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 (International Earth Rotation and Reference Systems Service - IERS; Earth Orientation Parameters - EOP; Combination of four (04) techniques - C04), from 1984 to 2023, to determine the variation of the Chandler wobble amplitude. To compare the results based on the C04 with the so-called Belgrade latitude data (Belgrade Lunette Zenithale - BLZ series 1949-1985) results, we calculated the latitude variations at the BLZ point using the C04 coordinates (x, y). The secular part of these latitude variations was determined by applying the least-squares method (LSM) and removed from the data to obtain the residuals. We used Direct Fourier transforms to extract annual and semiannual oscillations and to remove them from the residuals (resulting in a new set of residuals). These new residuals were divided into 33 independent 1.2-year subintervals. For each subinterval, we calculated the amplitude, period, and phase of the Chandler nutation using LSM. The quasi-periodic instability of 33 values of the Chandler wobble amplitude is detected with a period of 54.5 years using LSM (it was 38.5 years from the 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 between minimum of 0 .''012 (at 2019.3) and a maximum of 0 .''23 (at 1994.1); the period is stable, but the phase is not stable. We applied the Abbe's 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 based on the criterion. The obtained amplitude modulation is in accordance with previous studies, but also with our own results based on the BLZ data. Probably, the cause lies in the hydro-atmospheric circulation that could influence calculated quasi-periodic variation. A possible explanation can be found in the change in core-mantle electromagnetic coupling (in agreement with the last few years' investigations). In recent papers, it has been indicated that the effects of geomagnetic jerks are more important for exciting a free nutation than the net effect of atmosphere and oceans.