Thermal plasma parameters derived by the mutual impedance experiment on GEOS are described. The experiment is well suited to the measurement of the electron density and temperature of the outer plasmasphere (when kTe/Ne < 1.6 eV/cm³). This investigation of the whole set of data supplied by GEOS 1 (4 < L < 8, 0000–2400 MLT) covers three main regions : the plasmasphere, an intermediate region of ionospheric refilling, and the plasma trough. In the plasmasphere, we observe profiles with Ne ∝ L−4, while Te stands around 10,000° K or less. The intermediate region, situated next to the plasmasphere and above it, is always present in the day sector, where the ionospheric source plays a leadingpart. In that zone, the plasma parameters, poorly known up to now, exhibit Ne values ∼ 2 to 20 cm−3, together with Te values of 20,000° K on the average, dispersed over a 5,000 to 100,000° K range during disturbances. In the night sector, the intermediate region is seen only during the recovery phase. The region of depleted density is observed at the higher L values in the night and morning MLT sectors. There, plasmas out of Maxwellian equilibrium are seen under disturbed conditions. The dynamic response of the thermal plasma parameters to temporal variations of the am index of magnetic activity follows a known scenario as concerns Ne, making apparent a night-to-day, MLT dependent time delay. As concerns Te, the dynamical study reveals striking features, such as the persistance of the Te modifications into the dusk sector, the interpretation of which remains to be clarified.