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Volume 19, issue 10/12
Ann. Geophys., 19, 1289–1302, 2001
© Author(s) 2001. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: CLUSTER

Ann. Geophys., 19, 1289–1302, 2001
© Author(s) 2001. This work is distributed under
the Creative Commons Attribution 3.0 License.

  30 Sep 2001

30 Sep 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 K. Torkar et al.
  • 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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