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Abstract
Abstract. The payload of Equator-S was complemented by the potential control device (PCD) to stabilise the electric potential of the spacecraft with respect to the ambient plasma. Low potentials are essential for accurate measurements of the thermal plasma. The design of PCD is inherited from instruments for Geotail and Cluster and utilises liquid metal ion sources generating a beam of indium ions at several keV. The set-up of the instrument and its interaction with the plasma instruments on board is presented. When the instrument was switched on during commissioning, unexpectedly high ignition and operating voltages of some ion emitters were observed. An extensive investigation was initiated and the results, which lead to an improved design for Cluster-II, are summarised. The cause of the abnormal behaviour could be linked to surface contamination of some emitters, which will be monitored and cured by on-board procedures in future. The mission operations on Equator-S were not at all affected, because of the high redundancy built into the instrument so that a sufficient number of perfectly operating emitters were available and were turned on routinely throughout the mission. Observations of the effect of spacecraft potential control on the plasma remained limited to just one event on January 8, 1998, which is analysed in detail. It is concluded that the ion beam lead to the predicted improvement of the particle measurements even outside the low density regions of the magnetosphere where the effect of spacecraft potential control would have been much more pronounced, and that the similar instruments for the four Cluster-II spacecraft to be launched in 2000 will be very important to ensure accurate plasma data from this mission.Key words. Space plasma physics (active perturbation experiments; spacecraft sheaths · wakes · charging; instruments and techniques)
Highlights
Despite being a small spacecraft of less than 235 kg dry mass, Equator-S carried a complete payload for plasma and ®eld investigations in the Earth's magnetosphere
The plasma measurements were covered by the 3D-plasma analyser (3DA) instrument, based on electrostatic analysers for electrons and ions in the energy range from a few eV to 25 keV, and the ion composition instrument (ESIC) which could resolve the major ion species in the E/q range from essentially zero up to 40 keV
Plasma sources as on Polar and ion sources based on gas discharge as on Interball-2 require heavy tanks for the source material. This is not the case for a new generation of ion emitters, the working principle of which is based on the ionisation of a ®lm of liquid metal in a strong electric ®eld. These liquid metal ion sources (LMIS) are characterised by a small mass which is almost negligible in comparison to the required housing for protection and drive electronics
Summary
Despite being a small spacecraft of less than 235 kg dry mass, Equator-S carried a complete payload for plasma and ®eld investigations in the Earth's magnetosphere. This is not the case for a new generation of ion emitters, the working principle of which is based on the ionisation of a ®lm of liquid metal in a strong electric ®eld These liquid metal ion sources (LMIS) are characterised by a small mass which is almost negligible in comparison to the required housing for protection and drive electronics. They have been successfully implemented in an instrument aboard the Geotail spacecraft (EFD-iE) and have been selected for Equator-S as well as for the four forthcoming Cluster-II spacecraft. We have a dual objective to demonstrate the working principle of the applied technique under conditions encountered at the orbit of Equator-S, which is quite dierent from those of the spacecraft Geotail and Polar, and to show which lessons were learned for future missions by operating the potential control device
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