A spectral search for the inner core wobble in Earth's polar motion

Abstract

[1] The inner core wobble (ICW) is a normal mode of rotation of the inner core relative to the mantle predicted theoretically. While many papers have been devoted to its theoretical formulation, no attempt of its detection has been published in the literature. In this work, we first give a preliminary formulation for the variations of gravity and geopotential coefficients associated with the ICW that complements the formulation for the polar motion. The results show that the ICW is most likely detectable using polar motion data. Then we attempt to detect the ICW by comparing the spectrum of an Earth orientation parameter time series with that of the sum of an oceanic angular momentum time series and an atmospheric angular momentum time series. We found no firm evidence of the ICW using our approach to a resolvability of a few milliarc seconds. At such a low amplitude in polar motion, the ICW is far below the current detectability of the variations of gravity measured by absolute and superconducting gravimeters and of the variations of the geopotential coefficients obtained from the Gravity Recovery and Climate Experiment (GRACE).Citation: Guo, J. Y., H. Greiner-Mai, and L. Ballani (2005), A spectral search for the inner core wobble in Earth's polar motion,

Application of the AR‐z Spectrum to Polar Motion: A Possible First Detection of the Inner Core Wobble and Its Implications for the Density of Earth's Core

“…Although theoretical studies on the ICW have been ongoing for decades, only Guo et al (2005) have tried unsuccessfully to detect the ICW in the PM time series. Although theoretical studies on the ICW have been ongoing for decades, only Guo et al (2005) have tried unsuccessfully to detect the ICW in the PM time series.…”

“…In contrast with the study of Guo et al (2005), here, we convolve the AAMF/OAMF/HAMF to the observation domain. According to Munk and MacDonald (1960), the PM excited by the AAM and OAM can be written as…”

“…Unlike Guo et al (2005), we use the PM time series directly without deconvolution, but we convolve the AAM, OAM, and hydrological angular momentum (HAM) excitation functions to the observation domain and show that they do not have significant effects in the target frequency bands. Unlike Guo et al (2005), we use the PM time series directly without deconvolution, but we convolve the AAM, OAM, and hydrological angular momentum (HAM) excitation functions to the observation domain and show that they do not have significant effects in the target frequency bands.…”

Application of a non-linear damped harmonic analysis method to the normal modes of the Earth

“…This iterative approach is also similar to the iterative Fourier transform method used by Guo et al (2005) in the search for the inner core wobble in Earth's polar motion data. As for the second issue, we pick up a candidate pair of frequency and damping factor which maximizes the decrement of the objective function in the two-dimensional search of parameters in the range pre-assigned.…”

Commission 19: Rotation of the Earth

“…Guo et al (2005a) re-discuss the double-peak spectrum of the Chandler wobble. Guo et al (2005b) attempt to detect the inner core wobble (ICW) by comparing the spectrum of polar motion data with that of (AAM + OAM). Haas and Wünsch (2005) use the CONT02 VLBI data to search for terdiurnal and other terms in the residual time series (observations minus IERS model).…”