Search

SUMMARY The nutation of the Celestial Intermediate Pole can be considered as a retrograde diurnal polar motion. As the common polar motion, it presents a resonance, but with period TPM and quality factor QPM differing from the ones characterizing the Chandler wobble (TCW = 430.2−431.6 d, QCW in the interval (56 255) according to Nastula & Gross): according to the nutation analysis presented in a separate paper, this period is about TPM = 380 d and the quality factor becomes −10. In this study, we aim to revisit the geophysical interpretation of this result. Two complementary factors account for the observed values: the non-equilibrium response of the ocean to the pole tide potential in the diurnal band, and the resonance of the solid Earth tide at the free core nutation period. This leads to a resonance of TPM in the vicinity of the free core nutation period, confirmed by estimates derived from nutation analysis.

International audience; We compare nutation time series determined by several International VLBI Service for geodesy and astrometry (IVS) analysis centers. These series were made available through the International Earth Rotation and Reference Systems Service (IERS). We adjust the amplitudes of the main nutations, including the free motion associated with the free core nutation (FCN). Then, we discuss the results in terms of physics of the Earth’s interior. We find consistent FCN signals in all of the time series, and we provide corrections to IAU 2000A series for a number of nutation terms with realistic errors. It appears that the analysis configuration or the software packages used by each analysis center introduce an error comparable to the amplitude of the prominent corrections. We show that the inconsistencies between series have significant consequences on our understanding of the Earth’s deep interior, especially for the free inner core resonance: they induce an uncertainty on the FCN period of about 0.5 day, and on the free inner core nutation (FICN) period of more than 1000 days, comparable to the estimated period itself. Though the FCN parameters are not so much affected, a 100% error shows up for the FICN parameters and prevents from geophysical conclusions.

Because of its geophysical interpretation, Earth’s polar motion excitation is generally decomposed into prograde (counter-clockwise) and retrograde (clockwise) circular terms at fixed frequency. Yet, these later are commonly considered as specific to the frequency and to the underlying geophysical process, and no study has raised the possibility that they could share features independent from frequency. Complex Fourier Transform permits to determine retrograde and prograde circular terms of the observed excitation and of its atmospheric, oceanic and hydrological counterparts. The total prograde and retrograde parts of these excitations are reconstructed in time domain. Then, complex linear correlation between retrograde and conjugate prograde parts is observed for both the geodetic excitation and the matter term of the hydro-atmospheric excitation. In frequency domain, the ratio of the retrograde circular terms with their corresponding conjugate prograde terms favours specific values: the amplitude ratio follows a probabilistic gamma distribution centred around 1.5 (maximum for 1), and the argument ratio obeys a distribution close to a normal law centred around $$2 \alpha = 160^{\circ }$$ . These frequency and time domain characteristics mean an elliptical polarisation towards $$\alpha ={\sim } 80^{\circ }$$ East with an ellipticity of 0.8, mostly resulting from the matter term of the hydro-atmospheric excitation. Whatsoever the frequency band above 0.4 cpd, the hydro-atmospheric matter term tends to be maximal in the geographic areas surrounding the great meridian circle of longitude $${\sim }80^{\circ }$$ or $${\sim } 260^{\circ }$$ East. The favoured retrograde/prograde amplitude ratio around 1.5 or equivalently the ellipticity of 0.8 can result from the amplification of pressure waves propagating towards the west by the normal atmospheric mode $$\Psi _3^1$$ around 10 days.

International audience; From the maps of regional contribution to atmospheric angular momentum (AAM) over the period 1948-2011 (NCEP/NCAR reanalysis data) time domain excitation in Chandler frequency band was extracted by Panteleev's filtering method. This permits us to investigate the evolution of the regional atmospheric influence on Chandler wobble. It appears that the temperate latitudes bring the strongest inputs. For pressure term they are limited to continents, and highlight the role of Europe. For the wind term they mostly result from ocean area, encompassing in particular North Atlantic. A quasi-20 year cycle is found in the regional patterns of the atmospheric excitation. The integrated AAM is finally compared with the geodetic excitation reconstructed from the observed polar motion.

This study investigates the relationship between the equatorial atmospheric angular momentum oscillation in the nonrotating frame and the quasi-diurnal lunar tidal potential. Between 2 and 30 days, the corresponding equatorial component, called Celestial Atmospheric Angular Momentum (CEAM), is mostly constituted of prograde circular motions, especially of a harmonic at 13.66 days, a sidelobe at 13.63 days, and of a weekly broadband variation. A simple equilibrium tide model explains the 13.66 day pressure term as a result of the O1 lunar tide. The powerful episodic fluctuations between 5 and 8 days possibly reflect an atmospheric normal mode excited by the tidal waves Q1 (6.86 days) and σ1 (7.095 days). The lunar tidal influence on the spectral band from 2 to 30 days is confirmed by two specific features, not occurring for seasonal band dominated by the solar thermal effect. First, Northern and Southern Hemispheres contribute equally and synchronously to the CEAM wind term. Second, the pressure and wind terms are proportional, which follows from angular momentum budget considerations where the topographic and friction torques on the solid Earth are much smaller than the one resulting from the equatorial bulge. Such a configuration is expected for the case of tidally induced circulation, where the surface pressure variation is tesseral and cannot contribute to the topographic torque, and tidal winds blow only at high altitudes. The likely effects of the lunar-driven atmospheric circulation on Earth's nutation are estimated and discussed in light of the present-day capabilities of space geodetic techniques.

