Nearly simultaneous and conjugate measurements of the thermal plasma density and temperature structure in the early evening ionosphere and plasmasphere are obtained with the DE 1 and 2 satellites. The results indicate a number of significant relationships in plasmasphere-ionosphere thermal plasma interchange and coupling. The principal results are as follows: (1) A transition in the ionospheric electron temperature (Te) from a relatively smooth profile of low Te (<2000°K) at the base of the inner plasmasphere to enhanced and highly structured Te at higher invariant latitudes occurs near or along a plasmaspheric density gradient. (2) Plasmaspheric enhancements of the heavy ions O+ and O++ are often closely aligned with distinct ionospheric Te enhancements. (3) When the plasmaspheric light ion density profile contains a significant trough, there is a pronounced ionospheric Te peak aligned with the poleward plasmaspheric density enhancement or the poleward edge of the plasmaspheric trough. (4) The latitudinal structures of ionospheric Te and plasmaspheric TH+ are sometimes strikingly well-correlated. (5) The plasmaspheric density ratio He+/H+ is generally of the order 0.2 and appears to be insensitive to such parameters as the invariant latitude or the plasmaspheric and ionospheric densities and temperatures. (6) Equatorward of the plasmapause region/outer plasmasphere there is no consistent relation between ionospheric and plasmaspheric density structure, with the ionospheric electron densities frequently displaying smooth profile regions across field lines where highly complex plasmaspheric density structure is seen. (7) The evolution of the plasmasphere and F region/topside ionosphere during the recovery phase of a magnetospheric storm illustrates how the time scales for replenishment of the ionosphere are significantly shorter than those of the plasmasphere. Certain of these observations are interpreted qualitatively in terms of extensions of current ideas on plasmasphere-ionosphere coupling, in particular, ring current/plasmasphere/stable auroral red arc interactions and plasmasphere refilling. Other features are puzzles that await satisfactory theoretical explanation.