Image in header: visualisation of the Earth’s magnetic field interacting with the Sun’s magnetic field. Source: https://universemagazine.com/en/the-earths-magnetic-field-should-we-anticipate-issues-from-it/
A compass and an electromagnet
Who did not hold a compass in their hands as a child? Watching the little needle pointing somewhat hesitantly and spontaneously towards north was a memorable moment of wonder for me all round (especially when I discovered that yer could easily make one yourself). Our planet has an invisible feature that can be ‘felt’ everywhere. This is the global magnetic field. Much later, I learned that this magnetic field is vital. It protects life on Earth from dangerous radiation from the Sun and other cosmic rays. It also helps prevent molecules in our atmosphere from colliding with high-energy solar particles. Such collisions give some molecules enough speed (and direction) to escape from the planet. We will see later that this is how Mars lost most of its atmosphere. In the next entry, we will look at how unique our Earth is with its global magnetic field. But first we ask the question: how do you get such a field?
In high school, we learn that you can make a magnet by wrapping a coil of electrically conductive wire around an iron bar. Then you put electric current on that coil, and presto: you get a strong magnet on the iron bar. If I remember correctly, it was then told that the Earth’s core also forms a magnet on the same principle. The inner core is a solid sphere full of iron. The outer core also contains a lot of iron, but is more liquid. An iron-rich mass is an electrical conductor, and contains many freely moving charges (electrons with charge -1), so a spinning liquid iron core is actually something like a coil in which an electric current runs. So that’s how the inner core must be the same as the iron rod in our school lab? It sounded logical, but when, out of sheer curiosity, I started to explore the matter a bit more, this simple principle turned out not to be really the correct representation of the facts after all.
A recipe for a planetary magnetic field
Our planetary magnetic field is actually the total effect of a lot of smaller magnetic fields all arising in the liquid outer core of the Earth. And all the smaller magnets can arise because the iron-rich mass in the outer core is driven by two major motions: the rising hotter iron mass from the lower boundary of the outer core to the upper boundary with the mantle on the one hand, and the rotation of the Earth on the other. The combination of these two motions causes many local corkscrew-like flows. The core of such corkscrew will then create a local magnetic field. Of course, most corkscrews are parallel to the Earth’s axis, since the Earth’s rotation is perpendicular to it. Therefore, the sum of all those local magnetic fields will give a planetary field whose poles point roughly towards the geographical poles.
The rotation of the Earth is one of the two motions you needed to form the corkscrews. The other motion is a flow from the lower boundary to the upper boundary of the outer mantle (convection cells). These are present thanks to several reasons:
- The inner core has cooled less rapidly since Earth’s formation, and is therefore hotter than the outer core. Therefore, the lower boundary of the outer core is also warmer than above it. The warmer material against it is therefore lighter than the slightly colder material above it.
- The solid inner core is constantly growing. At its boundary, there is a layer where new mass is constantly solidifying. The solidification process releases heat.
- In addition, a lot of heat is released in the core due to decay of radioactive material. Because the heaviest elements sink to the centre when the earth is formed, there is extra large amount of radioactive matter.
- And finally, coagulation releases relatively more substances that are slightly lighter (sulphur, oxygen, silicon), and the iron sticks relatively more in the new boundary layer of the inner core. Therefore, the adjacent mass becomes relatively lighter than its surroundings, and so it also starts to rise.
The Earth: just right again
It may seem rather obvious that any planet with an iron core would develop a magnetic field that way. But that is certainly not the case. We will see in the next entry (1.17) which objects in the Solar System also have a global magnetic field.
For the planetary magnetic field to be possible in terrestrial planets, a lot of properties have to be just right:
- The chemical composition of the inner core and outer core must be just right so that they are solid/liquid at the prevailing pressure.
- The outer core must be sufficiently large (in height) to allow sufficient movement of iron-rich streams
- The height of the outer core reduces with age, so there should be enough reserve to last for billions of years.
- The inner core grows so that heat is given off (solidification heat): not too fast or the inner core would swallow up the whole core over time. And not too slowly, or there will be too little ascent of lighter material, and the system will shut down. The speed depends on temperature, pressure, and composition.
- The planet must rotate around its own axis. Not too fast, because that gives a lot of unpredictability and lability, and not too slow because then there is no more ‘coil’ effect, and thus no corkscrews in the convection cells.
For gas planets and ice giants, things are not quite the same. The electrical conducting matter there is not in an iron core, but in metallic water in the mantle, for example. But we are looking here specifically at terrestrial planets like Earth, because it is on such planets that we might hope to find life in the future. In any case, Earth has convincingly maintained its planetary field so far, and will very likely continue to do so for billions of years to come. So bravo again for our planet!