The Earth's rotation rate is not constant. The length of a day can change slightly over different time scales, and scientists have long known that various factors play a role in these changes. Short-term variations are usually related to the redistribution of mass and angular momentum among the atmosphere, the oceans, and the solid Earth, while the tidal effects of the Moon manifest over much longer intervals.
But the changes that occur on multi-decade scales have long remained one of the major puzzles of geophysics. New research shows the answer to this puzzle should be sought not at the Earth's surface, but thousands of kilometers deeper, in the interaction between the core and the mantle.
Three forces involved in the Earth's rotation
To explain multi-decade changes in day length, researchers examined three main mechanisms: gravitational coupling, electromagnetic coupling, and topographic coupling.
Gravitational coupling relates to the interaction between the inner core and the mantle. Deformation and motion of the inner core can produce gravitational torques and cause the transfer of angular momentum among different parts of the Earth.
Electromagnetic coupling also pertains to the interaction of the magnetic field and currents in the core with conductive layers near the core–mantle boundary. In contrast, topographic coupling relates to irregularities and structures present at the boundary between the core and the mantle.
Researchers' models show these three mechanisms are in a constant tug-of-war, but their contributions are not equal. Gravitational torque plays the largest role in producing multi-decade changes in day length, while electromagnetic and topographic effects partially oppose it.
Why does the Earth sometimes rotate faster and sometimes slower?
The Earth can be considered a collection of interacting components that do not necessarily all move at the same speed. The inner core, outer core, mantle, and crust can exchange angular momentum among one another.
In the new model, changes in the motion of the inner core lead to the generation of gravitational torque. This torque then affects the rotation rate of the Earth's outer parts through the interaction between the core and the mantle.
To investigate this issue, researchers combined seismic reconstructions of the relative rotation of the inner core with data on core flows. They then compared the history of different torques with the observed variations in day length. The results showed that gravitational torque can explain a large portion of the observed multi-decade changes.
Very small changes, but scientifically important
These changes are practically imperceptible on the scale of everyday life. The length of the day changes by only a few thousandths of a second, but this very small change is of great importance to scientists because it can provide information about the structure and physical processes deep within the Earth.
The new study goes beyond explaining changes in the Earth's rotation rate. The proposed model is consistent with features of the lower mantle structure, including regions of near-neutral density and a highly conductive layer near the base of the mantle. The findings also point to an inner core with relatively low viscosity and the ability to deform on a timescale of several years.
Why does knowing the Earth's rotation rate matter?
Changes in the Earth's rotation are not just a theoretical issue. Precise measurement of day length and Earth's motion is important for timekeeping systems, geophysical models, and understanding the planet's behavior. Moreover, rotation changes can help scientists probe parts of the Earth's interior that are practically inaccessible to direct observation.
In fact, geoscientists cannot directly access the Earth's core. Therefore, data such as seismic waves, the magnetic field, and very small changes in the planet's rotation rate act as indirect tools for studying the deep Earth.
The study shows that even a change of a few thousandths of a second in the length of the day can provide clues about processes occurring thousands of kilometers beneath the Earth's surface.