Celestial pole offsets are the displacements between the observed position of the Earth’s spin axis in space and its position predicted by the adopted models of precession and nutation. At present, the models are IAU2006 and IAU 2000, respectively. The celestial pole offsets are regularly measured by Very Long-Baseline Interferometry (VLBI), the observations being coordinated and published by the International VLBI Service for Geodesy and Astrometry (IVS). These offsets contain a mixture of several effects: the unpredictable free term, Free Core Nutation (FCN) that is due to the presence of the outer fluid core of the Earth, forced motions excited by the motions in the atmosphere and oceans, and also imperfections of the adopted precession-nutation models. The geophysical excitations are also available, as determined by several atmospheric and oceanographic services. The aim of this paper is to compare the time series of these integrated excitations with the observed celestial pole offsets and estimate the level of coherence between them., Cyril Ron and Jan Vondrák., and Obsahuje bibliografii
Recently the Earth System Modelling Group of GeoForschungsZentrum (ESMGFZ) in Potsdam started producing a new series of Effective Angular Momentum Excitation Functions (EAM). As a novelty, the data is given in 3-hour resolution for the influence of the atmosphere and dynamic ocean, and 1-day resolution for terrestrial hydrosphere and barystatic sea-level changes. In addition to this, IERS recently started publishing their new series of C04 solution for Earth Orientation Parameters (EOP), based on new combination of all observations and ITRF2014 terrestrial frame. We use the ESMGFZ data to numerically integrate Brzeziński’s broad-band Liouville equations in celestial frame and compare the results with IERS C04 solution for celestial pole offsets in the interval 1986.0-2018.4. Alternatively, we also add a possible influence of unevenly distributed Geomagnetic jerks (GMJ). In the process of integration we look for the best-fitting parameters (period T, Q-factor) of Free Core Nutation (FCN). It is demonstrated that the fit between integrated and observed values is much better when compared with our previous solutions, based on older models of geophysical excitations. The fit is improved significantly when GMJ quasi-impulse effect is included. The best fit is obtained for atmospheric, oceanic and GMJ excitations, the preferred parameters of FCN being.. . We also estimate new value for empirical prograde MHB Sun-synchronous correction.
Three variants of geophysical excitations and seven different VLBI solutions of celestial pole offsets (CPO) are used to determine period and Q-factor of Free Core Nutation (FCN). Brzeziński’s broad-band Liouville equations (Brzeziński, 1994) are numerically integrated to derive geophysical effects in nutation in time domain. Possible effect of geomagnetic jerks (GMJ) is also considered. Best-fitting values of FCN parameters are estimated by least-squares fit to observed CPO, corrected for the differences between the FCN parameters used in IAU 2000 model of nutation and newly estimated ones; MHB transfer function is used to compute these corrections. It is demonstrated that different VLBI solutions lead to FCN parameters that agree on the level of their formal uncertainties, but different models of geophysical excitations change the results more significantly. Using GMJ excitations always brings improvement of the fit between integrated and observed CPO. The obtained results show that the best fit is achieved when only GMJ excitations are used. Our conclusion is that GMJ are very probably more important for exciting FCN than the atmosphere and oceans. Empirical Sun-synchronous correction, introduced in the present IAU 2000 nutation model, cannot be explained by diurnal atmospheric tidal effects., Jan Vondrák and Cyril Ron., and Obsahuje bibliografii