The origin of large anomalies in the Earth’s magnetic field has long remained a mystery. However, researchers suggest that they may have a link with the abnormally hot zones present at the base of the mantle.
A protective bubble against solar radiation, the earth’s magnetic field has its source in the very heart of our planet. A process called geodynamo, which relies on the flow of matter that animates the outer core composed of liquid iron. A mechanism at work for several billion years and which, if it seems immutable, does not however produce a stable and regular magnetic field around the globe.
The magnetic field has thus been reversed an incalculable number of times since its origin which, by the way, is still poorly dated. In the last 200 million years alone, there have been no less than 300 reversals. A natural phenomenon and without consequence for living beings, which however is always difficult to explain.
Areas of weak magnetic field that pose problems for satellites
In addition to these reversals, the magnetic field strength is not uniform across the globe. There are regions where the intensity is very high, others where it is very low. We then speak of anomalies of the magnetic field. The most important is the negative South Atlantic Anomaly. Roughly, it extends from South America to the southern tip of Africa. The magnetic field there is relatively weak, a characteristic which is also not without consequences for the satellites which regularly cross this region of the globe. During their passage over the South Atlantic, the satellites in orbit are thus less well protected against solar radiation, to the point that some, like Hubble, simply do not take any measurements when they pass through this zone, in order to to limit the risk of damage.
If the phenomenon is well known, its origin is much less so. For Jonathan Mound and Christopher Davies, authors of a new study published in Nature Geosciencethe mantle and the thermal interactions that exist with the outer core would play a major role in this story.
A link to temperature anomalies at the base of the mantle
In general, the convection currents which produce the magnetic field within the outer core are intimately linked to the evacuation of the internal heat of the Earth. The heat is indeed transmitted from the seed to the mantle by passing through the outer core, which activates convection cells. The heat is then transmitted through the mantle to the Earth’s surface. This heat flux therefore seems to play a primordial role in the generation of the magnetic field. However, scientists at the University of Leeds have noticed that the areas where the magnetic field is weaker correspond to heat anomalies in the lower mantle.
These anomalies are well known. There are two main ones, named LLVP for Large Low Velocity Province (Large low-velocity provinces), located under Africa and under the Pacific, at the level of the core-mantle boundary.
An influence on the heat flow inside the outer core
By carrying out numerical simulations taking into account the presence of these areas of very hot mantle, the researchers realized that they reduced the flow of heat within the underlying outer core. In other words, the presence of temperature anomalies at the base of the mantle prevents the outer core from cooling in these regions, thus affecting the convection conditions, and therefore the generation of the magnetic field. It is therefore the differences in the escape velocity of the internal heat of the Earth which would be at the origin of the anomalies of the magnetic field observed on the surface.
It remains to be seen how long this phenomenon can last. The existence time of these anomalies would therefore depend on the evolution of the hot zones at the base of the mantle. However, the processes that drive the mantle are very slow and it can be estimated that the lifetime of temperature anomalies in the lower mantle is of the order of tens of millions of years. As a result, magnetic field anomalies are expected to persist over a similar time range. However, the authors highlight the fact that the outer core represents a very dynamic environment. It would therefore not be surprising to observe variations in the magnetic field on a smaller scale, over shorter periods, of the order of a few hundred or a few thousand years.
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