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Ann. Geophys., 24, 275-289, 2006
www.ann-geophys.net/24/275/2006/
© European Geosciences Union 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
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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