Comparison of thermal plasma observations of SCATHA and

Thermal plasma measurements on the SCATHA and GEOS satellites show the existence of the plasmasphere out to beyond geosynchronous orbit in the dusk bulge region. On at least one occasion, this plasma population persisted until local midnight, as observed by both satellites. These observations establish the context for the 'hidden ion' population reported by Olsen [1982] as the outer plasmasphere. The GEOS electrostatic analyzers, which were on biased booms, were able to measure the cold plasma with a-8 V bias. The SCATHA electrostatic analyzers were unable to measure this plasma in sunlight, but could in eclipse. The floating potential of the GEOS satellite in sunlight in this 50 cm-3 plasma was about +3 V, and it is apparent that SCATHA was at a similar potential. This potential was sufficient to prevent the measurement of the 0.6-0.7 eV temperature plasma. 1