The periodic motion of the Earth's spin axis in space (nutation) is dominantly forced by external torques exerted by the Moon, Sun and planets. On the other hand, long-periodic geophysical forces (with periods longer than several days), mostly caused by the changes in the atmosphere and oceans, have dominant effects in polar motion (in terrestrial frame) and Earth's speed of rotation. However, even relatively small short-periodic (near-diurnal) motions of the atmosphere and oceans can also have a non-negligible influence on nutation, thanks to the resonance that is due to the existence of a flattened outer fluid core. The retrograde period, corresponding to this resonance, is roughly equal to 430 days in non-rotating quasi-inertial celestial reference frame, or 23h 53min (mean solar time) in the terrestrial frame rota ting with the Earth. The aim of the present study is to use the geophysical excitations in the vicinity of this resonance to estimate their influence on nutation, based on recent models of atmospheric and oceanic motions. To this end, we use the numerical integration of Brzezinski's broad-band Liouville equations and compare the results with VLBI observa tions. Our study shows that the atmospheric plus oceanic effects (both matter and motion terms) are capable of exciting free core nutation; both its amplitude and phase are compatible with the observed motion. Annual and semi-annual geophysical contributions of nutation are of the order of 100 microarcseconds. They are slightly different for different at mospheric/oceanic models used, and they also differ from the values observed by VLBI - the differences exceed several times their formal uncertainties., Jan Vondrák and Cyril Ron., and Obsahuje bibliografické odkazy
In our study we find, from the analysis of VLBI observations, small quasi-periodic fluctuations of the period and quality factor of retrograde Free Core Nutation (FCN), ranging mainly between 429.8 to 430.8 days and 17000 to 21000, respectively. To this end, we use resonant effects in several dominant forced nutation terms to calculate the period and quality factor of FCN in running 6-year intervals. We also recently demonstrated that the atmospheric and oceanic excitations are capable of exciting FCN. Both amplitude and phase of the geophysically excited motion are consistent with the values observed by VLBI, in the interval of tens of years. The geophysical excitations are now numerically integrated, using Brzeziński’s broadband Liouville equations, and removed from the observed celestial pole offsets. The remaining part is then used to derive the period and quality factor of FCN in running intervals, and to study the temporal stability of these important Earth parameters. It is demonstrated that the observed quasi-periodic variations of both parameters are probably not caused by these geophysical excitations., Jan Vondrák and Cyril Ron., and Obsahuje bibliografii
In this review paper we study the atmospheric and oceanic effects in nutation. It is a continuation and summary of our previous studies that we made during the last five years or so. We use slightly modified methods and apply them to the most recent data (both atmospheric/oceanic excitation functions and combined solution of celestial pole offsets by International
VLBI Service for Geodesy and Astrometry - IVS). We find th at the atmospheric and oceanic excitations provide significant changes in nutation, mostly with annual and semi-annual periods. The numerical integration of Brzeziński’s broadband Liouville equations yield Free Core Nutation (FCN) that is consistent with VLBI-based observed values. The analysis of VLBI observations shows small quasi-periodic fluctuations of the period and qua
lity factor of retrograde FCN, ranging between 429.8 to 430.5 days and 17000 to 22000, respectively. To this end, we use resonant effects in several dominant forced nutation terms to calculate the period and quality factor of FCN in running 6-year intervals. Numerically integrated geophysical excitations are removed fro the observed celestial pole offsets, and the remaining part is used again to derive the period and quality factor of FCN in running intervals. Our conclusion is that the observed quasi-periodic variations of both parameters are not caused by these geophysical excitations, but another source should be searched for.