Earth's core undertones: detections from stacking global superconducting gravimeter data set and constraints on the stratification stability of the outer core

SUMMARY The stratification stability in the Earth's fluid outer core, which is challenging to constrain with seismology, could potentially be revealed by core undertones (typical inertial-gravity waves). Radial motions from internal inertial-gravity waves are believed to induce minuscule gravity variations at the Earth's surface, yet no conclusive observational evidence has been reported. In this study, we present a systematic search for core undertones within two intertidal bands by stacking a global data set of superconducting gravimeter observations, comprising 59 gravity residual sequences from 42 stations between 1997 July 1 and 2024 February 29. Under the hypotheses of seismic excitation and sustained excitation, the optimal sequence estimation is used to retrieve spatially coherent signals associated with low-degree (l ≤ 4) spherical harmonic patterns; the z-domain autoregressive power spectrum is used to highlight the weakly damped harmonic signal against the background noise. Rigorous statistical significance analyses indicate that seismic events are insufficient to excite core undertones to the currently observable level. Assuming that core undertones are sustainably excited, we identify three candidate undertones with periods of 9.93, 9.73 and 9.51 hr, associated with spherical harmonic patterns Y2,+1, Y2,–2 and Y3,+2, respectively, which suggest that the liquid outer core near the core surface may contain a strongly stabilized layer characterized by a relatively higher buoyancy frequency. The findings may contribute new insights into the structure and dynamics of the Earth's deep interior as inferred from surface gravimetry.