Electrostatic interaction between Interball-2 and the ambient plasma. 2. Influence on the low energy ion measurements with Hyperboloid

Abstract. The measurement of the thermal ion distributions in space is always strongly influenced by the ion motion through the complex 3D electrostatic potential structure built around a charged spacecraft. In this work, we study the related aberrations of the ion distribution detected on board, with special application to the case of the Hyperboloid instrument borne by the Interball-2 auroral satellite. Most of the time, the Interball-2 high altitude auroral satellite is charged at some non-negligible positive potential with respect to the ambient plasma, as shown in part 1; in consequence, the measurement of magnetospheric low energy ions (< 80 eV) with the Hyperboloid instrument can be disturbed by the complex electric potential environment of the satellite. In the case of positive charging, as in previous experiments, a negative bias is applied to the Hyperboloid structure in order to reduce this effect and to keep as much as possible the opportunity to detect very low energy ions. Then, the ions reaching the Hyperboloid entrance windows would have travelled across a continuous huge electrostatic lens involving various spatial scales from ~ 10 cm (detector radius) to ~ 10 m (satellite antennas). Neglecting space charge effects, we have computed the ion trajectories that are able to reach the Hyperboloid windows within their acceptance angles. There are three main results: (i) for given values of the satellite potential, and for each direction of arrival (each window), we deduced the related energy cutoff; (ii) we found that all ions in the energy channel, including the cutoff, can come from a large range of directions in the unperturbed plasma, especially when the solar panels or antennas act as electrostatic mirrors; (iii) for higher energy channels, the disturbances are reduced to small angular shifts. Biasing of the aperture is not very effective with the Hyperboloid instrument (as on previous missions with instruments installed close to the spacecraft body) because the potential environment is driven by effects from the spacecraft. Our results are used to explain some unexpected features of the low energy ion measurements, especially during polar wind events recorded by Hyperboloid. In conclusion, knowing the satellite potential from IESP measurements, we were able to reject any low energy doubtful data and to perform angular corrections for all higher energy ion data. Then the selected and corrected data are a reliable basis for interpretation and estimation of the thermal ion distributions.

Key words. Space plasma physics (charged particle motion and acceleration; numerical simulation studies; spacecraft sheaths, wakes, charging)