The Plasma Analyser Instrument (IAP) measures the ion composition, density, temperature and bulk velocity with a time resolution up to ~ 360 ms and the ion density fluctuations up to ~ 160 Hz. It makes use of two analyzers: APR (for Analyseur à Potentiel Retardateur, Retarding Potential Analyzer) performs the energy analysis of the rammed ionospheric ions and ADV (for Analyseur pour Direction de Vitesse, Velocity Direction Analyzer) determines the average angles of arrival of the ions. APR measurements allow to determine the density, temperature and bulk energy of the ionospheric ions. Along the ~ 700 kilometer altitude circular orbit of DEMETER, the ion species of interest are H+, He+ and O+ and, in periods of magnetic storms and at high enough latitudes, the molecular ion group N2+, NO+, O2+. ADV provides the direction of the ram velocity vector.