Improvement of plasma measurements onboard Cluster due to spacecraft potential control Original Research Article

The ion emitter experiment ASPOC (Active Spacecraft Potential Control) and the plasma electron experiment PEACE (Plasma Electron And Current Experiment) have been operating together in a large variety of plasma environments. Three cases on February 4, 2001, May 21, 2002, and June 12, 2002 have been analysed. A clear improvement of the electron measurements was found in regions with low plasma density in the lobes and the polar zones as well as near boundaries of these regions where the sustained spacecraft potential of less than ≈8 V helps to identify features in the electron spectra at low energy and facilitates the accurate calculation of moments. The variations of the electron distribution function were consistent with a change of the spacecraft potential from ≈40 to ≈8 V when ASPOC was turned on. A significant reduction of the count rates by photo-electrons generated at spacecraft surfaces was found. Also the low-energy ion measurements by the Cluster Ion Spectrometer improve using active spacecraft potential control. The refinement of the energy resolution of the electron detectors at low-energy levels is an important feature for accurate plasma measurements.