PEACE - A PLASMA ELECTRON AND CURRENT EXPERIMENT

An electron analyser to measure the three-dimensional velocity distrib ution of electrons in the energy range from 0.59 eV to 26.4 keV on the four spacecraft of the Cluster mission is described. The instrument c onsists of two sensors with hemispherical electrostatic energy analyse rs with a position-sensitive microchannel plate detectors placed to vi ew radially on opposite sides of the spacecraft. The intrinsic energy resolutions of the two sensors are 12.7% and 16.5% full width at half maximum. Their angular resolutions are 2.8 degrees and 5.3 degrees res pectively in an azimuthal direction and 15 degrees in a polar directio n. The two sensors will normally measure in different overlapping ener gy ranges and will scan the distribution in half a spacecraft rotation or 2 s in the overlapped range. While this is the fastest time resolu tion for complete distributions, partial distributions can be recorded in as little as 62.5 ms and angular distributions at a fixed energy i n 7.8 ms. The dynamic range of the instrument is sufficient to provide accurate measurements of the main known populations from the tail lob e to the plasmasheet and the solar wind. While the basic structure of the instrument is conventional, special attention has been paid in the design to improving the precision of the instrument so that a relativ e of the order of 1% could be attained in flight in order to measure t he gradients between the four spacecraft accurately; to decreasing the minimum energy covered by this technique from 10 eV down to 1 eV; and to providing good three dimensional distributions.