Yuichi Aoyama

Incessant excitation of the Earth’s free oscillations

We, for the first time, report the evidence of incessant excitation of the Earth’s free oscillations, mainly the fundamental spheroidal modes in a frequency range from 0.3 to 5 mHz, based on the three year record of a superconducting gravimeter at Syowa Station, East Antarctica. The frequency-time spectrogram of this record is striped by more than 30 lines at nGal level parallel to the time axis, mostly corresponding to the fundamental spheroidal modes. This spectrogram is characterized by relatively efficient excitation of gravest fundamental modes, enhancement of signal intensities in the austral winter and amplification of signal in the frequency band from 3 to 4 mHz. Assuming that earthquakes are only the sources for the free oscillations, we calculate the synthetic spectrograms, which have not shown such a series of parallel lines as observed. The result of this synthetic test and characteristics of the observed spectrogram suggest that the mode signals we found are not of earthquake origin. We tentatively suggest atmospheric or oceanic origin for this newly discovered phenomenon of the solid Earth.

On the observed annual gravity variation and the effect of sea surface height variations

Abstract Based on two datasets of sea surface height (SSH) variations, the Parallel Ocean Climate Model (POCM) [J. Geophys. Res. 101 (C10) (1996) 25779] and the TOPEX/POSEIDON (T/P) altimeter, we have analyzed the effect of SSH variations on gravity observations. For this purpose, we first estimated the steric component of SSH variations which we characterized by a thermal steric coefficient relating the sea surface temperature (SST) to SSH. The steric coefficient was estimated on each ocean mesh by assuming a simple linear relation ship between the time variations in the SSH and SST fields. We obtained a value of 0.60×10−2xa0m/°C averaged over the central parts of the Pacific and Atlantic Oceans. We then estimated the annual gravity change by calculating the effects of the solid tide, ocean tide, polar motion and SSH variations. The predicted values at the three observation sites (i.e. Esashi in Japan, Canberra in Australia and Syowa Station in Antarctica) were compared with the actual data obtained from the superconducting gravimeters installed at these three sites. The results of the comparison indicate that the predictions agree with the observations within 20% in amplitude (i.e. within 0.2xa0μGal, where 1xa0μGal=1×10−8xa0ms−2) and 20° in phase at each observation site for both SSH datasets of the POCM and T/P. We have also tested other values for the steric coefficient, i.e. 0.0×10−2 and 1.0×10 −2 m /° C , but find that the fit-to-gravity observations made at the mid-latitudes is clearly better at 0.60×10−2xa0m/°C. It is noted that our gravity observations point to a value of steric coefficient similar to that independently determined from the relationship between the SSH and SST data. We have also tried to investigate the effects of SSH on the gravity observations in other frequency bands. Among these effects, one of the interesting results is the gravity changes induced by ENSO-like ocean oscillations. Our computations suggest that the oscillations contribute 2–3xa0μGal in peak-to-peak amplitude to gravity variations in the equatorial Pacific at the maximum.

Effective expansion of satellite laser ranging network to improve global geodetic parameters

The aim of this study is to find an effective way to expand the ground tracking network of satellite laser ranging on the assumption that a new station is added to the existing network. Realistic numbers of observations for a new station are numerically simulated, based on the actual data acquisition statistics of the existing stations. The estimated errors are compared between the cases with and without a new station after the covariance matrices are created from a simulation run that contains six-satellite-combined orbit determination. While a station placed in the southern hemisphere is found to be useful in general, it is revealed that the most effective place differs according to the geodetic parameter. The X and Y components of the geocenter and the sectoral terms of the Earth’s gravity field are largely improved by a station in the polar regions. A middle latitude station best contributes to the tesseral gravity terms, and, to a lesser extent, a low latitude station best performs for the Z component of the geocenter and the zonal gravity terms.

Observed Gravity Change at Syowa Station Induced by Antarctic Ice Sheet Mass Change

Continuous observations with superconducting gravimeters (SG) TT-70 #016 and CT#043 have been on-going since 1993 to monitor Earth tides and Earth’s free oscillations at a gravity observation hut in Syowa Station, Antarctica. We obtained gravity residuals from the SG CT#043 data by subtracting Earth tides, effects of atmospheric pressure changes and polar motion, and instrumental drift from the original record. The smoothed gravity residuals obtained by taking a running mean of 33 days reveal variations from –5 to +5 μgal (10–8m/s2).

Near real-time monitoring of flow velocity and direction in the floating ice tongue of the Shirase Glacier using low-cost GPS buoys

The horizontal velocity vector of ice flow on the floating ice tongue of the Shirase Glacier, East Antarctica, was determined using two GPS buoys located on its east and west sides. The GPS buoys consisted of a single-frequency GPS receiver module and an Iridium satellite communication system. The instantaneous horizontal position of each GPS buoy was automatically obtained every 30 minutes, and the data were immediately transmitted to the National Institute of Polar Research (NIPR), Tokyo, Japan, via a satellite link. The location data demonstrated that the floating ice tongue moved primarily in a linear manner during the monitoring period between February and April, 2010. The speed and azimuth of the eastern buoy were (5.779 ± 0.004 m/day, N1.4°E ± 0.5°), respectively, while for the western buoy the speed and azimuth were (7.005 ± 0.006 m/day, N13.1°W ± 0.6°), respectively. Short-term variations about the mean speed and azimuth of the ice flow, with a period of 3–10 days, were also identified.