Effects of active spacecraft potential control on CLUSTER plasma observations - First results

The active spacecraft potential control devices ASPOC onboard the CLUSTER satellites off-set the spacecraft charging induced by the ultra-violet radiation from the sun by emitting positive indium ions. This clamps the spacecraft potential to within a few volts of the plasma potential. The lowering of the potential barrier between the spacecraft and the ambient plasma allows more low energy ions to reach the sensors. Hence, a larger part of the ion distribution is obtained. In addition, most photo-electrons can escape the vicinity of the satellite, whereby the contamination of the electron spectra is reduced. This paper concentrates on some of the first results from CLUSTER, presenting the initial current-voltage characteristics of the spacecraft, and illustrating the considerably improved plasma measurements due to the lower spacecraft potential and the dampening of the potential variations caused-by rapid changes in the ambient plasma environment. It is demonstrated that a further stabilisation of the potential can be obtained by varying the emitted ion current in an onboard control loop. The modifications of the sheath around the spacecraft due to the re-distribution of particles and the ion beam are addressed.