The Giotto three-dimensional positive ion analyser

The three-dimensional velocity distribution of positive ions in the neighbourhood of Comet Halley was measured by an instrument which included two complementary sensors. The fast ion sensor measured the energy/charge distribution from 10 eV/q to 20 keV/q once per revolution of the spacecraft. It obtained the characteristics of the solar wind flow near the comet. The implanted ion sensor measured the energy/charge distribution from 90 eV/q to 90 keV/q with discrimination into five mass groups in a period of 32 spacecraft revolutions. These observations provided the angular distribution of the cometary ions formed by the ionisation of gas molecules sublimed from the cometary nucleus. The relation between the raw count rates and the plasma parameters for the fast ion sensor is derived on the basis of a complete energy angle scan of the sensor in a calibration source. The accuracy of the analysis was tested by two techniques using data collected during the mission.