What ionospheric workers should know about the plasmapause--plasmasphere

Recent research on the structure and dynamics of the magnetospheric thermal plasma indicates that the vast region above an altitude of ∼1000 km rivals the underlying ionosphere in complexity and that it is coupled to the lower region in complicated, physically important ways. An example involves the relation of the electron content of magnetospheric tubes of ionization to the electron content of the regular ionosphere. Tube volume between ∼1000 km and the magnetic equator varies rapidly over a relatively small range of tube end point latitudes, which gives rise to correspondingly rapid variations with latitude in coupled effects that involve interchange of ionization between the upper and lower regions. In the past, some correlative studies involving the plasmapause have been hindered by lack of information concerning (1) the unsteady nature of the process by which the disturbed‐time plasmapause profile is established and (2) the fact that at most times and at most locations the plasmasphere‐plasmapause system is in a state of recovery. A series of equatorial density profiles is shown to illustrate the reduction of plasmapause radius during brief periods of increased disturbance and the recovery of the plasmasphere by various processes, particularly by filling from the underlying ionosphere. A number of research results are presented as part of a 'quick‐reference guide' to the plasmasphere. To the ionospheric observer, the plasmapause should appear to have a complex but generally predictable geometry as well as characteristic motions. A crude predictor of plasmapause L value ( L pp ) in the post‐midnight period as a function of magnetic disturbance is the formula L pp = 5.7‐0.47 Kp , where Kp is the maximum 3‐hour Kp value in the preceding 12 hours. A ground station at L ∼ 3.7 is recommended as optimum for observation of plasmapause‐associated effects directly overhead. The plasmasphere is regularly perturbed by substorm‐associated convection electric fields, and these apparently have important effects on the nightside ionosphere at middle latitudes. Other known departures of the plasmasphere from corotation are expected to have their counterparts in the ionosphere.