What is the origin of the Earth's magnetic field?

By Waleed Mouhali for The Conversation,

It is nicknamed "earth shield" by scientists. For many land and aquatic migratory animals, it is a landmark for long-distance movements. It forces the compasses to point the same direction all the time.

You have no doubt recognised it, it is the Earth's magnetic field (CMT).

The Earth already had a magnetic field 3.45 billion years ago. At that time, its intensity was only 50-70 per cent of its current value. But as early as 3.2 billion years ago, the Earth's magnetic field was as intense as it is today.

However, it is very complicated to have certainties in this area. In 2020, a work by MIT thus contradicts results published in 2014, based on the magnetisation of ancient zircon crystals and proving that the Earth's magnetic field already existed 4.2 billion years ago.

Although magnets have been known since Antiquity, it was the Chinese who, around the year 1000-1100, first used them for orientation: it was the birth of the compass.

The relationship between the magnets and the Earth's magnetic field was then discovered in 1600 by William Gilbert, English physicist and physician of Queen Elizabeth I republishing in 1600 of Magno Magnete Tellurium ("From the Great Magnet of the Earth"). He demonstrated how a compass placed on the surface of a magnetised ball (la terrella) always indicates the same point, as it does on Earth.

Then, in 1840, the mathematician and physicist Carl Gauss proposed the idea that the “terrestrial” magnet was at the centre of the Earth.

Since then, scientific progress has shed light on the functioning of the Earth's magnetic field, and its role in electromagnetic phenomena. Yet the origin of the Earth's magnetic field is probably one of the most surprising problems in modern physics. To the question "why does the compass point north?" "The answer refuses to physicists since the XVI th century.

The most conclusive hypothesis, that of the theory of self-excited dynamos, was first introduced by Sir Joseph Larmor in 1919. It has withstood the most severe criticism, but has not yet been able to be applied to the field. case of terrestrial parameters.

In the age of scientific computing, it may seem surprising that this model of a self-excited dynamo has not yet been fully modelled.

Recent numerical models certainly make it possible to study the complete system, but in a range of parameter regimes very far from physical reality, due to the limitation of the computing power directly linked to the mathematical complexity of the terms associated with physical phenomena involved in the problem.

Researchers are therefore working on the development of new digital approaches, more efficient, or based on models of the phenomena in play.

The Earth's magnetic field can be compared, roughly, to the magnetic field of a straight magnet (the magnets stuck on your refrigerators).

The central point of this magnet is not exactly at the centre of the Earth, it is located a few hundred kilometres from the geometric centre. The CMT still seems to be dominated by this dipole (two poles: North and South) which aligns on average with the axis of rotation of our planet (axial dipole).

The set of Earth's magnetic field lines located above the ionosphere, more than 1,000 km away, is called the magnetosphere. The influence of the Earth's magnetic field, for its part, is felt several tens of thousands of kilometres away.

Even if we observe that the compass indicates the magnetic North (and therefore the south pole of the terrestrial magnet ...) for hundreds of millions of years, the paleomagneticians have also shown that the pole of the magnetic needle which points towards the magnetic North is sometimes the north, as today, sometimes the south.

The Earth's magnetic field has indeed inverted more than 100 times over the past 50 million years, and the last inversion dates back 42,000 years.

Origin (s) of the magnetic field: the dynamo effect The Earth's magnetic field is created by the complex movements of fluid (called convection) in the outer core of our planet. Said outer core is in fact a veritable ocean of molten metal (in particular iron and nickel), located between the solid iron seed with a radius of 1,220 km and the bottom of the mantle with a radius of 3,500 km.

Convection is undoubtedly solutal (due to differences by place of concentration) rather than thermal (due to differences by place of temperature), and intimately linked to the growth of the inner core: solid iron-nickel being less rich in elements dissolved than the liquid, the crystallisation of this liquid enriches the base of the outer core in dissolved elements; these elements being lighter than iron and nickel, the deep metallic liquid tends to rise under the effect of Archimedes' thrust.

The inner core, however, is too young (its age is estimated to be between 165 million and 2.5 billion years, a recent estimate leans for 1.3 billion years) for the above mechanism to have worked there. over 1.5 billion years old.

Another process of solutal convection would then have been the exsolution (that is to say the separation of a homogeneous constituent into several distinct constituents without changing the overall composition of the mixture) of magnesium oxide (MgO) , due to the progressive cooling of the core (then entirely liquid). Magnesium oxide is indeed soluble in liquid iron at very high temperature.

To understand the Earth's dynamo, it is also necessary to be able to identify what links the rotation of the Earth on itself and the magnetic field.

In the absence of a magnetic field, we know that the Coriolis force (the force responsible for that hesitant step when you walk in a rotating merry-go-round) forces the flows (here fluids) to organise themselves into cyclones and anticyclones - like in the atmosphere - and opposes any variation along the axis of rotation, leading the convection of the nucleus to organise itself into immense columns parallel to the axis of rotation.

The Coriolis force therefore generates a winding of matter in the form of vortices. Because of the predominance of the Coriolis force, these vortices align with the Earth's axis of rotation. The viscous friction between the fluid of the outer core and the solid boundary of the mantle causes a localised secondary flow which gives a "sense" of entrainment to the vortices.

When convection movements are sufficiently vigorous, dynamo instability (a spontaneous "increase" in the magnetic field over time) is triggered and produces a magnetic field whose geometry naturally depends on that of the movements which give rise to it. The field increases until the Laplace forces (forces of magnetic origin) come to compete with the Coriolis force.

It is only very recently that this scenario has received the support of full numerical simulations. The magnetic field produced by these digital dynamos is dominated by a dipole aligned with the axis of rotation. The simulations produce a magnetic field that looks like Earth's, and many even show spontaneous reversals.

However, many questions arise: what role do the small scales of the flow and the magnetic field play, which cannot be modeled? Do they not dominate the dissipation? What then is the power necessary to operate the terrestrial dynamo? During its history, has the Earth always had sufficient power to maintain its dynamo? Even before the crystallisation of the solid seed begins, which today provides most of the Archimedean forces that nourish convection? Why does n't Venus have a dynamo?

Anyway, while the Sun's solar cycle 25 promises to be very intense (this 11-year cycle that characterises solar activity began in December 2019), we can count on our dear Earth's magnetic field to protect us.

And if ever solar flares were to deprive us of our satellites to locate us, we would only have to rely on this good old CMT to guide us. Hoping that the pole reversal does not confuse us for all that!

(The author is a teacher-researcher in Physics, ECE Paris)

Check out latest DH videos here