Measuring Chandler Wobble amplitude variations using IERS EOP C04 data

Abstract

We analyzed the Earth's long-term polar motion using the time series IERS EOP C04 (from 1984 to 2023) to determine the variation of amplitude of the Chandler wobble. To compare the results based on the C04 with the Belgrade latitude data (BLZ series 1949-1985) results, we calculated the variations of latitude at BLZ point using the C04 coordinates (x, y). The secular part of these latitude variations was determined by use of the least-squares method (LSM) and removed from the data to get residuals. We used the Fourier transforms (DFT) to obtain annual and semiannual oscillations and to remove them from the residuals (to get a new set of residuals). These new residuals were divided into 33 independent 1.2 years subintervals. For each subinterval, we calculated the amplitude, period and phase of the Chandler nutation using the LSM. The quasi-periodic instability of 33 values of the amplitude of the Chandler wobble is detected with a period of 54.5 years using LSM (it was 38.5 years from BLZ data 1949-1985); the amplitude of that quasi-periodic variation is 0.″087 (0.″06 from BLZ data). The amplitude of the Chandler nutation varies with a minimum of 0.″012 (at 2019.3) and a maximum of 0.″230 (at 1994.1); the period is stable, but the phase is not stable. We applied the Abbe criterion to explain the variability in 33 values of the Chandler wobble amplitude, and the hypothesis that there is no trend in these 33 values is rejected. The obtained amplitude modulation is in accordance with other published papers about similar subject (and with our results based on BLZ data). Probably, the cause is lying in the hydro-atmospheric circulation that could influence calculated quasi-periodic variation. A possible explanation can be found in changes of core-mantle electromagnetic coupling (in line with the last few years investigations). In recent papers, it has been noticed that the effects of geomagnetic jerks are more important for exciting free nutation than the atmosphere and oceans.