Investigation of Nutation beyond the IAU2000 Model

摘要

Precession and nutations are mainly generated by the luni-solar attraction on the Earth equatorial bulge. Diurnal variations in the atmospheric and oceanic angular momenta in an Earth-fixed reference system induce additional contributions to some nutation motions. Precession and nutations are observed through the variations of the Earth Orientation Parameters obtained using Very Long Baseline Interferometry (VLBI) data, assuming that the direction of observed quasars are fixed in space. We may consider different ways of interpretting the discrepancies between the observed nutations and the model MHB2000 ([5], model adopted by the International Astronomical Union): either the model needs to be improved, or that there are imperfections in the observed nutation series. In particular, we investigate in this study the time variable atmospheric and oceanic effects. The possible contamination of VLBI-derived nutation amplitudes by apparent changes in the directions of the extragalactic radio sources is examined in [3]. The residuals between the observed nutation using VLBI and the MHB2000 model adopted by the International Astronomical Union (IAU2000) are of the order of a few tens of microarcseconds for particular frequencies. Considering the relatively small mass of the external fluid, there are only two ways that their effects on nutation can reach the observable level: (1) at frequencies corresponding to special frequencies in the fluid dynamics such as the annual prograde nutation, corresponding to exactly one solar day in the terrestrial frame, and (2) by exciting the nutation free modes namely the Free Core Nutation (FCN) and the Free Inner Core Nutation (FICN). In the adopted model MHB2000, a constant prograde annual contribution attributed to the external geophysical fluids is adjusted. Additionally, a time variable FCN amplitude is adjusted on the observations. This partly accounts for the atmospheric and oceanic contributions.