2023-12-05 17:09:50
Terrestrial tides, atmospheric currents and ocean dynamics disrupt the rotation of the Earth, inducing variations in the length of days. Some of these variations, however, have a much deeper origin, as a new study reveals.
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The Earth completes one rotation around its axis over a 24-hour period. This is what defines the length of a day. However, this is only an average duration, as the Earth’s axial rotation is subject to some disturbances. Of course, these are tiny. These are delays of a few milliseconds, which occur periodically. Several oscillations in the length of the day are thus observed, over periods of the order of a month to a few decades.
External and surface disturbances
One of the main causes of these variations is well known. These are the earth’s tides induced by the attraction of the Moon and the Sun. Under the effect of this attraction, the Earth is periodically deformed. A movement of mass which regularly impacts its rotation and therefore the length of the day. To this disturbance of external origin, we can also add the movement of fluid masses on the surface. Oceans, atmosphere and continental waters in fact represent a very dynamic fluid envelope, capable of influencing the length of the day, particularly on a seasonal scale.
Gigantic whirlpools within the outer core
Another, more discreet actor would also participate significantly in these variations in day length. These are the fluid flows which animate the outer core. Unlike the crust, mantle, and inner core, the outer core is indeed liquid. However, this difference in physical state compared to the terrestrial envelopes which surround it means that the external core does not follow quite the same rotational movement as the mantle and the internal core (seed). The liquid iron that composes it is in fact subject to the Coriolis force, which, as in the atmosphere, will generate very specific convection currents. These currents are in fact organized in the form of immense columns parallel to the axis of rotation, a sort of gigantic spiral. Within the outer core, liquid iron therefore flows in the form of gigantic vortices, the direction of which is driven is influenced by the friction forces present at the interface between the outer core and the solid mantle.
Waves capable of disrupting the Earth’s rotation
These flows of metallic liquid participate in the dynamo effect, at the origin of the earth’s magnetic field. They also produce very specific waves recently identified, which propagate very slowly at the level of the equator of the nucleus. These are so-called magneto-Coriolis waves. The movements of the liquid also produce another type of waves, called Alfvén waves, which are torsion waves propagating from the solid seed towards the equator of the nucleus. By transferring angular momentum to the overlying solid mantle, these two types of waves might therefore disrupt the length of the day.
A hypothesis confirmed by a new study published in review Physics of the Earth and Planetary Interiors. Séverine Rosat of the Institut Terre et Environnement in Strasbourg and Nicolas Gillet of ISTerre in Grenoble observe a correlation between certain oscillations in the length of the day and the periods of propagation of these waves inside the outer core.
The variations in day length (of the order of 0.2 ms) observed every 6 and 8.5 years would be associated with the propagation of magneto-Coriolis and Alfvén waves. These results indicate that the Earth’s rotation and therefore the length of the day are influenced interannually by the dynamics of fluid flows in the outer core.
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