The rhythm of the Earth’s inner core for 8.5 years

The rhythm of the Earth’s inner core for 8.5 years

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Schematic diagram of internal oblique sway. Credit: Dr. Ding/Nature Communications. 10.1038/s41467-023-43894-9.

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Schematic diagram of internal oblique sway. Credit: Dr. Ding/Nature Communications. 10.1038/s41467-023-43894-9.

Researchers from China have confirmed the existence of an inner core wobble (ICW) for about 8.5 years in both polar motion and changes in day length, revealing a constant tilt of about 0.17 degrees between the Earth’s inner core and the mantle, challenging traditional assumptions. And provide insight into the Earth’s internal dynamics and density distribution.

The results of the study are published in Nature Communications.

The Earth’s inner core is a solid, dense sphere composed primarily of iron and nickel. It lies beneath the liquid outer core, and has a radius of about 1,200 kilometers (746 miles). This region plays a crucial role in Earth’s geophysical processes, influencing the planet’s magnetic field and contributing to the overall dynamics of Earth’s interior.

Understanding the properties and behavior of the inner core is essential to unraveling mysteries regarding Earth’s structure, seismic activity, and magnetic field.

ICW refers to the oscillatory motion of the Earth’s inner core around its axis of rotation. This phenomenon is characterized by periodic oscillation of the axis of the shape of the inner core.

A new study has confirmed that the Earth’s ICW has a periodic motion with a cycle lasting about 8.5 years. This oscillating motion has been observed in measurements of polar motion, the periodic motion of the Earth’s rotation axis, changes in the length of day (ΔLOD), and changes in the Earth’s rotation speed.

Professor Hao Ding, co-author of this research and Dean of the Department of Geophysics at Wuhan University, was inspired by the unconventional density structures revealed in the Earth’s free oscillation.

“My then-PhD student, Dr. Yachong An, and I discovered an 8.5-year signal in PM and ΔLOD, which prompted us to conduct the current study,” he told Phys.org.


Different layers of the earth. Credit: Amit Sengupta/YouTube

The freedom of the earth to shake and rotate

The Earth consists of four layers: crust, mantle, outer core, and inner core.

Traditionally, our understanding of the Earth’s rotation has been based on the assumption of a uniform distribution of density in the mantle and core along the radial direction (extending from the center outward). This assumption leads to the belief that the axis of rotation of the Earth’s core coincides with the axis of rotation of the mantle.

“However, the results of the Earth’s free oscillations (natural oscillations of the Earth as a whole) indicate that the density structures in the Earth’s interior are highly heterogeneous, so this assumption should not be realistic,” Dr. Ding explained.

When Professor Ding analyzed Earth’s fine particles in 2018, a signal with a period of approximately 8.5 years appeared, indicating the presence of the ICW. This unexpected discovery, later supported by a similar signal in the ΔLOD of Earth’s rotation, led to a paradigm shift.

Based on these discoveries, the researchers carefully analyzed the PM and ΔLOD of Earth’s rotation and identified a signal of approximately 8.5 years in the PM as a manifestation of the ICW.

This conclusion comes after excluding three sources of external excitation – atmospheric, oceanic and hydrological. Interestingly, the 8.5-year indication is not limited to precision medicine alone; It is also constantly present in the periodic motion of the Earth’s axis of rotation, or ΔLOD.

This simultaneous presence strongly suggests a deep relationship between the ICW and these rotational dynamics.

The static tilt between the inner core and the mantle

To explain the 8.5-year signal detected in PM and ΔLOD, the researchers examined the amplitude of the ICW in both. This led them to conclude that there is a constant tilt angle of 0.17 degrees between the axis of rotation of the inner core and the mantle.

“This means a possible eastward differential rotation angle of the inner core of less than one degree and a misalignment of the symmetry axes of the lower mantle/core boundary layer and the mantle with the upper mantle.”

“These deviations provide valuable constraints to the 3D density model of the mantle and question assumptions in the oblate liquid core, highlighting potential deviations from the ideal sphericity calculated using conventional theories,” Dr Ding explained.

Furthermore, the ICW’s periodicity of about 8.5 years reveals another layer of Earth’s complexity. The periodic motion indicates a jump in density of about 0.52 g/cm33 On the inner core border.

In simple terms, this means that there is a noticeable change in density at the boundary between the inner core and its surrounding layers.

While the research focuses primarily on the inner core, the identified constant inclination and ICW may extend their influence to broader geophysical phenomena. As Dr. Ding explained, “Static tilt may also lead to a certain change in the shape of the fluid core, leading to a change in fluid motion and a corresponding change in the geomagnetic field.”

Implications for future research

The study’s discovery of the Earth’s ICW and its associated constant tilt challenges traditional assumptions about the Earth’s rotation. The ICW’s 8.5-year periodicity, accompanied by a noticeable jump in density at the inner boundary of the core, reveals the complexities of our planet’s internal dynamics.

Future research by Dr. Ding and his team aims to delve deeper into the stratigraphic structure and density of the Earth’s core, and explore the patterns and periods of core movements.

“The layered structure and density of the Earth’s core has long been an issue in geoscience research. We aim to delve deeper into the periodic oscillation and differential rotation of the Earth’s core, seeking to clarify these different conceptual theories that can be difficult to understand.” coexistence.”

more information:
Yachong An et al, Internal static tendency inferred from intra-motional oscillation in Earth’s rotation, Nature Communications (2023). doi: 10.1038/s41467-023-43894-9.

Magazine information:
Nature Communications

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