Application of the AR‐z Spectrum to Polar Motion: A Possible First Detection of the Inner Core Wobble and Its Implications for the Density of Earth's Core

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 2S1.