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.
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