Terrestrial tides, atmospheric currents and ocean dynamics disrupt the Earth’s rotation and cause fluctuations in the length of days. But some of these variations have much deeper origins, a new study shows.
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The Earth completes one revolution on its axis within 24 hours. This defines the length of a day. However, this is only an average duration, as the Earth’s axial rotation is subject to some perturbations. Of course these are tiny. These are delays of a few milliseconds that occur periodically. Thus, multiple fluctuations in day length are observed over periods ranging from a month to a few decades.
External and superficial disorders
One of the main causes of these fluctuations is known. These are the Earth’s tides, caused by the gravitational pull of the moon and sun. Under the effect of this attraction, the Earth is periodically deformed. A mass movement that regularly affects its rotation and therefore the length of the day. In addition to this disturbance of external origin, there is also the movement of fluid masses on the surface. In fact, the oceans, atmosphere and continental waters represent a very dynamic fluid shell that can influence the length of the day, especially on a seasonal scale.
Gigantic whirlpools in the outer core
Another, more discrete actor would also be significantly involved in these fluctuations in day length. These are the fluid streams that animate the outer core. Unlike the crust, mantle, and inner core, the outer core is actually liquid. However, this difference in physical state compared to the Earth’s surrounding mantle means that the outer core does not follow quite the same rotational motion as the mantle and inner core (seed). The liquid iron it is made of is actually subject to the Coriolis force, which, like in the atmosphere, creates very specific convection currents. These streams are actually organized in the form of huge columns parallel to the axis of rotation, a kind of gigantic spiral. Within the outer core, the liquid iron flows in the form of huge vortices, the direction of which is influenced by the friction forces existing at the interface between the outer core and the solid shell.
Waves that can disrupt the rotation of the Earth
These streams of metallic fluid participate in the dynamo effect at the origin of the Earth’s magnetic field. They also produce very specific, recently identified waves that propagate very slowly at the core’s equator. These are so-called Magneto-Coriolis waves. The fluid’s movements also create another type of waves called Alfvén waves, which are torsion waves that propagate from the solid seed to the equator of the nucleus. Therefore, by transferring angular momentum to the solid mantle above, these two types of waves could disrupt the length of the day.
A hypothesis confirmed by a new study published in the journal Physics of the Earth and Planetary Interiors. Séverine Rosat from the Institut Terre et Environnement in Strasbourg and Nicolas Gillet from ISTerre in Grenoble observe a connection between certain fluctuations in day length and the propagation periods of these waves in the outer core.
The fluctuations in day length observed every 6 and 8.5 years (of the order of 0.2 ms) would be associated with the propagation of Magneto-Coriolis and Alfvén waves. These results suggest that Earth’s rotation, and therefore the length of the day, is influenced annually by the dynamics of fluid flows in the outer core.