The Shanghai Observatory has made progress in the study of the Earth's rotation changes

Day and night alternate, the sun, moon and stars rise in the east and fall in the west. The reason behind this is the rotation of the earth from west to east. The earth rotates around an imaginary axis that passes through the center of the earth and connects the north and south poles. This is the earth's rotation. The imaginary axis is called the rotation axis. Is the rotation of the earth always constant? Of course, the answer is no! The rotation of the earth is always changing, and the changes are quite complicated, including precession-nutation, pole shift and day length changes, which are physically expressed by Earth Orientation Parameters (EOP). They all reflect the overall movement of the earth and its important information about its changes with time. EOP is considered as another important means to study the physics of the deep interior of the Earth besides seismology. It is a "telescope" specially designed by astronomy to study the interior of the Earth and has always been one of the objects that scientists focus on measuring and studying.

Recently, Researchers from the Earth Rotation Change Research Group of the Astronomical Geodynamics Research Center of the Shanghai Observatory of the Chinese Academy of Sciences and the Key Laboratory of Planetary Science of the Chinese Academy of Sciences have made new progress in the research direction of day length change of EOP parameters. For the first time, it is found that there is a significant amplitude enhancement signal with a period of about 8.6 years in the daily length variation, and it is also found for the first time that the extreme value time of the oscillation has a close correspondence with the occurrence of the rapid change of geomagnetic field. At present, the work has been published online in the international journal Nature Communications. Another latest research on the detection of 6-year diurnal variation signals and the physical mechanism of attenuation and excitation is also published in the international geoscience journal Journal of Geophysical Research: Solid Earth.

From the outside to the inside, the inner circle of the earth includes the crust, mantle, fluid outer core and solid inner core. They have the physical structure of how. What is their motion state? What dynamic coupling effects are taking place between them? How does the magnetic field in the liquid core generate, maintain and change? These mysterious and important scientific information are crucial to understanding the Earth where we are located and further extending to the study of other Earth-like planets. Unfortunately, all this information cannot be directly observed from the surface or space. Fortunately, these problems can be indirectly "observed" and studied through the fine characteristics of EOP changes.

Take the change of day length as an example. Due to the tidal effect of the moon and the sun on the earth, the earth's rotation is getting slower and slower, and the day length is getting longer and longer, i.e. The time of day and night is getting longer, with an average of about 0.002 seconds per 100 years. "In addition to the slower and slower rotation rate, scientists have also found periodic changes in the rotation rate. The variation of day length has a variation spectrum from one day to decades or even longer term, corresponding to different astronomical and geophysical mechanisms respectively. It is generally believed that interdecadal changes with scales of several decades and sub-decadal changes with scales of about 5 to 10 years are probably related to the deep interior physics of the Earth. The characteristics and origins of these periodic changes are what we need to look for and explore, especially the characteristics of sub-decade changes. "Huang Chengli, a researcher at the Shanghai Observatory and a participant in the study, said.

At present, little is known about the fine time-varying characteristics of day length variation and related physical reasons in this field, but this part of research is of great significance for further understanding the interannual motion characteristics of the Earth's core and solving the origin problem of rapid geomagnetic field changes. The rapid change of geomagnetic field is called geomagnetic abrupt change ("jerk").

"We use that nearly 57-year diurnal variation data from 1962 to 2019 provide by the International Earth rotation service system (IERS), Based on the analysis of a large number of numerical simulation examples, Based on the standard wavelet time-frequency transform method (NMWT) and our independently developed strategy of 'de-wavelet edge effect' (BEPME), Although the NMWT method has high frequency resolution, It can distinguish different frequency components in the sub-ten-year variation of day length, but this method has significant edge effect problem, and BEPME strategy can make up for this deficiency, so that we can accurately separate the target harmonic signal. Based on this method, we first find the amplitude increase phenomenon of 8.6-year signal. " Duan Peng, the first author of the study and associate researcher of Shanghai Observatory, said.

The research results of this group show that there are actually two periodic signals of principal components in the sub-ten-year variation of day length, with periods of 6 years and 8.6 years respectively. The superposition of these two signals can well explain the time-frequency domain characteristics of the observed sub-ten-year variation of day length. Previous work has failed to clarify the different physical origins of the sub-decade variation of the diurnal length. Their research shows that the signal with a period of about 8.6 years in the diurnal length variation is probably related to the torsional Alfen wave oscillation near the equator on the surface of the liquid core.

The interior of the earth is full of magnetic fields. If the magnetic lines of force of the magnetic field are imagined as strings, when the magnetic field is disturbed, the strings of the magnetic lines of force will oscillate, and the oscillations will propagate along the magnetic field, forming Alfin waves. When the magnetic lines of force gather into tubular structures called 'magnetic flow tubes', the Alfin wave propagating in the magnetic flow tubes is the twisted Alfin wave. The torsional wave propagates outward and couples with the mantle, resulting in wave signals with the same period in the daily length. Researchers predict that a new geomagnetic emergency will probably occur in the next 1-2 years.

"Our work negates the widely accepted view that there are only 6-year periodic signals in the sub-ten-year periodic signals of diurnal variation, For the first time, we found that there is a significant amplitude enhancement signal with a period of about 8.6 years in the sub-decade variation of the daily length, and also clarified that there is a close correspondence between the extreme value time of the oscillation and the occurrence of geomagnetic abrupt change. "Duan Pengshuo concluded.

Regarding the significance of this work, Huang Chengli added: "At present, the prediction of geomagnetic abrupt changes is a difficult problem in the world. Our work provides a new research entrance, which can predict the occurrence time of geomagnetic abrupt changes in the future by accurately separating the sub-decade variation oscillation signals of the Earth's rotation. It is not difficult to see that through the study of the fine variation characteristics of the earth's rotation, we can deeply understand the magnetohydrodynamic problems in the deep interior of the earth. "

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Figure 1.   The comparison between the red line and the black line obtained by the method (NMWT + BEPME) shows that the two curves are generally in good agreement, which indicates that the sub-decade variation characteristics of the daily length can be well explained by the superposition of the 6-year and 8.6-year frequency signals.

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Figure 2.   The correspondence between the 8.6-year periodic oscillation of the target restored in this paper and the geomagnetic jerks time (shadow rectangular strip); The red curve is the result recovered in this paper, and the green line is the least square fitting result of the red line, which can be further used to predict the occurrence time of jerk in the future.