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.