Celestial frame instability in VLBI analysis and its impact on geophysics

Context. Derivation of Earth's orientation parameters through VLBI analysis needs to handle the positions of the radio sources and the celestial reference frame correctly, so that radio source positional instabilities will not unduly perturb the nutation estimates or all the subsequent geophysical analysis. Aims. This work aims at estimating the magnitude of these perturbations. It also yields new determinations of the resonant frequencies associated with the Earth's fluid outer and inner cores. Methods. We generate several VLBI global solutions, analyzing VLBI delays accumulated from 1984 to 2007, and using different constraints on the radio source positions, to obtain nutation offset time series. Then we analyze the nutation time series obtained from each global solution, from which we extract some prominent spectral components reflecting the non rigid Earth's response to the external tidal potential. Finally, we deduce values of the outer and inner cores' resonant frequencies from each solution. Results. We show that the error propagated in VLBI analysis that is due to instability in the celestial reference frame can produce an additional error in the estimates of nutation spectral components of 15 mu as for the 18.6-yr term, and decreasing for shorter periods. This leads to an uncertainty of a few tenth of a day on the RFCN period and of less than 200 on its quality factor, and to an uncertainty of roughly 100 days on the FICN period and of 100 on its quality factor. The values yielded for the outer and inner cores' resonant frequencies are nevertheless close to the MHB values within the error bars.