ELECTRIC-FIELD MEASUREMENTS ON THE GEOTAIL SATELLITE

The electric field detector (EFD) on board GEOTAIL measures the electr ic field by two different techniques, one by the probe technique and t he other by electron beam technique. The probe technique (EFD-P) gives electric field in the plane perpendicular to the satellite spin axis by measuring the voltage difference between the two sphere probes; eac h deployed 50 meters from the spacecraft in the opposite direction. Th e electron beam technique (EFD-B), measures the drift motion of the gy ration center of artificially emitted electrons to obtain the electric field. The drift motion of the electrons is measured by two methods, one by measuring the drift motion itself and the other by measuring th e time of the return flight of electrons to the spacecraft. To realize these measurements, EFD is equipped with two additional capabilities. One is the capability to measure the spacecraft potential relative to the ambient plasma and the other is that to control the spacecraft po tential by emitting ions. The reliability of the electric field measur ements can be improved greatly by employing the probe and beam techniq ues at the same time. The potential control of the spacecraft enables the plasma detectors onboard GEOTAIL to measure low energy ions which would otherwise be repelled by the positive potential of the spacecraf t. This article describes the outline of the electric field experiment s on GEOTAIL with emphasis on the principles of the measurements, the configuration of the hardware, the raw data processing, as well as the preliminary results from the initial operation with the intention of providing the basis for the studies which use the GEOTAIL electric fie ld data.