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Ann. Geophys., 23, 1723-1733, 2005
www.ann-geophys.net/23/1723/2005/
© European Geosciences Union 2005
The spherical segmented Langmuir probe in a flowing thermal plasma: numerical model of the current collection
E. Séran1, J.-J. Berthelier1, F. Z. Saouri1, and J.-P. Lebreton2
1CETP, 4 Avenue de Neptune, 94100 Saint-Maur, France
2ESA/ESTEC, Postbus 299, NL-2200 AG Noordwijk ZH, The Netherlands
Abstract. The segmented Langmuir probe (SLP) has been recently
proposed by one of the authors (Lebreton, 2002) as an instrument to derive
the bulk velocity of terrestrial or planetary plasmas, in addition to the
electron density and temperature that are routinely measured by Langmuir
probes. It is part of the scientific payload on the DEMETER micro-satellite
developed by CNES. The basic concept of this probe is to measure the current
distribution over the surface using independent collectors under the form of
small spherical caps and to use the angular anisotropy of these currents to
obtain the plasma bulk velocity in the probe reference frame. In order to
determine the SLP capabilities, we have developed a numerical PIC (Particles
In Cell) model which provides a tool to compute the distribution of the
current collected by a spherical probe. Our model is based on the
simultaneous determination of the charge densities in the probe sheath and
on the probe surface, from which the potential distribution in the sheath
region can be obtained. This method is well adapted to the SLP problem and
has some advantages since it provides a natural control of the charge
neutrality inside the simulation box, allows independent mesh sizes in the
sheath and on the probe surface, and can be applied to complex surfaces. We
present in this paper initial results obtained for plasma conditions
corresponding to a Debye length equal to the probe radius. These plasma
conditions are observed along the Demeter orbit. The model results are found to
be in very good agreement with those published by Laframboise (1966) for a
spherical probe in a thermal non-flowing plasma. This demonstrates the
adequacy of the computation method and of the adjustable numerical
parameters (size of the numerical box and mesh, time step, number of
macro-particles, etc.) for the considered plasma-probe configuration. We
also present the results obtained in the case of plasma flowing with mesothermal
conditions reproducing the case of measurements onboard a low altitude
spacecraft. Finally, we briefly discuss the capabilities of the SLP to deduce
the plasma bulk velocity under similar conditions and present the first
onboard measurements.
Keywords. Ionosphere (Modeling and forecasting; Instruments
and techniques)
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