Using PEACE Data to Study Charging of the Cluster Spacecraft

In most of the magnetosphere when the Cluster spacecraft are in sunlight and when the ASPOC (Active Spacecraft POtential Control) instrument is off, the spacecraft potential is positive and is determined by a current balance between emitted photoelectrons and incoming plasma electrons. The plasma ion current is almost always negligible in these circumstances. The PEACE (Plasma Electron And Current Experiment) measures the properties of both photoelectrons and plasma electrons. Therefore it should be possible to understand the usual charging of the spacecraft by analyzing the PEACE electron data to obtain the photoelectron and plasma electron properties. It is important to understand the charging of the spacecraft in order to obtain accurate estimates of the charged particle densities, especially at low energies. We have examined a number of intervals when the ASPOC instrument was turned on or off and have compared PEACE electron data before and after ASPOC switching. We report on these results and give a preliminary evaluation of the status of the charging of the Cluster spacecraft. We acknowledge use of the EFW (Electric Field and Wave) instrument to obtain values of the spacecraft potential and thank M. Andre, G. Gustaffson, F. Mozer and A. Pedersen for their help.