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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 24, issue 1
Ann. Geophys., 24, 275–289, 2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 24, 275–289, 2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.

  07 Mar 2006

07 Mar 2006

Electric field measurements on Cluster: comparing the double-probe and electron drift techniques

A. I. Eriksson1, M. André1, B. Klecker2, H. Laakso3, P.-A. Lindqvist4, F. Mozer5, G. Paschmann2,6, A. Pedersen7, J. Quinn8, R. Torbert8, K. Torkar9, and H. Vaith2,8 A. I. Eriksson et al.
  • 1Swedish Institute of Space Physics, Uppsala, Sweden
  • 2Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany
  • 3Solar System Division, ESA/ESTEC, Noordwijk, Netherlands
  • 4Alfvén laboratory, Royal Institute of Technology, Stockholm, Sweden
  • 5Space Sciences Laboratory, University of California, Berkeley, CA, USA
  • 6International Space Science Institute, Bern, Switzerland
  • 7Department of Physics, Oslo University, Norway
  • 8Space Science Center, University of New Hampshire, Durham, NH, USA
  • 9Space Research Institute, Austrian Academy of Sciences, Graz, Austria

Abstract. The four Cluster satellites each carry two instruments designed for measuring the electric field: a double-probe instrument (EFW) and an electron drift instrument (EDI). We compare data from the two instruments in a representative sample of plasma regions. The complementary merits and weaknesses of the two techniques are illustrated. EDI operations are confined to regions of magnetic fields above 30 nT and where wave activity and keV electron fluxes are not too high, while EFW can provide data everywhere, and can go far higher in sampling frequency than EDI. On the other hand, the EDI technique is immune to variations in the low energy plasma, while EFW sometimes detects significant nongeophysical electric fields, particularly in regions with drifting plasma, with ion energy (in eV) below the spacecraft potential (in volts). We show that the polar cap is a particularly intricate region for the double-probe technique, where large nongeophysical fields regularly contaminate EFW measurments of the DC electric field. We present a model explaining this in terms of enhanced cold plasma wake effects appearing when the ion flow energy is higher than the thermal energy but below the spacecraft potential multiplied by the ion charge. We suggest that these conditions, which are typical of the polar wind and occur sporadically in other regions containing a significant low energy ion population, cause a large cold plasma wake behind the spacecraft, resulting in spurious electric fields in EFW data. This interpretation is supported by an analysis of the direction of the spurious electric field, and by showing that use of active potential control alleviates the situation.

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