Inter-Calibration Between Plasma Instruments Onboard DEMETER

Abstract

The in-situ measurements of the ionospheric plasma that we use come from two instruments of the scientific payload of the satellite DEMETER; the plasma analyser IAP (Instrument d'analyse du plasma) and the Langmuir probe ISL (Instrument Sonde de Langmuir). DEMETER is a micro-satellite realized by the CNES(Centre National d'Etudes Spatiales, France) with a principal objective to seek a possible influence of the seismic activity on the electromagnetic waves in the ionosphere and on the ionospheric plasma. The satellite was placed on June 29, 2004, in a circular and quasi helio-synchronous orbit at ∼710 km altitude. The experiments function primarily at mid-latitudes (from +60° to −60°). The IAP data were analysed to deduce the ion population (densities of the dominant ions, i.e. generally O+, H+ and He+) therefore the total ion density. The use of data IAP thus requires some precaution to make sure that the electric equilibrium conditions of the satellite, such as the satellite potential (øsat), are obtained during the treatment of routine, does not induce an error of measurement. When this potential is negative, the minority light ions H+ and He+ can be measured in a reliable way when their proportion is above 3% to 5% of that of O+. The critical limitation is: under certain conditions, the satellite potential becomes positive and reach a value about ∼0.5 V so that it becomes impossible to measure H+ ions. This is likely to involve a significant error on the composition and the density of the plasma. Therefore we carried out a calibration to estimate the missing density. The ISL experiment (Langmuir probe) provided the collected current/polarized tension characteristics of a cylindrical probe from which both electron density Ne and temperature Te were obtained. In some situations it is necessary to examine the accuracy of the electron density using another technique, for instance the high frequency (HF) spectrogram, provided by ICE (instrument champ electrique) instrument. This technique could give precise information about the accuracy of the data provided by the plasma analyser and the Langmuir probes. The observed satellite surface potential was found to be remarkable and subject of question. It displayed negative values at daytime and positive at night.