Core Oscillations and Their Detection in Superconducting Gravimeter Records

Abstract

We review recent theoretical and numerical advances which allow the accurate calculation of modes of oscillation of the Earth's liquid core with periods beyond the range of acoustic radiation effects. In particular, the forms of the governing equations and boundary conditions at long periods, the use of effective internal Love numbers, and variational methods for the calculation of core modes, are considered. The Product Spectrum is introduced as a method of comparing, in the frequency domain, common features of gravity records taken at different observatories and at different times. It is applied to four long superconducting gravimeter records from Brussels, Bad Homburg and Strasbourg totalling more than 111,000 hours of observations. A method of detection and identification of core oscillations, based on rotational splitting, is presented and applied to three resonances found in the subtidal band of the European Product Spectrum. While the central frequencies of these resonances are in excellent agreement with the resonant frequencies for the translational modes of the solid inner core, computed ignoring its inertial drag, the two equatorial modes are oversplit compared to observation when computed in the conventional free oscillations approximation, which includes inertial drag, but ignores other important dynamical effects such as viscous drag and the Ekman, and possibly, the Ekman-Hartmann, boundary layer surrounding the inner core. Recent results from an improved fully dynamical theory of the translational oscillations. are described. The problem of the detection and identification of other core modes, particularly those in the intertidal band, is discussed.

A global experimental model for gravity tides of the Earth

“…Due to their low drift, high sensitivity and high precision, the SG gravity data have successfully contributed to the studies on geodynamics such as the Earth's tides (Melchior et al, 1996;Xu et al, 1997;Sun et al, 1999Sun et al, , 2001, oceanic and atmospheric loading effects (Warburton et al, 1975;De Meyer and Ducarme, 1991;Merriam, 1993;Sun and Luo, 1998;Xu et al, 1999a;Kroner and Jentzsch, 1999), accurate retrieval of the resonance parameters of the FCN (Merriam, 1994;Defraigne et al, 1994;Xu et al, 1999bXu et al, , 2002, polar motion (Loyer et al, 1999;Xu et al, 2004), free oscillations (Van Camp, 1999;Banka and Crossley, 1999), gravity signals originating from the Earth's fluid outer core (Melchior and Ducarme, 1986;Aldridge and Lumb, 1987;Smylie, 1992;Smylie and Jiang, 1993;Hinderer et al, 1995;Courtier et al, 2000). Most of the previous studies concern only gravity data recorded at individual stations.…”

Experimental detection of the inner core translational triplet

“…In 1992, Smylie affirmed that the weak signatures related to the ST were detected in the gravity measurements from three SG stations in central Europe using the stacking technique in the frequency domain (i.e. the product spectra) [15][16][17][18], and then the density near the Earth's center and the viscosity at the bottom of the fluid outer core were estimated [15,16]. These works gathered special attention of the geoscientists.…”

RATGP95: a harmonic development of the tide-generating potential using an analytical method

“…The tidal contribution to the gravimetric signal is the largest one that is observed, so its removal from the record is essential before starting any search for more subtle signals. These signals may be, for example, the Earth's core oscillations ), or Slichter modes (Smylie & Jiang 1992;Smylie et al 1992). However, the observations of these effects are challenged by a part of the scientific community (Crossley, Hinderer & Legros 1991;Crossley, Rochester & Peng 1992); it is for this reason that the prediction of Earth tides with an accuracy below the sensitivity of the gravimeters is a pre-requisite to the search for sub pgal anomalous effects like those eventually induced by excitation of core oscillations, or even by excitation of ocean normal modes (Francis 1993).…”