We derive the Chandler wobble excitation from the polar motion (PM) observations by using the Panteleev corrective filtering. The latter method is based on inversion of the Euler–Liouville equation, with additional filtering in the Chandler frequency band. The excitation reconstruction reveals amplitude changes different from the one observed in the Chandler wobble itself. Their main feature, well observable over the length of the day (LOD), is the presence of a 18.6-year amplitude modulation synchronous with the lunar orbital precession cycle and tidal effects. The filtering of oceanic and atmospheric excitation in the Chandler frequency band also reveals a coherent 18.6-year oceanic pattern. Most probably the ocean provide a channel for the tidal energy transfer.

Since the beginning of the 20th century, the observation of the Earth rotation variations through astro-geodetic techniques enables to investigate the global rheological properties of the Earth, in particular, the resonance parameters of the free rotation modes reflect the solid Earth anelasticity, the ocean response to an external forcing, and the properties of the fluid inner core, eventually of the solid inner core. Better constraints on these resonance parameters can be obtained by confronting the observed terrestrial motion of the rotation pole (the so-called polar motion) - including nutation as a retrograde diurnal polar motion - to the modelled excitation producing it. The more precise the modelled excitation and the observed polar motion are, the better the Earth rheological properties will be determined. For now, the best precision are reached in the nutation band. So, our work is first dedicated to a direct adjustment of the nutation components from VLBI delays, then the adjustment of the resonance parameters in the transfer function between the observed nutation terms and the corresponding rigid nutation terms that reflects the luni-solar forcing. The obtained resonance parameters confirms in particular the shortening of the polar motion resonance period of about 40 - 50 day in the retrograde diurnal band. Then, we show that the dynamical behaviour of the oceans in the diurnal band is mostly responsible for that. We also predicted a supplementary change of the resonance parameters in the vicinity of the free core nutation resonance, as expected from the solid Earth response, and confirmed by the adjustment of these parameters through the nutation terms. In addition to the nutation band, we revisit the estimation of the polar motion resonance parameters in the seasonal band, dominated by the Chandler wobble, in light of the most recent global circulation models of the hydro-atmospheric layers. Finally, we extend the investigation of polar motion resonance to the prograde diurnal polar motion, where the excitations mostly result from the ocean tides. We obtain a resonance period of about 393 days, and confirmed by our prediction based on the ocean tidal models. These results allow us to impose constraints on the frequency dependence of the body Love number and the Love number oceanic, characterizing respectively the response of the solid Earth and the oceans to an external potential of degree 2., Depuis le début du vingtième siècle, l’observation des variations de rotation de la Terre par les techniques astro-géodésiques permet de dévoiler les propriétés rhéologiques globales de la Terre. En particulier, les paramètres des modes d'oscillation libre de l'axe de rotation, qui sont aussi des paramètres de résonance - reflètent les défauts d'élasticité de la Terre solide, la réponse de l’océan au forçage externe et les propriétés de la graine fluide, éventuellement du noyau solide. On peut obtenir de meilleures contraintes sur ces paramètres de résonance en confrontant le mouvement terrestre du pôle de rotation observé (mouvement du pôle) – et aussi la nutation en tant que mouvement du pôle en rétrograde diurne - à l’excitation modélisée qui le produit. Les propriétés rhéologiques de la Terre sont d'autant mieux déterminées que le mouvement du pôle (y compris dans la bande diurne) est observé précisément et l’excitation modélisée correctement. A l'heure actuelle, si la précision du mouvement du pôle est légèrement meilleure au delà de 2 jours, la meilleure reconstitution du forçage concerne la bande de nutation (la bande rétrograde diurne), car il est procède alors des marées gravimétriques luni-solaires, lesquelles sont modélisées avec une grande précision). C'est pourquoi notre travail est d'abord consacré à l'ajustement direct des composantes luni-solaires périodiques de la nutation à partir des retards du VLBI, puis à l'estimation des paramètres de résonances dans la fonction de transfert entre ces termes de nutation observés et ceux modélisés pour une terre rigide, lesquels reflètent le forçage luni-solaire. Les résultats obtenus confirme notamment le raccourcissement d’environ 40-50 jours de la période de résonance du mouvement du pôle dans la bande rétrograde diurne. Nous montrons comment cette diminution s'explique par la réponse dynamique des océans à la marée du pôle, laquelle est étudiée à l'aune des modèles de marées océaniques diurnes . Nous prédisons également un changement des paramètres de résonance du mouvement du pôle à proximité de la fréquence de résonance de la nutation libre du noyau; cette modification semble confirmée par la détermination de ces paramètres à partir des termes de nutation. Notre étude des paramètres de résonance s'étend au delà de la bande de nutation. D'une part, nous réexaminons l’estimation des paramètres de résonance du mouvement du pôle dans la bande saisonnière, dominée par l’oscillation de Chandler, en utilisant les derniers modèles hydro-atmosphériques. D'autre part, nous étudions la résonance du mouvement du pôle à la bande prograde diurne, où l’excitation est dominée par la marée océanique. Nous obtenons une période de résonance de l’ordre de 393 jours, corroboré par notre modélisation fondée sur les modèles de marée océanique. L'ensemble des résultats obtenus permet d'apporter des contraintes sur la dépendance en fréquence du nombre de Love et du nombre de Love océanique caractérisant respectivement la réponse de la Terre solide et des océans à un potentiel externe de degré 2.