The Slichter mode of the Earth: Revisit with optimal stacking and autoregressive methods on full superconducting gravimeter data set

The Slichter mode is the triplet oscillational mode of the 3‐D translation of the Earth's solid inner core; despite much effort it has so far eluded observation. Here we revisit the search by applying new data processing methods upon the latest set of available superconducting gravimeter (SG) data. The new methods are the AR ‐z spectrum that is sensitive in detecting weak harmonic signals, and the OSE data stacking scheme proven to be effective in identifying seismic normal mode singlets. The SG data consist of 19 records from 14 worldwide stations spanning up to 15 years. We arrive at three candidate sets of frequencies for the Slichter triplet that satisfy the theoretical splitting rule, namely (3.952, 4.432, 4.908) cpd, (5.136/5.032/5.020, 5.592, 6.040) cpd, and (5.704, 6.208, 6.748) cpd, of which the first set is regarded to be the more likely. A series of synthetic experiments shows that the Slichter triplet signals with RMS received amplitude of ~0.3 nGal (10 −11 m/s 2 ) can be detected to the extent of the present combination of data and methodology.