Projet de recherche BL/33/C18 (Action de recherche BL)
1/1/2001-31/12/2006
Context and objectives
The Earth’s nutations are changes of orientation of the rotation axis of the Earth in space as induced by the gravitational attraction of the Moon, the Sun and eventually the other planets. They are Earth orientation parameters which link the space inertial reference frame and the terrestrial reference frame on which our astronomy observation is based. These nutations can be observed by geodetic techniques like the Very Long Baseline Interferometry (observation of the position of distant celestial objects like quasars which can be considered fixed in space). They can also be computed from a model considering precise modelling of the interior of the Earth, such as electromagnetic coupling at the fluid outer core boundaries. Therefore, nutation study is very important in astronomy, geophysics and astronautics. The present project was in that context. It aims at incorporate electromagnetic core-mantle coupling in the theoretical computation of nutations. Both institutes, the Royal Observatory of Belgium (ROB) and the Shanghai Astronomical Observatory (SHAO) have developed codes allowing these computations. We have revisited the theoretical background, recomputed the equations and their corresponding solutions, and incorporated them in the codes.
Methodology
Both teams have computer codes available for computing nutation models, with different options, and, it was of interest to both teams to incorporate electro-magnetic coupling into the computer programs. In addition, when working on the fine details during their cooperation, Huang and Dehant have observed the existence of instabilities in the solutions. They suspected that these instabilities are related to the existence of normal modes in the fluid outer core influencing the solutions.
The methodology used by the teams was thus to compare and work out together the theoretical background and to compare the behavior of the solutions from the different codes in order to better interpret the results. They have as well incorporated new internal models coming from seismology as input for the rheological properties of the interior of the Earth.
