Evidence for a Generalized Core Resonance Phenomena in Tidal Gravimetry

It is well established that a resonance effect involving the Earth's fluid outer core affects gravimetric responses of tidal modes in the diurnal band. In this Earth-fixed case, the gravimeter is indirectly sensing the presence of the Nearly Diurnal Free Wobble (NDFW). Also well established with observations from Very Long Baseline Interferometry (VLBI) is the Free Core Nutation (FCN) - the corresponding interaction viewed from a space-fixed frame. Because contained, rotating fluids like the Earth's outer core support an infinite set of oscillatory modes, resonance effects in addition to the NDFW/FCN are possible in the diurnal band. In principle, these same oscillatory modes exist at semi-diurnal periods, where additional possibilities for resonance effects exist. Although this generalized resonance phenomena was suggested over a decade ago (e.g., Lumb et al., AGU Monograph 72, 51-68, 1993), observational evidence remains absent. The situation is further complicated by the presence of the Earth's solid inner core which is also expected to participate in the resonance process. With data from the globally distributed network of superconducting gravimeters involved in The Global Geodynamics Project (GGP), observational prospects appear enhanced. In fact Merriam (Geophys. J. Int., 123, 529-540, 1995) reports four residuals of 4-10 ngals in the semi-diurnal band after the removal of all known effects - including loading effects of shallow-water non-linear tides. Given that the spectrum of oscillatory modes is dense throughout this tidal band, these residuals may already be an indication of resonance effects at semi-diurnal periods. Merriam also reports validation of his single-station results from Cantley, Quebec (Canada) in single-station records from both Europe and Japan, thus emphasizing the global presence of these residuals. After briefly reviewing the generalized core resonance phenomena, and the theoretical basis for its detection in gravimetric data, attention is focused on a modified approach for data analysis. In particular, this approach exploits the fact that the oscillatory modes are traveling disturbances in the Earth-fixed frame. Because use of novel approaches like this one could be greatly accelerated, there is ample motivation for opportunities to Grid-enable the GGP (Lumb & Aldridge, Eos. Trans. AGU, 85(17), Joint Assembly Suppl., Abstract G34A-03, 2004).