GEOPHYSICAL ANALYSIS OF ZONAL TIDAL SIGNALS IN LENGTH OF DAY

The Earth's zonal response coefficient kappa is estimated from the tidal signals in the observed length-of-day (LOD) data. Its magnitude and phase are functionals of the Earth's internal structure and dynamics. In this paper, an analysis of 13 years of precise LOD data (1980-1992) reveals strong signals for nine zonal tidal groups ranging from 5 to 35 days in period. Numerical estimates of kappa for 27 major tides are thus obtained, 11 among which are considered sufficiently high in signal-to-noise ratio to provide meaningful geophysical constraints on the Earth's rotational dynamics. The results favour a kappa magnitude close to, but somewhat smaller than, 0.315, which is the theoretical value for an elastic mantle completely decoupled from the fluid core plus equilibrium oceans. A small amount of dispersion is also detectable, where shorter periods tend to have lower kappa magnitude and larger phase lag. Our kappa magnitude estimates are consistent with two recently published non-equilibrium ocean-tide models and an anelastic response in the mantle, although an equilibrium response in the ocean and a purely elastic response in the mantle is not disallowed. Phase lags of a few degrees are required by both ocean-tide models, and by our data.