2 Gravity field and Earth System Lustrumboek "The 5th Element" atmosphere, cryosphere, oceans, and the exchange of momentum between the various subsystems. For this, space geodetic techniques such as VLBI, GPS, SLR, LLR, and DORIS are used. Finally, the physical component consists of the static and time varying global gravity field and geoid. Its main task is to improve the unification of height systems and to quantify models of absolute ocean circulation and of the transport of mass and heat in the oceans. Gravimetry, radar altimetry, and Earth orbiting satellites are the major source of information. To a large extent the ISGN is already operational: it makes use of various existing services of the International Association of Geodesy (IAG) such as the International Earth Rotation Service (IERS), the International GPS Service (IGS)the International VLBI Service for Geodesy and Astrometry (IVS), the International Laser Ranging Service (ILRS), and the Permanent Service for Mean Sea Level (PSML). Besides, the system is supported by a number of radar altimeter and SAR interferometry sensors. Whereas the geometric-kinematic component and the Earth rotation component have attained about the same level of accuracy and resolution, this does not hold true for the physical component, the static and time-varying gravity field both with respect to accuracy and spatial/temporal resolution. It is expected that in the next years the gravity field missions Challenging Mini-satellite Payload for Geophysical Research and Application (CHAMP), Gravity Recovery and Climate Experiment (GRACE) and the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) will change this situation dramatically and will make the ISGN complete. Purpose of this article is to discuss several issues in the ISGN that played a major role in the research in the section fysische meetkundige and ruimtegeodesie (FMR) in the last 5 years. One aspect will be that of the need of understanding the Earth's static gravity field for which the GOCE gravity mission proposed by ESA will play a major role. Other aspects are the advances in modelling the Earth ocean tide field and understanding related issues in atmospheric pressure and wind forcing of the Earth's surface. Information about the static and temporally varying gravity field helps in various ways to understand the Earth's interior, the hydrosphere, the cryosphere and their interaction. The gravity field is closely related to processes occuring in the Earth's core, the mantle and the lithosphere since the gravity field is mainly caused by the Earth's internal mass distribution. In order to understand this we simply have to remember that the mass distribution inside the Earth and Earth rotation generate the gravity field. If there would be no geodynamics at all, the Earth gravity field would be that of a slowly rotating fluid in hydrostatic equilibrium. Therefore, any difference between the gravity field of the real Earth and that of the equilibrium figure reflects the anomalous density structure inside the Earth. These density anomalies are due and related to a number of processes and features over a wide range of scales from global to regional. Examples are the structure of the lithosphere, orogenic processes, the existence and characteristics of sedimentary basins, and the temperature and viscosity variations in the upper mantle (cf. figure 1). They can be solved for by combining gravity data with seismic tomography, magnetic data, topographic data and information about the physical behaviour of the Earth's interior. This allows to address specific issues such as the structure and thermal state of ridge trench configurations and the patterns of mantle convection, the discrimination between active and passive models of rifting, determination of the deep density structure beneath the continents, identification of the contribution of post-glacial rebound to sea level change, and determination of the mechanical strength of the continental lithosphere. 4

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Lustrumboek Snellius | 2000 | | pagina 17