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Major anomaly in Earth’s magnetic field finally explained?

The origin of the large anomalies in Earth’s magnetic field has long been a mystery. However, the researchers suggest that they may have a connection to the abnormally hot regions found at the base of the mantle.

A protective bubble of solar radiation, Earth’s magnetic field has its source in the core of our planet. A process called geodynamo, which relies on the flow of matter moving an outer core of liquid iron. A mechanism that has been operating for many billions of years and which, if it seems unalterable, does not, however, produce a stable and uniform magnetic field around the world.

Thus the magnetic field has been reversed countless times since its inception which, by the way, is still poorly dated. In the past 200 million years alone, there have been at least 300 reversals. It is a natural phenomenon and has no consequences for living beings, which is always difficult to explain.

Weak magnetic field areas that cause problems for satellites

In addition to these reversals, the magnetic field strength is not uniform across the globe. There are areas where the intensity is very high, and others where it is very low. Then we talk about magnetic field anomalies. The most important is the negative South Atlantic Anomaly. Roughly speaking, it extends from South America to the southern tip of Africa. The magnetic field there is relatively weak, a characteristic that is also not without consequences for the satellites that regularly cross this region of the globe. As they pass over the South Atlantic, orbiting satellites are less shielded from solar radiation, to the point where some, like Hubble, simply don’t take any measurements when they pass through this region, in order to reduce the risk. from damage.

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If the phenomenon is well known, its origin is much less likely. For Jonathan Maund and Christopher Davies, authors of a new study published in Natural Earth Sciencesthe mantle and thermal interactions with the outer core will play a major role in this story.

Correlation to temperature anomalies at the base of the mantle

In general, the convection currents that produce the magnetic field within the outer core are closely related to the evacuation of Earth’s internal heat. Heat is actually transferred from the seed to the mantle by passing through the outer core, which activates the convection cells. The heat is then transmitted through the mantle to the Earth’s surface. So it seems that this heat flux plays an essential role in generating the magnetic field. However, scientists at the University of Leeds note that regions where the magnetic field is weaker correspond to thermal anomalies in the lower mantle.

These anomalies are well known. There are two main types, named for LLVP Big county low speed (large low-velocity subdivisions), located under Africa and under the Pacific Ocean, at the level of the core-mantle boundary.

Effect on heat flow within the outer core

By running numerical simulations taking into account the presence of these regions of the superheated mantle, the researchers realized that they reduced heat flux within the underlying outer core. In other words, the presence of a temperature anomaly at the base of the mantle prevents the outer core from cooling in these regions, thus affecting convection conditions, and thus magnetic field generation. It is thus differences in the escape velocity of Earth’s internal heat that are at the root of the deviations in the magnetic field observed at the surface.

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It remains to be seen how long this phenomenon can last. Thus, the time when these anomalies exist will depend on the evolution of hot regions at the base of the mantle. However, the processes driving the mantle are very slow and the lifetime of the temperature anomalies in the lower mantle can be estimated to be in the tens of millions of years. As a result, magnetic field anomalies are expected to persist over a similar time scale. However, the authors highlight the fact that the outer core represents a highly dynamic environment. So it wouldn’t be surprising to observe variations in the magnetic field on a smaller scale, over shorter periods, on the order of a few hundred or a few thousand years.