Searching for Free Core Nutation Effects on Two Long Baseline Tiltmeters

Retrieval of FCN parameters (period, Q factor and resonance strength) has a clue role for our knowledge of deep Earth’s properties (e.g. dynamic ellipticity of the core, dissipation through electro-magnetic coupling at the core-mantle boundary, etc.).

The resonance associated with the FCN has been widely studied in VLBI observations as well as in superconducting gravity records, but few experiments have been done with tiltmeters. In this study we use records coming from two different installations of long baseline pot-and-tube tiltmeters respectively at:  1) Sainte Croix aux Mines (Alsace-Eastern France), with a pair of about 100 m long hydrostatic silica tiltmeters, orthogonally installed in a dismissed silver mine; 2) Black Forest Observatory (Schiltach, Germany), with a differential fluid pressure (DFP) tiltmeter (baselength about 110 m) in an underground installation.

Main difficulties in retrieving FCN parameters from tidal analysis arise from the weak amplitude of PSI1 tidal wave (the closest in frequency to the FCN), as well as from the inaccuracy of the available ocean loading correction. Moreover because of the closeness in frequency of the single constituents of the diurnal tidal band, long (> 1 year) records are needed for resolving K1, PSI1 and PHI1 waves. Hence we analyze a ten-year dataset of tilt records, which has preliminarily required a critical review and a relevant editing for making records suitable for tidal analysis and subsequent inversion of the tidal parameters.

A Bayesian inversion is finally used for the retrieval of the FCN parameters.