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CAVEATS for the Data supplied by the CIS Experiment Onboard the Cluster Spacecraft

Caveats for specific data intervals

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General Caveats

The user of the CIS data needs to be cautious (cf. CIS Experiment Description, in Rème et al., Annales Geophysicae, 2001).

The CSDS parameters from the CIS experiment are moments of the distribution functions, that result from summing counting rates. Thus they do not convey information on the detailed structure of the 3D distributions.

Counting statistics are essential for reliable results. 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.

Inappropriate operational mode is not without consequences on the parameters.

Solar wind (SW) modes :

Solar wind modes (modes 0-5) in the magnetosphere exclude a large portion of the ion distribution. This is particularly important for HIA moments obtained in the magnetosheath while the instrument is in a solar wind mode. The moments then come from the 45° x 45° centred in the solar wind direction, resulting in largely under-sampled distributions.

• Incorrect HIA moment values in the magnetosheath when the instrument is  in a SW mode

CODIF energy sweeping during solar wind modes has a reduced energy range when the high-sensitivity side ("high G") 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 (energy cut-off just above the main solar wind energy). In the magnetosheath the ions coming from the sunward pointing direction (45° in azimuth over 360° ) are also not detected below the solar wind cut-off energy.

• Incorrect CODIF moment values in the solar wind when data from the high-sensitivity side and in a SW mode
• Incorrect CODIF moment values in the magnetosheath when data from the high-sensitivity side and in a SW mode

Magnetospheric (MAG) modes :

Magnetospheric modes (modes 8-14) in the solar wind can result in a detector saturation.

• HIA data in a magnetospheric mode can suffer from partial detector saturation when in the solar wind (slightly underestimated densities)
• CODIF is saturated in the solar wind and in the magnetosheath when the data come from the high-sensitivity side and in a MAG mode

CODIF data in the solar wind, if acquired from the low-sensitivity side but with the instrument in a MAG mode, can suffer from cross-talk with the high counting rates of the high-sensitivity side, which is then saturated by the solar wind

• Overestimated CODIF densities in the solar wind when data from the low-sensitivity side and in a MAG mode
• Underestimated CODIF velocities in the solar wind when data from the low-sensitivity side and in a MAG mode

Magnetosheath modes (modes 12 and 14) :

Magnetosheath modes are like magnetospheric modes. However, modes 12 and 14 have been redefined for CODIF, starting from 1 November 2003. From this date, the CODIF energy sweeping during these modes has a reduced energy range when the high-sensitivity side ("high G") faces a 180° sector centered in the solar direction (180° in azimuth over 360° ). This implies that if the data come from the high-sensitivity side, the mainstream magnetosheath flow is only partially detected (energy cut-off  above 2 keV).

• Incorrect CODIF moment values in  magnetosheath modes 12 and 14  when data from the high-sensitivity side (starting from November 2003).

CIS at Perigee :

Background due to penetrating particles, from the radiation belts.

• High background on HIA around the perigee pass (in the radiation belts)

Similar HIA high background, due to penetrating particles, also during some SPE (solar particle events).

CODIF presents much better immunity to background from penetrating particles, due to the time-of-flight technique. However, in some cases, CODIF data can also suffer from some contamination by penetrating particles.

He++ data from CODIF :

• The He⁺⁺ data can be contaminated by H⁺ ions, resulting in over-estimated He⁺⁺  densities

O+ data from CODIF :

O⁺ data can be contaminated by penetrating particles in the radiation belts (cf. caveat on radiation belts). They can also be slightly contaminated by H⁺ ions in very high H⁺ flux plasmas, as for example in the magnetosheath. This is due to occasionally two uncorrelated H⁺ ions, one generating only a "start" time-of-flight signal and the other generating only a "stop" time-of-flight signal, and the time difference between the two being that of an O⁺ ion.

• O+ data can be slightly contaminated by a "ghost" of the H+ data, in very high H+ flux plasmas

CIS calibrations :

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 an hysterisis between the detector efficiency drift and the calibration updates.

• Only reprocessed CIS data are based on the updated calibrations; for the near-real-time data the calibrations are preliminary

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), where one MCP quadrant efficiency decreases faster than the other one. This results in a degraded accuracy of the Vz component of the plasma flow velocity measured onboard this spacecraft (improved in September 2001 with onboard software patches).

• Reduced accuracy of CODIF data on sc3

Spacecraft potential effects :

Spacecraft charging to a positive floating potential repels low-energy ions, which in these cases cannot be detected by CIS. The operation of the ASPOC ion emitter provides a helpful contribution in the detection of low-energy ions by CIS (particularly in low-density regions), by reducing the spacecraft potential. The user needs to check the spacecraft potential, provided by the EFW experiment and available at the CSDS, and/or the ASPOC ion beam operational times, before analysing low-energy ions.

• Spacecraft potential effects have to be taken into account for low-energy ions

Data compression effects :

Data compression of the ion distribution functions by the onboard processor, in some modes, can result in an artificial increase of the particle counts at the lowest energy channel (border effect of the data compression algorithm). This appears in the Energy-time spectrograms as a red line at the lowest energy channel, particularly in the CODIF data.

• The "red line" that appears in the lowest energy channel, in some CODIF spectrograms, is the artefact of data compression

Time tagging  :

The HIA onboard calculated moments can rarely, in high-density regions, suffer from a saturation of the onboard processor buffer. In these cases repeating short data gaps can appear, that concern only the HIA onboard moments, and that are a few spins each (e.g. on 16 March 2001, around 16:18 UT, on sc1). The corresponding data, between the gaps, can have a time tag error of a few spins. Time tagging is again re-synchronised after the disappearance of the gaps (a maximum of 16 spins after).

• In case of short repeating data gaps of HIA onboard calculated moments: inaccurate time tagging of  the corresponding data  between the gaps

Temperature calculation in the solar wind  :

Calculation of the parallel and perpendicular temperatures, in the CSDS data, is performed not by a rotation around the measured magnetic field direction, but by diagonalising the pressure tensor (after having subtracted the plasma bulk velocity), and by assuming that the distribution is gyrotropic and thus the symmetry axis of it is the magnetic field direction. T_par is then parallel to the most variable direction. In the case of very cold plasmas, as in the solar wind, where thermal velocities are very small compared to the plasma bulk velocity and to the instrument intrinsic energy (and thus velocity) resolution, the relative error in T_par can be large.

• Large fluctuations in the parallel temperature calculated in the cold solar wind

CIS on spacecraft 2 :

• The CIS instrument is not operational on spacecraft 2 (Salsa)

CSDS data quality :

• The CSDS data quality is not for publication, and the users should contact the PIs.