POTENTIAL CONTROL METHOD FOR THERMAL PLASMA MEASUREMENTS ON THE DE-1 SPACECRAFT.

Thermal plasma measurements from the Dynamics Explorer 1 spacecraft using an aperture bias in the outer plasmasphere and over the polar cap show the existence of cold plasma (T less than equivalent to 1 ev) which is otherwise hidden from the particle detector by spacecraft potentials between plus 1 and plus 3 v. In eclipse, the photoelectron current drops and the spacecraft potential drops to near zero volts. At density levels between 10 and 100 cm** minus **3, the cold plasma becomes visible in eclipse, along with a warmer tail to the distribution. Operations of the RIMS instrument with the external aperture plane biased up v minus 8 V with respect to the spacecraft show the existence of low energy field-aligned flows over the polar cap.