Thermal/suprathermal plasmas observed by the S-302 experiment on GEOS-1

The S-302 experiment has benefited more than most from the non-geostationary nature of the GEOS-1 orbit in so far as additional regions of quite different thermal and suprathermal plasmas were made accessible. Electron and positive ion spectra from three regions, the plasmasphere, plasmatrough and boundary layer, are described in order to highlight the variety of thermal plasmas observed.We show how, even in the presence of the local photoelectron sheath and its associated electric potential, the thermal characteristics can be derived. The success of this technique during active periods is demonstrated by the observation of both a heating of the thermal population and the appearance of a second field aligned thermal component during a period of intense wave activity.The detailed structure of the boundary layer adjacent to the magnetopause as observed on the 2nd December, 1977 shows only slow systematic development over a period of more than an hour. Where the thermal plasma density peaks the suprathermal positive ions show considerable drift motion as well as non-Maxwellian characteristics. This region is identified from the thermal particle data as being very close to the magnetopause, however, whether in fact the magnetopause was crossed requires corroborative data from the magnetometer and other experiments.