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Ann. Geophys., 17, 1582-1591, 1999
www.ann-geophys.net/17/1582/1999/
© European Geosciences Union 1999
Spacecraft potential control aboard Equator-S as a test for Cluster-II
K. Torkar1, W. Riedler1, M. Fehringer2, F. Rüdenauer2, C. P. Escoubet3, H. Arends3, B. T. Narheim4, K. Svenes4, M. P. McCarthy5, G. K. Parks5, R. P. Lin6, and H. Rème7
1Space Research Institute, Austrian Academy of Sciences, Inffeldgasse 12, A-8010 Graz, Austria
E-mail: klaus.torkar@oeaw.ac.at
2Austrian Research Centre Seibersdorf, A-2444 Seibersdorf, Austria
3Space Science Division, ESA/ESTEC, NL-2200 AG Noordwijk, The Netherlands
4Norwegian Defence Research Establishment, N-2007 Kjeller, Norway
5University of Washington, Seattle, WA 98195, USA
6Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
7CESR, Université Paul Sabatier, BP 4346, F-31028 Toulouse, France
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)
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