Contribution de la France à l'observation du champ magnétique terrestre Rapport quadriennal 1995-1998 Bitterly, J. Mandea Alexandrescu, M. Schott, J.J. /Vassal, Jacques CHAMP MAGNETIQUE TERRE CONDUCTIVITE OBSERVATOIRE VARIATION TEMPORELLE VARIATION SECULAIRE VARIATION SPATIALE MODELISATION The aim of our paper is to show the importance of geomagnetic data to the knowledge of the main geomagnetic field and its secular variation, and also of the temporal, in order to survey the Earth's environment. Firstly, we recall the results obtained on the high-resolution secular variation of the geomagnetic field in two sites situated in Western Europe (London and Paris) over the last four centuries. The declination data are remarkably well correlated between the two sites and can be used to construct a more complete and accurate synthetic series, in which many gaps or times with scattered or uncertain data can be amended. The built declination synthetic series is believed to be valid for much Western Europe, and information about the secular variation in this area are obtained. But, to characterise the geomagnetic field, the spatial variations are also important. To monitor the field variations, an international program has been developed since 1988. Under the INTERMAGNET program, high quality data from a global network of geomagnetic observatories are sent in near real time via satellite and computer links to world-wide collection and disseminatin points called "Geomagnetic Information Nodes" or GINs. All INTERMAGNET observatories operate with the same common specifications (resolution, filtering, sampling rate, data formats etc.) Today, the INTERMAGNET observatories data-sets of 1.0 minute values with good baselines are available within minutes and hours from about 60 observatories. Another INTERMAGNET goal is to fill out the global geomagnetic observatory distribution. In fact, a recent study specified the minimum number of equally spaced observatories needed to do a degree and order 10 spherical harmonic model of the main field. The model required a minimum of 92 observatories on about a 2,000 km spacing. Eight sites would require ocean bottom observatories... (D'après résumé d'auteur) CNFGG Barriot, J.P. (ed.) 1999 text https://www.documentation.ird.fr/hor/fdi:010020073 fdi:010020073 Bitterly J., Mandea Alexandrescu M., Schott J.J., Vassal Jacques. Contribution de la France à l'observation du champ magnétique terrestre. In : Barriot J.P. (ed.), . Rapport quadriennal 1995-1998 CNFGG, 1999, 145-157 Assemblée Générale de l'Union Géodésique et Géophysique Internationale, 23., Birmingham (GBR), 1999/07/18-30 FR