Abstract
In recent decades much effort has been devoted to searching observations for evidence of free core nutations predicted by theoretical models. Polar phototube data, tidal gravity observations and very long baseline nutation measurements have all been used. In this paper we report the first direct detection of the free core nutation in the frequency domain. A data set which consists of nutation series from 1100 very long baseline interferometry observing sessions from January 1984 to May 1992 is used in the spectral analysis. The results of the spectral analysis of the nutation series show that the period of the retrograde free core nutation mode is −431.0 (−425.5 to −436.7) solar days with an estimated amplitude of 0.19 ± 0.07 milliarcseconds. The recovered Q factor is 2000 ± 100. Based on an Ekman boundary layer on the surface of the core, this Q value implies a kinematic viscosity at the core-mantle boundary of 1.2 × 10 5 cm 2 s −1. A spectral peak at the predicted location of the prograde free core nutation is below the 95% confidence interval and cannot be considered significant.
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