Aperture Plane Potential Control for Thermal Ion Measurements

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 ~1 eV) which may otherwise be hidden from the particle detector by positive spacecraft potentials. Operations of the retarding ion mass spectrometer instrument with the external aperture plane biased up to-8 V with respect to the spacecraft reveal the existence of low-energy (few eV) field-aligned flows over the polar cap. Electrostatic models of the spacecraft indicate that if the spacecraft potential is more than a few volts positive, there can be a saddle point, or an electrostatic barrier, inhibiting the effectiveness of the aperture bias. Comparison of the aperture bias experiments with the electrostatic models shows that the aperture bias is successful in allowing the measurement of those field-aligned flows with kinetic energy greater than the predicted barrier height but is less effective in aiding the measurement of the isotropic background. The addition of electron measurements from the high-altitude plasma instrument allows the determination of the spacecraft potential when it is greater than +5 V. This additional information allows us to determine that the dayside polar wind (reported by Sojka et al., 1983) is supersonic, up to Mach 5