Free Core Resonance parameters from strain data: sensitivity analysis and results from the Gran Sasso (Italy) extensometers

SUMMARY We analyse eight years of discontinuous strain records from two crossed 90‐m long laser interferometers (extensometers) operating in the Gran Sasso underground observatory (Central Italy) to investigate the FCN parameters. At first, inverting realistic synthetic tidal parameters (amplitude and phase of eight diurnal tidal components) we show that the resolving power of strain tides is comparable to that of gravity tides if tidal parameters are inverted minimizing the misfit (as usually done). Both resolving powers improve if data are inverted minimizing the misfit, and this improvement is particularly notable for gravity tides. The analyses of strain records have been performed after correcting measured strain for local distortion of the regional strain field and ocean loading. Data are inverted minimizing the misfit. Parameter uncertainty assessment is performed both inverting synthetic tidal parameter sets, obtained from the real one by adding a random noise, and estimating the Bayesian marginal probabilities from an ensemble of parameter space samples. Even if we analyse data from a single station, our results for the period of the Free Core Nutation (about 429 sidereal days) are robust and comparable to those from gravity tides, obtained from the joint inversion of data from several stations. The quality factor is not well constrained because of the large uncertainty on the Ψ1 phase; however our results are consistent with recently published values (≈20 000). The agreement between observations and predictions looks better than in any previous work that makes use of strain tides. Our inversions provide also an independent estimate of Re(h1− 3l1), which is the dominating term in the resonance strength expression, in agreement with IERS Conventions (2003).