Current collection by a segmented Langmuir probe in the ionospheric plasma.

The segmented Langmuir probe (SLP) has been recently proposed by one of the authors(Lebreton, 2002) as an instrument for the newly prepared ionospheric mission DEMETER todetermine the plasma bulk velocity in addition to the electron density and the temperaturethat are routinely deduced from the Langmuir probe data. The basic idea of the SLP conceptis to measure the current distribution on the probe surface by means of the individualsegments and then to use the current anisotropy to estimate the amplitude and the direction ofthe plasma bulk speed in the probe frame. With the aim to evaluate the performances of sucha probe we have developed a numerical particle in cell (PIC) model which provides a tool tocalculate the current collection by spherical probe and its segments. This model is based onthe simultaneous determination of the charge densities in the near-probe sheath and on thesurface of the probe which are then used to compute the potential distribution in the sheathregion. This scheme is well adapted to the SLP problem and has an advantage with respect toother classical probe models, i.e. it provides a natural control of the charge neutrality insideof simulation box. Comparison of the results of our model with the exact solution given byLaframboise for the spherical probe in thermal non-flowing plasma demonstrates an excellentagreement. Here we present the results for the bulk thermal plasma in the case when Debyelength and sphere radius have approximately the same values, i.e. few cm. These conditionsare expected to be observed in the ionosphere at the altitude of ~700 km and therefore ourmodel may be directly applied for the interpretation of the current measurements on the boardthe DEMETER satellite