Tidal variations in UT1 observed with very long baseline interferometry

Abstract

Nine years of very long baseline interferometry (VLBI) observations have been analyzed to determine the magnitudes of the tidal variations in UTl for periods between 5 and 35 days. Corrections for variations in atmospheric angular momentum (AAM) significantly reduce the scatter of the measured amplitudes across both time and frequency. The AAM corrections are found to reduce the scatter in the observed tidal amplitudes by as much as 60% for periods as short as 5.6 days; in contrast, earlier studies have shown a loss of coherence between AAM and length‐of‐day (LOD) for periods shorter than about 10 days. The tidal amplitude measurements place bounds on both the nonequilibrium ocean and mantle anelasticity effects. The in‐phase k/C determination is found to agree to better than 0.5% with the value of 0.944 from Yoder et al. (1981). The out‐of‐phase values are found to have a frequency dependence that can only be explained by nonequilibrium ocean effects. The observed slope is larger than the theoretical by about 3 times the expected error. This result may indicate that the ocean is farther out of equilibrium for the higher frequencies than present models permit. Significant improvements are needed in both ocean and atmosphere modeling to exploit the full capability of the VLBI observations.