NEW TRANSFER-FUNCTIONS FOR NUTATIONS OF A NONRIGID EARTH

There are differences between the observed values of nutation and the computed ones based on the International Astronomical Union (IAU) 1980 adopted nutation series. These differences can be expressed in the fr equency domain where they may reach several milliarc seconds, a level that is too large for practical use. This paper aims to resolve part o f these differences by computing a new theoretical model accounting fo r additional geophysical effects. A new transfer function is computed, based on an Earth initially in a nonhydrostatic equilibrium correspon ding to the steady state associated with the present mantle convection . The mantle mass anomalies are deduced from seismic tomography data, and the flow-induced boundary deformations are computed from internal loading for an Earth made up of a viscous inner core, a liquid outer c ore, a viscous mantle, and a solid lithosphere. In this way, a new cor e-mantle boundary (CMB) flattening is obtained, which gives the observ ed free core nutation (FCN) period. Furthermore, the global Earth dyna mical flattening induced by the mass anomalies in the mantle associate d with tomography and by the mass anomalies due to the computed bounda ry deformations, is in agreement with the J(2) form factor (or the obs erved precession constant). In addition to this nonhydrostatic initial state, the rheology of the mantle is considered as inelastic. The tra nsfer function for nutation is then obtained by numerical integration of motion equations from the Earth's center up to the surface to provi de a model which is completely self-consistent. In order to validate o ur model, the transfer function is convolved with new rigid Earth nuta tions, ocean corrections are applied and the final results are then co mpared with the observed nutations or with the International Earth Rot ation Service (IERS) nutation series. The residuals between our model and the observation are about 3 times smaller than those between the I AU 1980 adopted model and the observation. However, our model still pr esents residuals above the observational error; this is particularly t rue for the out-of-phase part of the residuals, while the in-phase par t gives very small residuals (improvement of about I order of magnitud e). A further step in this study is a refinement of the modeling of ge ophysical fluids (core, ocean, and atmosphere).