Proceedings of Journees 2019
Journees 2019 have been marked by record numbers of participants and communications. Therewere 118 attendants coming from 21 countries over the 5 continents. The scientific programincluded 58 oral presentations and 38 posters. Presentations were split into five sessions, alternatingastrometry (sessions I and III), Earth rotation changes (Session II/IV) and solar system dynamics(session V). Of particular interest were the communications dedicated to the results of the Gaiamission and its possible future prolongation in IR band (Session I), the realization of the InternationalCelestial Frame III (session III). In session II, many interesting talks were devoted to the centennialof the ”Earth rotation” IAU commission
The works that we carried out during the last ten years lead to significant progress inthe knowledge of the Chandler wobble (CW) of the Earth polar motion (PM). In Zotov and Bizouard(2012) we reconstructed the excitation of this resonant mode by using Panteleev’s correctivefiltering. Now we are sure, that this filtering is a regularizing algorithm asfar as its parameters areconsistently selected with the uncertainty affecting the resonance parameters and the observations.The excitation demonstrated a quasi 20-year amplitude modulation. In theframework of the firstorder differential linear equation describing the polar motion, it is easyto show that this modulationaccounts for the 40-year change of the CW amplitude as well as the splitting ofthe CW spectra.A simple model of the CW, composed of 80 and 40-year harmonics, accounts for the presentminimum in Chandler wobble amplitude, like in 1930s, and also predicts that its amplitude will startto increase in the nearest future with a phase shift ofπ. On the other hand, geodetic excitationof the CW well matches the ocean-atmospheric excitation (Bizouard, 2020) over recent 50 yearswith a dominant role of the ocean, producing the 20-year modulation. Thus, thephysical cause ofit could stem from climatic or tidal process influencing the oceanic circulation.
The decadal instabilities in Earth’s rotation (DIER) are thought to be caused bythe interactions between the Earth’s core and its mantle. This hypothesissuccessfully explains whythere is a close correlation between DIER and the variations in the rate of the westward drift of thegeomagnetic eccentric dipole, since it is successfully reproduced by modeling of the redistribution ofthe angular momentum between the fluid core and the mantle of the Earth. However, the hypothesiscan not explain the close correlations of DIER: with the observable variations in the masses of theAntarctic and Greenland ice sheets; with the decade oscillations of thetypes of synoptic processes(i.e. the epochs of the atmospheric circulation); with the anomalies ofthe global temperature;and with regional anomalies of the cloudiness, precipitations, and otherclimatic characteristics. Analternative to the core-mantle interaction hypothesis is presentedhere. This alternative hypothesisclaims that the DIER are actually caused by fluctuations in the angular velocity of lithospheric driftover the asthenosphere. The sliding of the lithosphere over the asthenosphere is possible due to ofthe vibrational displacement mechanism produced by tidal forces. The lithospheric plates exhibitvibrational displacements over the asthenosphere in the horizontal direction by shear stresses causedby friction, wind, and ocean currents. There is abundant evidence supporting thislithospheric driftmodel.