Improvement of Nonrigid-Earth Nutation Theory by Adding a Model Free Core Nutation Term

摘要

From an analysis of Very Long Baseline Interferometry (VLBI) observational data compiled by the U.S. Naval Observatory from MJD 44089.994 to 51618.250 (McCarthy, 2000), the authors showed that a strong peak around -400 sidereal days in the spectrum of its differences from the IERS96 nutation theory could be explained by adding a model Free Core Nutation (FCN) term in the form of a single damped oscillation. Then they developed a new analytical theory of nonrigid-Earth nutation that included the derived FCN model. They adopted RDAN98 (Roosbeek and Dehant, 1998) as the rigid Earth nutation theory. It was convolved with a transfer function using numerical convolution in the time domain (Shirai and Fukushima, 2000). The form of the transfer function was the same as that of Herring (1995). However, its free parameters, such as the complex amplitude and frequency of the FCN, were readjusted by fitting to the above VLBI data. Even after truncating the forced nutation series so as to contain only 180 terms, the Weighted Root Mean Square (WRMS) of the complex residuals for the new nutation series is 0.312 mas, which is significantly smaller than 0.325 mas, that of the IERS96 nutation theory. As for the FCN term, they estimated its oscillatory period as -430.8 +/- 0.6 sidereal days, and its Q-value as 16200 +/- 1600. They also estimated the correction of the precession constants as -0.29297 +/- 0.00047' /cy in longitude and -0.02430 +/- 0.00019' /cy in obliquity, respectively.