GEOPHYSICAL ANALYSIS OF ZONAL TIDAL SIGNALS IN LENGTH OF DAY

The Earth's zonal response coefficient kappa is estimated from the tid al 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 ar e thus obtained, 11 among which are considered sufficiently high in si gnal-to-noise ratio to provide meaningful geophysical constraints on t he Earth's rotational dynamics. The results favour a kappa magnitude c lose to, but somewhat smaller than, 0.315, which is the theoretical va lue for an elastic mantle completely decoupled from the fluid core plu s equilibrium oceans. A small amount of dispersion is also detectable, where shorter periods tend to have lower kappa magnitude and larger p hase lag. Our kappa magnitude estimates are consistent with two recent ly published non-equilibrium ocean-tide models and an anelastic respon se 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 .