Active spacecraft potential control for Cluster – implementation and first results

Abstract

Abstract. Electrostatic charging of a spacecraft modifies the distribution of electrons and ions before the particles enter the sensors mounted on the spacecraft body. The floating potential of magnetospheric satellites in sunlight very often reaches several tens of volts, making measurements of the cold (several eV) component of the ambient ions impossible. The plasma electron data become contaminated by large fluxes of photoelectrons attracted back into the sensors.The Cluster spacecraft are equipped with emitters of the liquid metal ion source type, producing indium ions at 5 to 9 keV energy at currents of some tens of microampere. This current shifts the equilibrium potential of the spacecraft to moderately positive values. The design and principles of the operation of the instrument for active spacecraft potential control (ASPOC) are presented in detail.Experience with spacecraft potential control from the commissioning phase and the first two months of the operational phase are now available. The instrument is operated with constant ion current for most of the time, but tests have been carried out with varying currents and a "feedback" mode with the instrument EFW, which measures the spacecraft potential . That has been reduced to values according to expectations. In addition, the low energy electron measurements show substantially reduced fluxes of photoelectrons as expected. The flux decrease in photoelectrons returning to the spacecraft, however, occurs at the expense of an enCorrespondence to: K. Torkar (klaus.torkar@oeaw.ac.at) larged sheath around the spacecraft which causes problems for boom-mounted probes.

Spacecraft potential effects on electron moments derived from a perfect plasma detector

“…Also, the minimum of this index occurs for potentials close to the critical potential (for which N 0 =N ) which typically lies in the range 2-6 V, the higher value corresponding to the solar wind. This range agrees well with the capabilities of existing active potential control devices as employed, for example, on Polar (PSI experiment, Moore et al (1995), with a bias potential ∼2 V) and Cluster (ASPOC experiment, Torkar et al, 2001, with a bias potential ∼3-7 V). Such devices basically emit a positive ion beam to counter the positive charge and manage to dynamically stabilize the potential to values close to the bias value.…”

Survey of cold ionospheric outflows in the magnetotail

“…(The sketch shows the simplified case, when the magnetic field and the wake is in the spacecraft spin plane.) special circumstances: low spacecraft potential due to operation of the artificial spacecraft control, ASPOC (Torkar et al, 2001), and operation of CODIF (Rème et al, 2001) in lowenergy mode (0.7-25 eV). Events where these circumstances are fulfilled on one spacecraft at the same time as wakes are seen in the electric field signature on another spacecraft are rare.…”

Plasma transport along discrete auroral arcs and its contribution to the ionospheric plasma convection

“…differences in current emitted by the EDI instrument, particularly notable for spacecraft 3 that is used here in six cases) lead to significant variations of a. For seven events, the spacecraft potential data are ignored, as in these cases the density determination becomes more complicated because of an artificial spacecraft potential control by ASPOC (Torkar et al, 2001). …”