DOI: 10.24400/802406/pnp7-1wdf
title: Cluster 4 CIS Prime Parameters

description: The Cluster Ion Spectrometer (CIS) instrument is a comprehensive ionic plasma spectrometry package onboard the four Cluster spacecraft, capable of obtaining full three-dimensional ion distributions with good time resolution (one spacecraft spin) and with mass-per-charge composition determination. Since the scientific objectives cannot be met with a single detector, the CIS package therefore consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion Composition Distribution Function (CODIF), plus a sophisticated dual-processor based instrument control and data processing system (DPS), which permits extensive onboard data-processing. Both analysers use symmetric optics resulting in continuous, uniform, and well-characterised phase space coverage. The CODIF instrument is a high-sensitivity mass-resolving spectrometer with an instantaneous 360° × 8° field-of-view to measure full three-dimensional distribution functions of the major ion species (in as much as they contribute significantly to the total mass density of the plasma), within one spin period of the spacecraft. Typically these include H+, He+, He++ and O+, with energies from ~0 to 40 keV/e and with medium (22.5°) angular resolution. The CODIF instrument combines ion energy-per-charge selection, by deflection in a rotationally symmetric toroidal electrostatic analyser, with a subsequent time-of-flight analysis after post-acceleration to ~15 keV/e. The energy-per-charge analyser is of a rotationally symmetric toroidal type, which is basically similar to the quadrispheric top-hat analysers and has a uniform response over 360° of polar angle. In the time-of-flight section the velocity of the incoming ions is measured. Microchannel plates (MCPs) are used to detect both the ions and the secondary electrons, which are emitted from the carbon foil during the passage of the ions and give the start signal, for the time-of-flight measurement, and the positional information (22.5° resolution). In order to cover populations ranging from magnetosheath/magnetopause protons to tail lobe ions (consisting of protons and heavier ions), a dynamic range of more than 105 is required. CODIF therefore consists of two sections, each with 180° field of view, with geometry factors differing by a factor of ~100. This way, one section will always have counting rates which are statistically meaningful and which at the same time can be handled by the time-of-flight electronics. However, intense ion fluxes can in some cases saturate the CODIF instrument (particularly if data are acquired from the high sensitivity side), but these fluxes are measured with HIA. The sensor primarily covers the energy range between 0.015 and 40 keV/e. With an additional RPA device in the aperture system of the sensor, and with pre-acceleration for the energies below 25 eV/e, the range is extended to energies as low as the spacecraft potential. The RPA operates only in the RPA mode. The analyser has a characteristic energy response of about 7.3, and an intrinsic energy resolution of ΔE/E ~ 0.14. The deflection voltage is varied in an exponential sweep. The full energy sweep with 31 contiguous energy channels is performed 32 times per spin. Thus a partial two-dimensional cut through the distribution function in polar angle is obtained every 1/32 of the spacecraft spin (125 ms). The full 4π ion distributions are obtained in one spacecraft spin period. Including the effects of grid transparencies and support posts in the collimator, each 22.5° sector has a respective geometry factor of 2.4 × 10^-3 cm^2 sr keV keV^-1 in the h igh sensitivity side, and 2.6 × 10^-5 cm^2 sr keV keV-1 in the low sensitivity side, depending on the flight model. The HIA instrument does not offer mass resolution but, also having two different sensitivities, increases the dynamic range, and has an angular resolution capability (5.6° × 5.6°) adequate for ion-beam and solar-wind measurements. HIA combines the selection of incoming ions, according to the ion energy-per-charge ratio by deflection in an electrostatic analyser, with a fast imaging particle detection system. This particle imaging is based on MCP electron multipliers and position-encoding discrete anodes. Basically the analyser design is a symmetrical quadrispherical electrostatic analyser which has a uniform 360° disc-shaped field-of-view and narrow angular resolution capability. The HIA instrument has two 180° field-of-view sections with two different sensitivities, with a 20-30 ratio (depending on the flight model but precisely known from calibrations), corresponding respectively to the high G and low g sections. The low g section allows detection of the solar wind and the required high angular resolution is achieved through the use of 8 sectors, 5.625° each, the remaining 8 sectors having 11.25° resolution. The 180° high G section is divided into 16 sectors, 11.25° each. For each sensitivity section a full 4π steradian scan, consisting of 32 energy sweeps, is completed every spin of the spacecraft, i.e., 4 s, giving a full three-dimensional distribution of ions in the energy range ~5 eV/e to 32 keV/e. The geometry factor is ~8.0 * 10^-3 cm^2*sr*keV*keV^-1 for the high G half (over 180°), and ~3.5 * 10^-4 cm^2*sr*keV*keV^-1 for the low g half, depending on the flight model. Caveats The user of the CIS CSDS parameters needs to be cautious. These parameters are only moments of the distribution functions, that result from summing counting rates. Thus they do not convey information on the detailed structure of the three-dimensional distributions. Counting statistics are essential for obtaining reliable results. Preliminary information on inadequate counting rates, dead or saturated detectors, is given in the Caveats attribute. Besides instrument sensitivity and calibration, the accuracy of computed moments is mainly affected by the finite energy and angle resolution, and by the finite energy range of the instruments. An inappropriate choice of an operational mode is not without consequences for the accuracy of the parameters. Solar wind modes in the magnetosphere exclude a large portion of the ion distribution. This is particularly important for HIA moments obtained i n the magnetosheath while the instrument is in a solar wind mode. The moments then come from the 45° × 45° centered in the solar wind direction, resulting in largely under-sampled distributions. This data set contains Cluster 4 CIS Prime Parameters. CODIF energy sweeping during solar wind modes has a reduced energy range when the high sensitivity side faces the solar wind (45° in azimuth over 360°). This implies that if the data come from the high sensitivity side, the solar wind is then not detected. Magnetospheric modes in the solar wind result in a probable detector saturation. The He++ data can be contaminated by some H+ ions, resulting in over-estimated He++ densities. The CIS calibration values are regularly updated to take into account the detector efficiency evolution. However, as the evaluation of the detector efficiency requires some time history, necessary for a statistical analysis, there is a hysteresis between the detector efficiency drift and the calibration updates. Furthermore, an inhomogeneous evolution of the detection efficiency between the different anode sectors can result in a bias in the calculated direction of the bulk plasma flow. This phenomenon has been observed on the CODIF data obtained onboard Spacecraft 3 (Samba), resulting in a degraded accuracy of the Vz component (corrected in September 2001 with onboard software patches). The CIS instrument is not operational on Spacecraft 2 (Salsa).