The four PEACE (Plasma Electron And Current Experiment) electron analysers measure the electron velocity distribution function at the satellite in three dimensions with good time, energy, and angular resolution. Each instrument consists of two sensors mounted on opposite sides of the spacecraft. Each sensor is built to the ''top hat'' design: with hemispherical electrostatic energy analysers and a semi-annular microchannel plate (MCP) with a position-sensitive readout as the detector. The sensors are mounted unconventionally, with the plane of the field of view containing the spin axis direction but lying perpendicular to the spacecraft body, rather than tangentially. Each sensor separately provides complete 4pi solid angle coverage in one spin. The only difference between the sensors is the geometric factor. LEEA is better suited to studying the high fluxes usually found at lower energies, and vice versa. The instrument dynamic range is therefore extended beyond that which a single sensor could provide. In most other respects the sensors are essentially identical and they can be used interchangeably. The Particle Correlator experiment, part of DWP, takes measured counts directly from HEEA.

The instruments have been designed with special care in two areas: they are intended to make high quality measurements of the low energy electron distribution (below 10 eV) and the eight sensors (on the four spacecraft) are to have the best possible relative accuracy.

Good, low energy measurements are facilitated by a design which minimises the amount of direct UV, internally generated photoelectrons and secondary electrons which can reach the MCPs, and by the radial mounting of the sensors on the spacecraft. A sunlit spacecraft immersed in a plasma will usually become electrically charged and will become surrounded by a cloud of low energy photo- electrons originating from the spacecraft surface. Due to acceleration in the electric potential of the charged spacecraft, the measured arrival directions and energies of low energy electrons from the plasma will differ from their natural values (i.e. the values we seek to measure). Also, the sensors will detect both the natural plasma electrons and the photo-electrons originating from the spacecraft. The active spacecraft potential control, provided by ASPOC is intended to hold the spacecraft potential to a small and steady level, which will minimise the distortion of the velocity distribution function of the natural electron population and which will make it easier to characterise the spacecraft photo-electron population so that it can be separated from the plasma electrons during data analysis. The radial field of view (as opposed to tangential) also minimises the time an arriving electron spends in the vicinity of the spacecraft surface where the electric field associated with the charging is strongest.

Careful design and manufacturing work have sought to produce sensors that are identical to within the tolerances set as a design goal, such that relative accuracies of better than 1% are expected in good conditions (e.g., suitable electron flux levels, etc.)should allow reliable data analysis, combining and comparing data from all four satellites.

The full instrument energy range is divided into 88 different energy levels. Data is accumulated by counting the number of arriving electrons in each anode for intervals of 1/1024th of a spin, known as accumulation bins. Depending on user choice, the energy level is changed either by one step (from level n to level n-1) or two steps (level n to level n-2) during an accumulation bin. The set of consecutive measurements at ever-reducing energies is called a sweep. The lowest 16 steps are approximately linearly spaced and cover the region below 10 eV. The remaining steps are between logarithmically spaced levels (each differs from the next by a factor 1.17) up to the highest energy level at approximately 26.5 keV. A single sensor cannot sample the full energy range. It may sample a range of only 60 levels (or, in another mode, 30 levels). The two sensors together can cover the full energy range in one spin and where their energy ranges overlap there is full 4 pi solid angle every half spin.

The position sensitive readout in each sensor is divided into 12 equal parts, each seeing a 15° sector in the plane containing the spin axis and the sensors. The satellite spins while the sweep occurs, steadily rotating the field of view around the spin axis. The angle of rotation during a sweep and hence the angular resolution in the spin plane, depends on the sweep duration which may take one of three values. The three sweep modes are called LAR, MAR and HAR.

In the default standard operational mode, the sweep mode is MAR mode for both sensors. LEEA covers the energy range below about 1.2 keV and HEEA covers the range 34 eV-26.5 keV. The full 4π angular coverage is composed of a grid of 12 polar bins by 32 azimuthal bins. Each bin is 15° in polar by 11.25° in azimuth. The data are acquired simultaneously in all 12 polar bins while the azimuthal data are gathered sequentially as the satellite spins. The sweep covers 60 levels in a series of 30 steps, two in each accumulation bin, so that a set of 30 energy bins are generated. Thus the measured data consist of a matrix of (30 energies x 12 polars x 32 azimuths, for each sensor).

The default standard operational mode data products transmitted to ground are LEEA and HEEA moment sums (calculated on board), LEEA and HEEA pitch angle distributions and a LEEA three dimensional distribution with heavily reduced angular resolution covering only the energy range below 10 eV (called LER: 8 energies x 3 polars x 8 azimuths, for one sensor). When burst mode telemetry is available, a reduced energy and angular resolution three- dimensional distribution (called 3DR-D: 15 energies x 6 polars x 16 azimuths, for each sensor) is transmitted. All the above are transmitted once per spin. In addition, in burst mode, a LEEA 3D distribution of full energy and angle resolution, from the energy range below 10 eV, is transmitted at the rate of once per spin. The default mode is intended for use when the spacecraft potential is held well below 10 eV, which is not always true.

Note that PEACE is not operated exclusively in this way during the mission. The energy range coverage of the sensors may be varied. For example they may both be set to cover exactly the same range, so as to provide half spin all-sky coverage over the greatest possible energy range. Quite often the LEEA energy range is selected so that the lowest measured energy is above or near the anticipated spacecraft potential, to avoid unnecessary counting high fluxes of spacecraft electrons (which accelerates detector aging). The sweep rate may be varied. Sweeps lasting half the usual time give doubled azimuthal resolution, but the energy range is halved (this is the HAR). Sweeps lasting twice the usual time give one step per accumulation bin, i.e. doubled energy resolution, but halved azimuthal resolution (this is the LAR). The sweeps may be stopped to allow measurements at a fixed energy, though this option is not available at energies above 1.8 keV and will rarely be used. In burst mode there are a variety of alternative ways of selecting a subset of the full resolution 3D distribution data for transmission to the ground. The distributions are all partial forms of the measured (30 energies x 12 polars x 32 azimuths) distribution, having limited angular or energy coverage, and in some cases halved polar resolution.

This description has been obtained from Section 3.5 of the ''Users Guide to the Cluster Science Data System'', DS-MPA-TN-0015.