Abstract
In order to verify the time variability of free core nutation (FCN) period, global superconducting gravimeter (SG) observations were analyzed based on synthetic test data. The gravity data series were synthesized to check the detectability of resonance variation caused by FCN period change. The tests indicate that the discrepancy between the FCN periods determined by SG and VLBI observations is caused by the high correlation between the FCN parameter and the amplitude factor of the ψ1 wave. The K1 wave is more sensitive to the FCN period change than other diurnal waves. The limit of the standard deviation of the K1 wave is found for more precisely observing the FCN period change. Tidal parameters of diurnal waves estimated from long series of 20 global SG stations were analyzed. A common variation trend is found in the amplitude factor of both K1 and ψ1 waves in all 8 stations above the limit, which indicates the FCN period may be not so stable in time. Furthermore, the variation in the K1 and ψ1 waves constrains the FCN period change to between 2.5 and 4 sidereal days, which also agrees with the possible variation from the current VLBI and SG observations.
Highlights
The interaction between the elliptical liquid core and the solid mantle, due to the misalignment of their instantaneous rotation axes, produces one of the Earth’s rotational modes, called free core nutation (FCN) in the celestial reference system and nearly diurnal free wobble (NDFW) in the terrestrial reference system.The FCN causes resonance enhancement to nearby Earth tidal waves/nutation terms in the diurnal/annual frequency band
In this paper, the detectability of FCN period variation in Earth tides at the current precision level of superconducting gravimeter (SG) instruments was evaluated by synthesizing time series
The results indicate that: 1. In FCN determination, the large discrepancy in the FCN period determined by SG observations is caused by the inaccuracy of the ψ1 wave and the high correlation between the ψ1 wave and the FCN period
Summary
The interaction between the elliptical liquid core and the solid mantle, due to the misalignment of their instantaneous rotation axes, produces one of the Earth’s rotational modes, called free core nutation (FCN) in the celestial reference system and nearly diurnal free wobble (NDFW) in the terrestrial reference system. According to previous research (Cui et al 2014), the standard deviation of the amplitude factor for diurnal waves can achieve Q1–2 × 10−4, O1–4 × 10−5, P1–9 × 10−5, K1–3 × 10−5, ψ1–4 × 10−3, φ1–2 × 10−3, which could represent high precision data in the current global SG stations This is shown in the tidal analysis results for 20 global stations, which is analyzed in more detail in “Global SG data” section. In order to avoid errors in the scale factor and changes in the sensitivity of the gravimeter, it is very effective to use the O1 wave as a reference (e.g., Ducarme et al 2007) due to its relatively large amplitude, high signal-to-noise ratio and resistance to the FCN resonance. No effective information can be found in the amplitude factors from these stations
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