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1 Reports on Geodesy

1 2005

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Six-year cycles of the Earth rotation and gravity

Chapanov Y., Vondrak J., Gorshkov V., Ron C.

Reports on Geodesy

2005

z. 2/73

221-230

Six-year cycles of the pseudo-periodical oscillations are discovered in the series of astrometrical observatioos of the latitude and universal time, tidal observations and Earth rotation series. The variations of the latitude with period about 6 years are.due to oscillations of the local vertical. At observatory Washington the 6-year oscillations of the vertical for the period 1919-1953 are with amplitude 20-30 mas and 5.5-year oscilIations of the vertical at observatory Plana are with amplitude about 10 mas for the period 1987.5-2005.3. A part of the last oscillations are highly correlated with the 5.5 year cycles of nontidal changes of the gravity at observatory Brussels with amplitude 50 nm/s**2, determined from superconducting gravimeter measurements. ln the case of even distribution of the gravity effects, due to the six-year oscillations of the Earth, the corresponding depth of the resulting central 'force of the disturbances (pendulum model) will be about 600 km. The residuals UTO-TAX of the approximation of the universal time observations at observatory Washiogton consist of several 6-year cycles for the period 1953-1992. The Singular Spectrum Analysis of the LOD series from the solution C04 of the IERS shows the presence of six-year oscillations for the period 1962-1987 and no existence of six-year oscillations after 1987. Some of the six-year oscillations are with close phases. Probably the six-year oscillations of the gravity are due to the core-mantle interactioo aod correspoodiog redistribution of the heavy and light masses in the mantle convective flows and these oscillations are observable on the Earth surface but they affect very insignificantly the Earth rotation. These results are obtained in the frame of Joint research projects between scientific institutions of the Academy of Sciences of three European countries - Republic Bulgaria, Czech Republic, and Russia.