The numbers
- How strong it is
- 65microtesla25 to 65 microtesla
- How tilted it is
- 9.34°
How it happens, step by step
A heart of iron
In the Earth's outer core there is liquid iron, very hot, that cannot stay still for a moment.
The metal moves
Heat makes it rise and sink, and the planet's spin bends those motions into big spirals.
Currents form
That flowing metal carries electric charges, and huge electric currents form deep inside the core.
The field is born
Every electric current makes a magnetic field. Added together, the ones in the core wrap the whole planet.
The shield deflects
The field stretches into space and bends the Sun's particles, which hardly touch the ground.
To get an idea
A compass needle moves thanks to the same shield that protects us from the Sun's wind.
Without that shield the Earth's air would have worn away little by little, as happened to Mars.
Fun facts
These are not in the book: there was no room on paper.
The polar auroras are born where the Sun's wind meets Earth's magnetic shield.
Around the Earth the field traps particles in two rings called the Van Allen belts.
Other bodies in space have their own magnetic field too, for example the Sun and Jupiter.
The first description of Earth's magnetism dates back to 1269, in a letter about the magnet.
The mathematical shape of the field and its lines of force were worked out by Gauss in 1832.
Mars no longer has a magnetic field, and that helped it lose its atmosphere.
Test yourself
The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.
Question 1 of 5★ is the one from your book
Where does Earth's magnetic field come from?
Question 2 of 5
Why can't the Earth be a magnet?
Question 3 of 5
What does the magnetic field do?
Question 4 of 5
What are polar auroras?
Question 5 of 5
Where is the field strongest?
Riddles
Nobody sees me and nobody hears me, and yet I shelter you from a wind that blows from ninety-three million miles away. A metal needle is the only thing that openly obeys me. What am I?
I am not born from a stone but from a motion: melted metal that turns and turns and never stops. If that motion stopped, I would disappear. What am I?
I give myself away only near the two ends of the world, where I let a few intruders in and the air lights up green. The rest of the time I stay invisible. What am I?
Try it yourself
Bring a magnet near the compass and watch the needle go wild. Earth's field is very weak: it takes little to cover it up.
Look for a photo of a polar aurora: that light is where the Sun's particles manage to come down along the field lines.
The words
- electric current
- Electric charges all moving in the same direction.
- magnetosphere
- The region of space around the Earth ruled by its magnetic field.
Sources: Wikidata (Q178061), Wikipedia (Campo geomagnetico).