Frequency dependence of long-period tidal factors determined from gravity and space geodetic data

The effects of ocean tides, a fluid core and mantle inelasticity of the earth cause the frequency dependence on zonal tidal signals. In particular, because inelastic effects of the medium of the Earth become larger for lower frequencies (Wahr, 1985) and the long-periods tides have their maximum values at the poles, tidal observations and analysis of long-period at high latitudes is advantageous for studying the dissipative properties in the Earth (Iwano et al., 2005). From this view point, Sato et al. (1995) and Iwano et al. (2005) analyzed the data of the gravity observations using superconducting gravimeter (SG) at Syowa Station in Antarctica. However, they concluded that the residuals from theory is mainly due to the inaccurate ocean models for ocean tidal loading correction. Recently, new two ocean tide models (FES2012 and TPXO8-atlas) were developed, which provide long-period ocean tide information. Thus, we reanalyzed the gravity data of Syowa Station using these new ocean tide models for ocean loading correction, and evaluated the gravimetric factors (δ). The estimated gravimetric factors are 1.15822 (FES2012) and 1.15541 (TPXO8-atlas) for Mm and 1.15396 and 1.15405 for Mf. Meanwhile, the zonal response coefficient κ defined by Agnew and Farrell (1978) were also estimated using zonal tidal signals with periods from 5 to 35 days in dUT1 (UTC-UT1), a tidally induced variation of the rotation rate of the Earth. This coefficient show also the frequency dependent zonal response. In this study, the values of estimated κ for different tidal frequencies from dUT1 observed by VLBI were compared to theory and to the results of previous determinations of κ from observations of space geodetic techniques. We used VieVs (Vienna VLBI Software) program for the estimation of dUT1 and processed all 24 hours from 1984 to 2012 and also processed the IERS C04 dUT1 series. These time series of dUT1 data were subtracted by the UT1 variations from atmospheric, hydrologic, and oceanic angular momentum estimated from ECMWF data.