The electric-field and wave experiment (EFW) on Cluster is designed to measure the electric-field and density fluctuations with sampling rates up to 36000 samples s -I. Langmuir probe sweeps can also be made to determine the electron density and temperature. The instrument has several important capabilities. These include (I) measurements of quasi-static electric fields of amplitudes up to 700 m V m -I with high amplitude and time resolution, (2) measurements over short periods of time of up to five simualtaneous waveforms (two electric signals and three magnetic signals from the seach coil magnetometer sensors) of a bandwidth of 4 kHz with high time resolution, (3) measurements of density fluctuations in four points with high time resolution. Among the more interesting scientific objectives of the experiment are studies of nonlinear wave phenomena that result in acceleration of plasma as well as large-and small-scale interferometric measurements. By using four spacecraft for large-scale differential measurements and several Langmuir probes on one spacecraft for small-scale interferometry, it will be possible to study motion and shape of plasma structures on a wide range of spatial and temporal scales. This paper describes the primary scientific objectives of the EFW experiment and the technical capabilities of the instrument.

The European Space Agency's Cluster programme is designed to study the small-scale spatial and temporal characteristics of the magnetospheric and near-Earth solar wind plasma. The programme is composed of four identical spacecraft which will be able to make physical measurements in three dimensions. The relative distance between the four spacecraft will be varied between 200 and 18 000 km during the course of the mission. This paper provides a general overview of the scientific objectives, the configuration and the orbit of the four spacecraft and the relation of Cluster to other missions.

Abstract.The potential of ISEE 1 spacecraft has been used to derive the plasma density in the magnetosphere and its environment. First, we show that using the equilibrium of currents flowing in and out of a spacecraft, we could numerically derive a relation between the spacecraft potential and the density of the surrounding plasma. After verifying that this relation was in good agreement with the measurement of the density made by other instruments during selected periods of time, we apply this relation to the spacecraft potential measured continuously from 1977 to 1984 on ISEE 1. An image of the plasma density in the magnetosphere and its environment is obtained as a result. All principal magnetospheric regions are clearly identified: the solar wind with a density around 5 cm -3 , the magnetosheath with a density around 50 cm -3, the magnetosphere with a density around i cm -3, the plasma sheet with a density around 0.5 cm -3, and finally, the more tenuous tail lobes with a density below 0.1 cm -3. The plasma density was observed slightly higher on the dawnside than on the duskside of the magnetosphere. In addition, the magnetosheath was closer to the Earth on the dawnside than on the duskside. When the magnetic activity increased (recorded by the AE index), the dayside magnetosphere was compressed/eroded by about i to 2 RE, while the plasmasphere/inner magnetosphere became quite irregular and expanded in the dawn-midnight and in the dusk-noon sectors. In addition, during high magnetic activity, the plasma sheet at YCSM --0 was thicker and slightly denser than at low-activity. On the other hand, the flanks of the plasma sheet were thinner and slightly less dense during high-activity than during low-activity.

The European Space Agency's Cluster programme is designed to study the small-scale spatial and temporal characteristics of the magnetospheric and near-Earth solar wind plasma. The programme is composed of four identical spacecraft which will be able to make physical measurements in three dimensions. The relative distance between the four spacecraft will be varied between 200 and 18 000 km during the course of the mission. This paper provides a general overview of the scientific objectives, the configuration and the orbit of the four spacecraft and the relation of Cluster to other missions.

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