Free core nutation observed by VLBI

Abstract

Aims. The signature of free core nutation (FCN) is found in the motion of the celestial intermediate pole in the celestial reference frame and in the resonance behaviour of the frequency-dependent Earth tidal displacement in its diurnal band. We focus on estimation of the FCN parameters, i.e. the period and amplitude. Methods. We run several global adjustments of 27 years of very long baseline interferometry (VLBI) data (1984.0-2011.0) to determine the FCN period from partial derivatives of the VLBI observables with respect to the FCN as contained in the nutation of the celestial intermediate pole and in the solid Earth tidal displacement in the diurnal band. Finally, we estimate the FCN period by a global adjustment from both phenomena simultaneously, which has not been done before. Results. We find that our estimate of the FCN period of −431.18 ± 0.10 sidereal days slightly deviates from the conventional value of −431.39 sidereal days. Additionally, we present our empirical model of the FCN with variable amplitude and phase compatible with the estimated period.

Testing a new Free Core Nutation empirical model

“…The period estimates range between 425-435 days, rather than the predicted 460 days based on the hydrostatic theory for the Earth, implying a core more elliptical than a core in hydrostatic equilibrium would be. In a recent study Krásná et al (2013) performed the first simultaneous adjustment of solid tides and nutation data to estimate the FCN period, which was determined as 431.18 ± 0.10 sidereal days (-430.00 solar days), close to the value of 430.21 solar days estimated by Lambert (2007).…”

“…We adopted a constant period of -431.18 sidereal days (-430.00 solar days) since the phase and period would be correlated in the fitting. This value was determined recently by Krásná et al (2013) from VLBI and gravity data, with an accuracy of more than twice the one estimated by Mathews et al (2002), -430.21 solar days. Upon considering a fixed period, the possibility to estimate the amplitudes from window lengths NL lesser than the FCN period had to be checked and examined.…”

“…In this section, we focused on the comparisons with other FCN models (Krásná et al, 2013;Lambert 2007;Malkin 2013) (2013), is computed by using the running reference interval of 431 days with a shift of one day between sequential reference intervals. All the FCN series estimated with the aforementioned models and the model parameters Ac and As that appear in equation 1 are depicted in Fig.…”

“…IERS Conventions (2010) (Petit and Luzum, 2010) recommends an FCN empirical model of Lambert (2007). However, other alternative models are available today (Krásná et al, 2013;Malkin, 2013). In the time domain, the FCN signal is "buried"; it is thus simpler to conceptualize it in the frequency domain.…”

“…In this research we have developed a new empirical FCN model with higher temporal resolution by fitting M a n u s c r i p t the amplitude parameters directly to the Very Long Baseline Interferometry (VLBI) solution calculated with the GeoForschungsZentrum (GFZ) version (Nilsson et al, 2015) of the Vienna VLBI Software (VieVS) (Böhm et al, 2012). A comparison with other recently determined empirical FCN models: Malkin (2013), Krásná et al (2013) and Lambert and Dehant (2007) was included by means of the weight root mean square (WRMS) of the residuals during the entire period of VLBI data. Moreover, we assessed the sensitivity of our empirical FCN model with respect to different a priori Earth Orientation Parameter series, namely IERS 08 C04 and USNO finals.…”