Thursday 25 August 2005

G1
1530-1700 hours

424
Geophysical excitation of nutation: time domain comparisons
Brzezinski, Aleksander¹, Bolotin, Sergei^1,2
1 Space Research Centre, Polish Academy Of Sciences, Warsaw, Poland
2 Main Astronomical Observatory, National Academy of Sciences of Ukraine, Kiev, Ukraine
The celestial motion of the Earth's pole, that is precession-nutation monitored by the very long baseline interferometry (VLBI), is influenced by the dynamically coupled atmosphere-ocean system. A part of this influence is the regular variation which can be expressed by the incremental amplitudes of the conventional precession-nutation model (Bizouard et al., 1998; Brzezinski et al., 2004). The remaining part, which is considered here, is the irregular fluctuation contributing to the free core nutation (FCN) signal and to a broad-band variability with power concentrated around the prograde annual frequency. Our approach is similar to that applied routinely in the excitation studies of polar motion - from the time series of the celestial pole offsets observed by VLBI we compute the corresponding excitation series, the geodetic excitation of nutation, which is then compared to the available subdiurnal estimates of the atmospheric and oceanic angular momentum functions. A general observation from this comparison is that there is a rough agreement in magnitude between the geodetic and geophysical excitations but the overall correlation is low, below 0.3. A sliding window correlation analysis reveals the periods with high correlation, up to 0.8, but also the periods with negative correlation. The cross-spectral analysis shows a high coherence around the prograde annual frequency. At the resonant FCN frequency, the atmospheric and the oceanic processes contain more power than could be inferred from the observed FCN signal. The highest coherence magnitude at the FCN frequency is for the atmosphere alone, about 0.75, but the corresponding coherence phase has an unexpected value of about 180 degrees. References: Bizouard et al., 1998, J.Geod., Vol.72, 561-577; Brzezinski et al., 2004, JGR, Vol.109, doi: 10.1029/2004JB003054.

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