Recent experience with spacecraft potential control

Abstract The stabilisation of the electric potential of a spacecraft, which would otherwise amount to several tens of volts positive in tenuous plasma regions of the magnetosphere, significantly improves the conditions for measurements of the thermal plasma. One of the possible techniques relies on the controlled emission of a positive ion beam of several keV and has first been demonstrated on the spacecraft Geotail operating since 1992. More recently similar instruments have been developed for the spacecraft Interball-2 (Auroral Probe) and Equator-S launched in 1996 and 1997, respectively, which have quite different sizes and orbital characteristics. Initial observations related to ion beam operation onboard these spacecraft are presented, demonstrating the working principle of spacecraft potential control using ion beams of high energy. In the light of the experience from Interball and Equator-S the design of the ion emitters for the four Cluster-II spacecraft to be launched in 2000 could be optimised.