The Electron Drift Instrument on Cluster: overview of first results

EDI measures the drift velocity of artificially in- jected electron beams. From this drift velocity, the perpen- dicular electric field and the local magnetic field gradients can be deduced when employing different electron energies. The technique requires the injection of two electron beams at right angles to the magnetic field and the search for those directions within the plane that return the beams to their as- sociated detectors after one or more gyrations. The drift ve- locity is then derived from the directions of the two beams and/or from the difference in their times-of-flight, measured via amplitude-modulation and coding of the emitted elec- tron beams and correlation with the signal from the returning electrons. After careful adjustment of the control parame- ters, the beam recognition algorithms, and the onboard mag- netometer calibrations during the commissioning phase, EDI is providing excellent data over a wide range of conditions. In this paper, we present first results in a variety of regions ranging from the polar cap, across the magnetopause, and well into the magnetosheath. 1997) in that the EDI measurements are essentially unaf- fected by the spacecraft environment and include the compo- nent of the field along the spacecraft spin axis. The present paper provides an overview of first results obtained with EDI. After a brief description of the technique and its implemen- tation in Sect. 2 and 3, the in-flight performance and analysis methods are discussed in Sect. 4, before results, predomi- nantly from the dayside, are presented in Sect. 5. Results from the night sector are described in a companion paper (Quinn et al., 2001, this issue).