Detection of Long-Period Core Modes by Using the Data from Global Superconducting Gravimeters

Abstract

Possibility of detection of the long-period core modes has been discussed in this paper by using the long-term gravity observations recorded with 13 superconducting gravimeters (SGs) in a worldwide network of the Global Geodynamics Projects. Identical techniques are used to remove the luni-solar gravity tides, the long-term trends of instrument drift, and the effects of atmospheric pressure from all 13 SG data sets. The power spectral density of each data set, as well as the product spectral density of all the data sets, is evaluated. The background noise levels in various non-tidal bands are estimated and analyzed. The results indicate that the background noise level of the global SG observations is 0.0649nm/s2 in inter-tidal band from 0.047 to 0.075cph, 0.0350nm/s2 in another inter-tidal band from 0.089 to 0.117cph, and 0.0138nm/s2 in the sub-tidal band from 0.172 to 0.333cph. The magnitude thresholds of any global harmonic signals, which may be detected by the global SGs, in these three non-tidal bands are 0.0416, 0.0231 and 0.0098nm/s2, respectively. It implies that the signals, related to the long-period core modes, may be identified in the global SG observations, if they exist. The spectral peaks of the global harmonic signals with periods of 16.55, 15.79, 11.00 and 10.09h are detected in the two inter-tidal bands of the global SG records, and they may come from the long-period oscillations in the fluid outer core of the Earth. In the sub-tidal band, there are no obvious peaks relating to the Slichter triplet claimed by Smylie in 1992. Instead, 8 spectral peaks of global harmonic signatures significantly emerge from the background noise. Comparing their periods with those of the Slichter triplet predicted theoretically, it may be concluded that the Slichter triplet is the possible sources of three peaks of them. Of course, a more certain conclusion cannot be drawn now until further knowledge of the deep interior of the Earth is obtained and more acceptable theoretical technique for the dynamic study of the fluid outer core is developed.