Spacecraft potential control on the POLAR satellite by the plasma source instrument

Summary form only given. The importance of low energy core plasma to magnetospheric plasma transport has been increasingly recognized in recent years. However, in low density space plasmas, spacecraft charging makes observation of low energy plasma particles difficult. The Plasma Source Instrument (PSI) on the POLAR satellite effectively controls both positive and negative spacecraft charging through the emission of electrons and ions, as needed, to maintain the potential near that of the ambient plasma. This paper provides information on the nature of the instrument, its operation, and the consequences for low energy ion observations. Data from the Electric Field Instrument (EFI) on POLAR is used to demonstrate how well PSI controls the spacecraft potential. High latitude observations from the Thermal Ion Dynamics Experiment (TIDE) provide a clear view of the plasma dynamic environment, which in the past has been obscured by the normally present spacecraft potential.