Are the free core nutation parameters variable in time?

The free Core Nutation (FCN) is a normal mode of the Earth which induces re sonance in the diurnal tides as well as in the nutations. This is particula rly true for frequencies near the resonance frequency, i.e., for the retrog rade annual nutation and the psi(1) tide. The paper aims at analyzing Very Long Baseline Interferometry (VLBI) data and superconducting gravimeter (SG ) data in order to gain more information on the FCN period, damping, resona nce strength and free mode amplitude. Section 2 deals with the variability as seen in precise tidal gravimetric observations from consecutive subsets of an SG located near Strasbourg (France). We will show that the apparent v ariations appearing in the eigenperiod, quality factor and strength are rel ated to a variable noise level in the tidal gravimetric data. Section 3 is devoted to the analysis of the time variability of the VLBI series. In part icular we show that the amplitude variations found in the forced nutational response of the Earth are not real but rather induced by a variable free m ode excitation. We show that the eigenperiod is stable within a range of 3 days. (C) 2000 Elsevier Science B.V. All rights reserved.