Variations of the Earth’s figure axis from SLR and GRACE data

The principal figure axis of the Earth is the axis of maximum inertia for the deformed (oblate) Earth. Satellite Laser Ranging (SLR) data from 5 geodetic satellites were used to determine the variations in the Earth’s figure axis represented by the degree two and order one geopotential coefficients: C21 and S21. Strong variations at the annual and Chandler wobble frequency appear in the SLR time series when the pole tides (i.e. the solid Earth and ocean pole tide) were not included in the model for SLR data analysis. Modeling the pole tides removed the power at the Chandler period and pole tides induced annual variation in both C21 and S21. The contribution of the pole tides is small for the annual variation in S21. Contribution of the ocean pole tide is estimated to be only ~8% of the total annual variation in C21, based on the SLR data analysis. The estimates of the non-tidal annual variation in S21 are in a good agreement with SLR, GRACE and the Earth orientation parameters. It appears that the observed non-tidal annual variation in the S21 is due to the mass redistribution in the atmosphere and oceans, evaluated using an Atmosphere-Ocean de-aliasing (AOD) model, which appears to overestimate the annual power in S21. The behavior of the linear trend for the variations of Earth’s figure axis is in general agreement between the estimate from EOP, SLR and GRACE. Comparison with the prediction from GIA models suggests there are significant mass changes during recent years and other excitations in the geodetic observations. The mean figure axis from the recent GRACE and SLR solution could provide improved constraints on the properties of the core and core-mantle boundary, and the Love number related to mean pole.