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The density jump at the inner core boundary of the Earth is poorly constrained by seismic data. One sensitive measure of the jump is the eigenperiod of the inner core wobble (ICW). Although theoretical studies on the ICW have been ongoing for decades, no observed ICW signal has been reported. To increase the detection level, here we use a recently proposed AR-z method to analyze the 1960-2017 polar motion time series. We finally identify a +8.7 +/- 0.2 year prograde motion with an 2.67 +/- 0.04 milliarcsecond amplitude. After confirming that this signal is almost a stationary oscillation and that atmospheric/oceanic/hydrological effects cannot seem to excite this signal, we carefully suggest that this 8.7 year signal is possibly the ICW. According to this suggestion, we reconstruct a new 1-D density model with a 507 +/- 15 kg/m(3) inner core boundary density jump. This new model can also provide a better fit than Preliminary Reference Earth model to the seismic overtone 2 S1.

Plain Language Summary The eigenperiod of the ICW is directly related to the density jump at the inner core boundary (ICB) (which is still poorly known). Although theoretical studies on the ICW have been ongoing for decades, only Guo et al. (2005) have tried unsuccessfully to detect the ICW in the PM time series. In this study, we use the AR-z spectrum, which has significantly higher sensitivity and frequency resolution than the Fourier (and maximum-entropy) spectrum, especially when the signal-to-noise ratio is low. We identify a +8.7 +/- 0.2 year prograde motion from the PM as the possible ICW. Our results provide useful constraints on the density jump at the ICB and can further strengthen the understanding of Earth's deep structure and dynamics.