Short period variations in Earth rotation as seen by VLBI

An overview of the abilities of Very Long Baseline Interferometry (VLBI) to measure the variable Earth rotation and of the international VLBI collaboration is given. The paper concentrates on the short-period, i.e. subseasonal variations of Earth rotation which can be seen in VLBI measurements of length of day (lod) and polar motion between 1981 and 1999. The wavelet transform allows the time localisation of an irregular quasi-harmonic signal within a given data set. The wavelet analysis of lod series yields in the high-frequency range periods of similar to 28 days, similar to 14 days down to 6.86 days caused by the lunisolar tides and irregular quasi-periodic variations between 40 and 130 days. These are mainly associated with global zonal wind changes which can be seen when looking on the wavelet cross-scalogram between the lod series and the atmospheric angular momentum (AAM) time series. In polar motion variable periods between two and five months and even down to 7-10 days can be made visible by the wavelet scalograms.Today it is possible by VLBI to determine polar motion and UT1-UTC with a temporal resolution of as short as 3-7 minutes. The results of parallel VLBI sessions which took place since 1998 using two independent VLBI networks were analyzed in the subdiurnal period range and compared by computing the wavelet cross-scalograms, the covariance spectrum and the normed coherency. Periods between 5 and 7 hours can be seen in many of the UT1-UTC data sets besides the well-known diurnal and semi-diurnal periods. The wavelet analyses reveal interesting patterns in the subdiurnal range in polar motion, too.