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

Abstract

SUMMARY The nutation harmonic terms are commonly determined from celestial pole offset series produced from very long baseline interferometry (VLBI) time delay analysis. This approach is called an indirect approach. As VLBI observations are treated independently for every session, this approach has some deficiencies such as a lack of consistency in the geometry of the session. To tackle this problem, we propose to directly estimate nutation terms from the whole set of VLBI time delays, hereafter referred as a direct approach, in which the nutation amplitudes are taken as global parameters. This approach allows us to reduce the correlations and the formal errors and gives significant discrepancies for the amplitude of some nutation terms. This paper is also dedicated to the determination of the Earth resonance parameters, named polar motion, free core nutation, and free inner core nutation. No statistically significant difference has been found between the estimates of resonance parameters based upon ‘direct’ and ‘indirect’ nutation terms. The inclusion of a complete atmospheric-oceanic non-tidal correction to the nutation amplitudes significantly affected the estimates of the free core nutation and the free inner core nutation resonant frequencies. Finally, we analyzed the frequency sensitivity of polar motion resonance and found that this resonance is mostly determined by the prograde nutation terms of period smaller than 386 d.

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

“…For this parameter, the CIs for opa2018b and ivs15q2X are not fully compatible with the CIs of the other inversions. We will not expand on the PM resonance here but it is noteworthy that our estimates of T 1 and Q 1 are in relatively good agreement with the computed theoretical and inverted values given in (Bizouard et al 2019) andNurul Huda et al (2019), respectively, even if the data were prepared and inverted quite differently in the latter study. For the FCN and FICN, our results differ to varying degrees from both the results of Mathews et al (2002) andNurul Huda et al (2019), with credible intervals and confidence intervals which do not always overlap.…”

“…We will not expand on the PM resonance here but it is noteworthy that our estimates of T 1 and Q 1 are in relatively good agreement with the computed theoretical and inverted values given in (Bizouard et al 2019) andNurul Huda et al (2019), respectively, even if the data were prepared and inverted quite differently in the latter study. For the FCN and FICN, our results differ to varying degrees from both the results of Mathews et al (2002) andNurul Huda et al (2019), with credible intervals and confidence intervals which do not always overlap. Of course, our estimates rely on much more data than what was available at the time of Mathews et al (2002) study but the recent result of Vondrák & Ron (2017), which is not compatible with ours, is closer to MHB values.…”

“…The FCN period we find from the VLBI data or joint inversion, approximately −430.2 to −429.8 solar days, is not fully compatible with the results of other studies. Rosat et al (2017) give a lower bound at −430.1 d, Mathews et al (2002) andNurul Huda et al (2019) provide estimates ranging from −430.4 to −430.0 d and Vondrák & Ron (2017) suggest an even larger period (in absolute value), at −430.28 ± 0.04 d. In most of these works, the FCN quality factor is significantly larger than our values as well.…”

Parameters of the Earth’s Free Core Nutation from Diurnal Strain Tides

“…Empirical FCN models give temporal variations of the FCN term amplitude and phase, mainly related to geomagnetic jerks and atmospheric and oceanic excitations (e.g. [12][13][14][15][16] ); temporal variations (few sidereal days) of T FCN have also been suggested (e.g. 17,18 ).…”

“…Rosat et al 10 gravity 429 ± 3 (a) 7,762 to 31,989 (b) Krásná et al 12 (a) 19,500 ± 200 (a) Rosat et al 11 VLBI 430.48 to 431.28 (d) 15,392 to 16,866 (d) Rosat et al 11 gravity 397 to 516 (d) 7,763 to 323,888 (d) Nurul Huda et al 16 σ σ σ − ≈ − ) and focus on the FCN contribution to A, S and C, i.e. A 2 , S 2 and C 2 .…”