A 6-year westward rotary motion in the Earth: Detection and possible MICG coupling mechanism

Seeking geophysical explanations for the periodic ∼six-year oscillation (SYO) previously found in the Earth's length-of-day variation (ΔLOD), we analyze the global GPS displacement and geomagnetic data using the array processing technique of OSE (Optimal Sequence Estimation), and find clear evidences of the 6-yr signals which manifest as a westward rotary propagating wave of the sectoral spherical-harmonic pattern of degree-2 order-2 (Y22). Based on the period, the spatial pattern, and the amplitudes and the estimated phases that exhibit consistent synchronicity among the three datasets, we propose the following scenario: The mantle-inner core gravitational (MICG) coupling gives rise to a 6-yr axial torsional libration of the inner core controlled by the sectoral Y22 density anomalies, or the equatorial ellipticities, in the inner core and the (lower) mantle, the angular momentum exchange between which manifests as the SYO in ΔLOD. It forces into action a pressure wave-2 cyclic in 6 yr through the fluid outer core, which in turn produces the corresponding GPS and geomagnetic variations. Our findings provide insight into the dynamics of the deep Earth, and corroborate a positive density anomaly for the lower-mantle Large Low-Shear-Velocity Provinces.