The GEOS electron beam experiment S 329

In order to measure the electric field and the gradient of the magnetic field, both perpendicular to the local magnetic field, an entirely new instrument was flown onboard the geostationary satellite GEOS of the European Space Agency ESA. This instrument is described in this paper, together with a presentation of the initial scientific results. Electrons are emitted perpendicular to the local magnetic field and are detected after one gyrocircle. Owing to the electric field and the magnetic field gradient there is a detectable displacement of the returning beam. In the magnetosphere close to the geostationary orbit, the sensitivity of the instrument appears to be 100μV/m. Such fields may be defined with accuracies of a few percent if the magnetic field is known with sufficient accuracy and if the magnetosphere is sufficiently quiet. (Auth.)