Contribution of a joint Bayesian inversion of VLBI and gravimetric data to the estimation of the free inner core nutation and free core nutation resonance parameters

Abstract

SUMMARY The rotational motions of the internal Earth layers induce resonances in the Earth nutations and tidal gravimetric response to external luni-solar gravitational forcings. The characterization of these resonances is a mean of investigating the deep Earth properties since their amplitudes and frequencies depend on a few fundamental geophysical parameters. In this work, we focus on the determination of the free core nutation and free inner core nutation periods and quality factors from the Bayesian inversion of VLBI and gravimetric data. We make a joint inversion of data from both techniques and show that, even if the results are only slightly different from the inversion of VLBI data alone, such approach may be valuable in the future if the accuracy of gravimetric data increases. We also briefly discuss the polar motion resonance, which is related to the Chandler Wobble as seen from the diurnal frequency band. Our overall estimates of the FCN period and quality factor, TFCN = (−430.2, −429.8) solar days and QFCN = (15 700, 16 700), respectively, are in good agreement with other studies, albeit slightly different for unclear reasons. Despite some concerns about the detection and characterization of the FICN, it seems that we could also successfully estimate its period, TFICN = (+600, +1300) solar days, and give a loose estimate of the upper bound on its quality factor.

Nutation terms adjustment to VLBI and implication for the Earth rotation resonance parameters

“…So, a frequency dependence of the PM resonance parameters is observed in the diurnal band, especially in the vicinity of the FCN. This is in accordance with the theoretical modeling of Bizouard et al (2019), providing more insights on this issue.…”

“…The FICN parameters have decreased by 50 d and 90 for its period and quality factor, respectively. More importantly, the PM resonance parameters are mostly determined by the prograde nutation terms of period smaller than 386 d. The PM period and quality factor seem to be frequency dependent in the diurnal band in reason of the FCN resonance, in agreement with the theoretical prediction of Bizouard et al (2019).…”

“…For FCN and FICN we also provide the equivalent parameters (P , Q ) in the non-rotating frame. It should be noted that the negative value Q PM is not associated with the damping of the resonance, but stems from the dynamical response of the oceans to the pole tide in the retrograde diurnal band, as modeled in Bizouard et al (2019).…”

“…The Table 4 also shows that the nutation estimates obtained by the direct and indirect approaches lead to almost the same results. By way of comparison, we report in Table 4 the results of Ziegler et al (2019), which used a Bayesian inversion technique to estimate the nutation components. It shows that the results match ours except for the quality factor of FCN.…”

Polar motion resonance in the prograde diurnal band

“…Meanwhile, from nutation analysis, many studies (see e.g. Mathews et al 2002;Rosat et al 2016;Nurul Huda et al 2020;Ziegler et al 2020) evidenced that, in the retrograde diurnal band, the PMR has a period shorter than in the seasonal band (∼380 d) and that the complex part becomes negative (corresponding to an equivalent quality factor of −10). As shown in Bizouard et al (2020), the shortening of the PMR in the retrograde diurnal band is mainly caused by the dynamical response of the oceans to the pole tide potential, and the associated negative quality factor reflects the strong phase-shift of this response with respect to the pole tide.…”