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Consistent Intradecadal/Interdecadal Oscillations in the Surface Geomagnetic Observations and in the $Δ$LOD: New Findings and Unresolved Problems

Surface geomagnetic observations and length of day variations (dLOD) have played an important role in interpreting the long-period motions of the Earth's core. Focusing on the 5-30yr period band, we use the optimal sequence estimation method to analyze the global geomagnetic vertical observations, and newly find that the about 6yr/8.6yr signals, the about7.6yr/13.6yr/22.5yr signals, the about 15.4yr and about 18.6yr signals respectively have the Y2,2, Y2,-2, Y2,-1 and Y2,0 spatial patterns; in which, the about 7.6yr/8.6yr/13.6yr/15.4yr/18.6yr signals are clearly detected for the first time. We also find that the five Y2,+/-2-related "equivalent" excitation signals have good phase consistency with the corresponding signals in the dLOD, and they have the same about 0.042 amplitude scaling factor. Determining the physical mechanisms behind those seven signals still leaves many unresolved problems, and our new findings make those problems more complicated. As a preliminary thought, we suggest the five Y2,+/-2-related signals may be originated from the Magnetic-Archimedes-Coriolis waves