former Soviet Union and United States, has been reported in Chapter 1. Those who worked in this field in the 1960s will remember some of the episodes, but probably will have discovered other aspects of the story that are not published anywhere else.