The HIA is a symmetric quadrispherical analyzer of top-hat geometry, and uses microchannel-plate electron multipliers and position encoding by discrete anodes. A 2D distribution is obtained once per 62.5 ms, and a full 3D distribution of ions in the energy range ~5 eV/charge to 32 KeV/charge is obtained every 4 s. For more details of the Cluster mission, the spacecraft, and its instruments, see the report Cluster: mission, payload and supporting activities,'' March 1993, ESA SP-1159, and the included articleThe Cluster Ion Spectrometry Experiment,'' by H. Reme et al., from which this information was obtained.