RESONANCE PARAMETERS OF THE FREE CORE NUTATION MEASURED FROM 3 SUPERCONDUCTING GRAVIMETERS IN JAPAN

The resonance in the diurnal tide due to the Free Core Nutation (FCN, Toomre, 1974), is investigated by using data from three superconductin g gravimeters (SCG) in Japan. To estimate the resonance parameters, i. e. the eigenfrequency, Q-value and strength of the resonance, tidal pa rameters are normalized to the O1 constituent at each site and simulta neously fitted to a damped harmonic oscillator model. Before fitting, five corrections were applied to the observed tide data: an atmospheri c pressure effect, the phase delay introduced by the electronic filter in the SCG, the so-called ''inertia correction'', ocean tide effects, and the effect of unconserved mass in the ocean tide models. The pres ent study strongly suggests that the unconserved mass in ocean models used for the correction is responsible for low Q-values for the FCN ob tained from the previous tidal estimates (Levine et al., 1986; Neuberg et al., 1987; Richter and Zurn, 1988; Sato, 1984, 1991). We found tha t the parameters obtained by taking into account mass conservation giv es a value for Q which is more consistent with nutation studies based on VLBI data (Gwinn et al., 1986; Herring et al., 1986, 1988) and with the result of Cummins and Wahr (1993) obtained by stacking global gra vimeter data. The values of resonance parameters obtained here are: 1. 00229+/-0.0007, 6000 (3240 to infinity), -4.62+/-0.28x10(-4) and 0.01/-0.17x10(-4) for the frequency in cycle per sidereal days, Q-value, r eal part of the resonance strength and its imaginary, respectively.