Quantification of corrections for the main lunisolar nutation components and analysis of the free core nutation from VLBI-observed nutation residuals

The attempt to quantify the corrections of lunisolar nutation components was made after analysis of six sets of Earth's orientation parameters (EOP). The deviations of the long-term nutation components about IAU2006/IAU2000A precession-nutation model are consistent with the uncertainties suggested by Mathews et al. (J Geophys Res Solid Earth, 2002. ), but they exceed the errors determined in this work. The corrections are validated using the IERS 14C04 and IVS 19q4e combined solutions. After applying the corrections found in this work to the 14C04 nutation residuals, we analyzed the remaining signals, which contain the signature of the free core nutation (FCN). The eigenperiod of the FCN is fixed to the value derived from the resonance of the non-hydrostatic earth model in a priori. The amplitude of FCN is computed by fitting observations to the empirical model using a sliding window, the length of window is determined by taking into account the interference between those close nutation components and the FCN. In addition, we also fitted the nutation residuals by a viscous damping function; both methods produce the same results in the amplitudes of FCN. The magnitude of the free core nutation bears a V-shape distribution, and furthermore, the oscillation of the FCN shows a decay and a steady reinforcement before and after 1999. In order to examine the origin of the modulation in FCN's magnitude, we briefly analyzed the possible damping or beating mechanism behind it. We diagnosed the magnitude and running phase changes of FCN by comparing it with the occurrence of the transient geomagnetic jerks. The weighted root mean square errors of nutation residuals are minimally reduced about 36% when the corrections to the 21 nutation components and the FCN signature are considered together.

Automated summary

The study attempted to quantify corrections to lunisolar nutation components, and after analyzing Earth's orientation parameters and validating with standard solutions, corrections to the free core nutation were derived. The amplitude of the free core nutation, determined by fitting observations, has a V-shape distribution, and the oscillation of the FCN shows a decaying and steady reinforcement pattern before and after 1999.