THE ELECTRON-DRIFT INSTRUMENT FOR CLUSTER

The Electron Drift Instrument (EDI) measures the drift of a weak beam of test electrons that, when emitted in certain directions, return to the spacecraft after one or more gyrations. This drift is related to t he electric field and the gradient in the magnetic held, and these qua ntities can, by use of different electron energies, be determined sepa rately. As a by-product, the magnetic field strength is also measured. The present paper describes the scientific objectives, the experiment al method, and the technical realization of the various elements of th e instrument.