Basic Earth Parameters from VLBI observations using Bayesian inversions in the time domain: updated insights of the Earth's interior

Abstract. We present updated estimates of Basic Earth Parameters (BEP) from VLBI Celestial Pole Offset (CPO) time series spanning 1980-2025 using ensemble Markov Chain Monte-Carlo Bayesian inversion. Building upon Koot et al. (2008), we employ enhanced sampling algorithms and incorporate recent advances in ocean tidal modeling (Cheng and Bizouard, 2025). Key improvements include: (1) implementation of piece-wise cubic spline modeling for Free Core Nutation (FCN) amplitude variations, which significantly reduces multimodality in MCMC sampling compared to linear modeling; (2) integration of updated Ocean Tidal Angular Momentum (OTAM) values from FES 2014 ocean tidal atlas (Lyard et al., 2021) without the empirical 0.7 scaling factor previously applied; and (3) utilization of five diverse CPO series from different analysis centers spanning up to 45 years of observations.

Our results show good consistency across different CPO series, with estimated dynamical ellipticity values at the edge of the 1σ range of MHB 2000. Notable findings include a larger absolute value for the imaginary part of the core-mantle boundary coupling constant (Im(KCMB)), approaching the 2σ boundary of Mathews et al. (2002), which may reflect contributions from multiple coupling mechanisms, including topographic coupling through the "form drag" effect caused by wave interactions with irregular boundaries (Rekier et al., 2025). The real part of the inner core boundary coupling constant (Re(KICB)) is approximately half the MHB 2000 value, potentially indicating the need to revisit hydrostatic assumptions for the inner core given recent seismic evidence of viscous deformation. Compliance estimates suggest that frequency extrapolation methods from seismic to nutation bands require revision. The enhanced FCN free mode modeling successfully captures amplitude variations that differ from empirical models, particularly after 2000, though the physical interpretation of these differences requires further investigation.

The systematic discrepancies across multiple parameters suggest that the current nutation theory needs substantial updates to incorporate more realistic models of core-mantle coupling and inner core behavior.