Observations of plasmasphere-ionosphere coupling are made through thermal plasma measurements by the dual-spacecraft mission Dynamics Explorer. DE 1 measurements of ion composition and temperatures at 1.4--3.5 RE in the plasmasphere are combined with DE 2 measurements of the ion composition and electron and ion temperatures in the upper F region/topside ionosphere closely spaced in universal and local time for cases in the period November 6--11, 1981. Our findings include the following: (1) In some cases, there is evidence that the light ion latitudinal density profiles in the upper ionosphere resemble those for the plasmasphere even though the dominant O+ density profile in the ionosphere is typically much smoother and flatter. (2) There is strong evidence that the O+ density profile variations within the plasmasphere correlate with the ion and electron temperature latitudinal variations in both the plasmasphere and ionosphere, but have very little correlation with the comparatively smooth ionospheric O+ density profile. (3) The He+/H+ density ratio is typically much greater than unity in the upper ionosphere sampled by DE 2, but is around 0.2 in the plasmasphere data taken by DE 1. (4) The latitudinal variations of ionospheric O+ temperature and plasmaspheric H+ temperature are broadly similar in that both show general increases with latitude and occasional correlated peaks in the outer plasmasphere, although the plasmaspheric temperatures are typically a factor of 3 or more larger than the ionospheric temperatures. The observations are compared directly with calculations of altitudinal ion density and temperature profiles from the field-line interhemispheric plasma (FLIP) model. When the FLIP model permits fractional trapping of ionospheric photoelectrons and consequent plasmaspheric heating, generally quite good agreement with the observations is obtained. ¿ American Geophysical Union 1990