EDI: Electron Drift Instrument
Principal Investigator: Roy B. Torbert, Department of Physics, University of New Hampshire, USA
EDI fires two weak beams of electrons 10 km or more into the space around each spacecraft. When properly aimed, the electrons eventually return to receivers on the opposite side of the spacecraft. The returning beams are affected by the strength of the electric field in space and by the gradient in the ambient magnetic field. From beam firing directions and electron travel times, scientists can determine the strength of the electric field. As a by-product, the magnetic field strength is also measured.