Short‐period variations in the length of day: Atmospheric angular momentum and tidal components

Abstract

Using very long baseline interferometry (VLBI) observations of extragalactic radio sources, it is now possible to determine the rotational phase angle of the Earth (UT1) with an accuracy of 0.1 ms in an observing period of about 1 hour. We analyzed a 3‐month series of daily length‐of‐day (LOD) values produced from VLBI observations collected under project International Radio Interferometric Surveying (IRIS) during April–July 1984. Comparison of the IRIS LOD values with variations in the atmospheric angular momentum (AAM) during the same time period revealed quasi‐periodic components in both series at about 9, 33, and 58 days, with closely matching phases. We removed the AAM components from the IRIS LOD values, filtered the resulting series to remove variations with periods longer than 35 days, and analyzed the resulting time series to estimate the scaling parameter k/C from the 9.12, 13.66, and 27.56‐day tidal components. The results are consistent with similar results from other methods based on much longer time series.