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Ann. Geophys., 19, 1289-1302, 2001
www.ann-geophys.net/19/1289/2001/
© European Geosciences Union 2001
Active spacecraft potential control for Cluster – implementation and first results
K. Torkar1, W. Riedler1, C. P. Escoubet2, M. Fehringer2, R. Schmidt2, R. J. L. Grard2, H. Arends2, F. Rüdenauer3, W. Steiger4, B. T. Narheim5, K. Svenes5, R. Torbert6, M. André7, A. Fazakerley8, R. Goldstein9, R. C. Olsen10, A. Pedersen11, E. Whipple12, and H. Zhao13
1Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
2Space Science Department of ESA/ESTEC, 2200 AG Noordwijk, The Netherlands
3Now at: International Atomic Energy Agency, Safeguards Analytical Laboratory, 2444 Seibersdorf, Austria
4Institute for Physics, Austrian Research Centers Seibersdorf, 2444 Seibersdorf, Austria
5Forsvarets Forskningsinstitutt, Avdeling for Elektronikk, 2007 Kjeller, Norway
6Space Science Center, Science and Engineering Research Center, University of New Hampshire, Durham, NH 03824, USA
7Swedish Institute of Space Physics, Uppsala Division, 75121 Uppsala, Sweden
8Dept. of Physics, Mullard Space Science Laboratory, University College London, Dorking, Surrey, UK
9Southwest Research Institute, San Antonio, Texas 78238, USA
10Physics Department, Naval Postgraduate School, Monterey, California 93943, USA
11Dept. of Physics, University of Oslo, Blindern, Norway
12University of Washington, Geophysics Department, Seattle, Washington 98195, USA
13Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100080, P. R. China
Abstract. Electrostatic charging of
a spacecraft modifies the distribution of electrons and ions before the
particles enter the sensors mounted on the spacecraft body. The floating
potential of magnetospheric satellites in sunlight very often reaches several
tens of volts, making measurements of the cold (several eV) component of the
ambient ions impossible. The plasma electron data become contaminated by large
fluxes of photoelectrons attracted back into the sensors. The Cluster
spacecraft are equipped with emitters of the liquid metal ion source type,
producing indium ions at 5 to 9 keV energy at currents of some tens of
microampere. This current shifts the equilibrium potential of the spacecraft to
moderately positive values. The design and principles of the operation of the
instrument for active spacecraft potential control (ASPOC) are presented in
detail. Experience with spacecraft potential control from the commissioning
phase and the first two months of the operational phase are now available. The
instrument is operated with constant ion current for most of the time, but
tests have been carried out with varying currents and a "feedback"
mode with the instrument EFW, which measures the spacecraft potential . That
has been reduced to values according to expectations. In addition, the low
energy electron measurements show substantially reduced fluxes of
photoelectrons as expected. The flux decrease in photoelectrons returning to
the spacecraft, however, occurs at the expense of an enlarged sheath around the
spacecraft which causes problems for boom-mounted probes.
Key words. Space plasma physics
(spacecraft sheaths, wakes, charging); Instruments and techniques; Active
perturbation experiments
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