What is Earth’s magnetic field?
A magnet has a magnetic field: the space around it where it can pull or push. Earth has one too. It looks like the field of a bar magnet placed inside the planet and tilted a little from the spin axis.
A free-hanging magnet or a compass needle lines up along this field. Its red end points to the north. Note: the magnetic pole in the north is really the south pole of the Earth magnet. It pulls the north end of the needle.
The field is weak: about 50 microtesla on the ground. A fridge magnet is far stronger. But it reaches tens of thousands of kilometres into space.
Where does the field come from? The dynamo
The inside of Earth is too hot to be a solid magnet. Iron loses magnetism above about 770 °C. So a different idea is used: the geodynamo.
- The outer core is liquid iron and nickel. It is very hot.
- Heat rising from the inner core makes the liquid churn, like boiling soup.
- Earth’s spin twists the flowing liquid into spirals.
- Moving, electrically conducting liquid makes electric currents. Currents make magnetic fields. The field helps keep the currents going.
This is a self-sustaining dynamo. Rocks that cooled long ago keep a record of the field direction. They show that the poles have flipped many times, on average every few hundred thousand years, with no fixed schedule.
Compass and the three elements: declination, dip, strength
To describe the field at a place we need three numbers (the magnetic elements).
- Declination (D): the angle between true north and magnetic north. It exists because the magnet axis is tilted about 11°. It changes from place to place and slowly with time.
- Dip or inclination (I): the angle the field makes with the ground. At the equator it is flat (0°). At the magnetic poles it points straight down (90°).
- Horizontal component (H): the part of the field that is parallel to the ground. It is what turns a normal compass. H = B cos I, and the vertical part is Z = B sin I.
For a simple dipole, tan I = 2 tan λ, where λ is the magnetic latitude.
Magnetosphere, solar wind and aurora (protection)
The Sun sends out a stream of charged particles called the solar wind. A moving charged particle feels a sideways force in a magnetic field, so it curves instead of going straight.
Earth’s field makes a huge bubble, the magnetosphere. On the Sun side it is pressed in to about 10 Earth radii. On the night side it stretches into a long tail. The solar wind flows around it like water around a stone.
This protects air, water and life. Solar particles cause radiation damage and could erode the atmosphere over time. Particles that do get in follow the field lines to the poles. There they hit air molecules and make light: the aurora (northern and southern lights). Oxygen glows green or red; nitrogen glows blue or purple.
Above about 60 km sits the ionosphere, a layer of charged air. It reflects some radio waves, so shortwave radio can travel far. A big solar storm can disturb it and trouble satellites, GPS and power grids.
Try it: make your own compass
- Rub a steel needle on a magnet 20 times, always in one direction.
- Push the needle through a small cork or leaf. Float it in a bowl of water.
- Predict first: which way will it settle? Then watch. Compare with a phone compass.
- Bring a magnet close. The needle turns toward it. A close magnet beats Earth’s weak field.
In the 3D above, drag the place slider and read the dip angle.
Key formulas and definitions
- Dip at magnetic latitude λ (dipole): tan I = 2 tan λ
- Horizontal part: H = B cos I Vertical part: Z = B sin I
- B = √(H² + Z²), tan I = Z / H
- Surface field strength ≈ 25–65 μT (1 μT = 10⁻⁶ T)
- Axis tilt from spin axis ≈ 11°
Worked examples
1. At a place on the magnetic equator the field is 40 μT. What is the dip and the horizontal part?
At the equator λ = 0, so tan I = 0 and I = 0°. The field is flat, so H = B cos 0 = 40 μT and Z = 0.
2. The horizontal part of the field at a place is 30 μT and the dip is 60°. Find the total field B.
H = B cos I, so B = H / cos I = 30 / 0.5 = 60 μT.
3. Why does a compass needle in the far north stop working well?
Near the magnetic pole the field points almost straight down. The horizontal part H becomes very small, so the needle gets almost no sideways push and may point in any direction.
Common mistakes
- Thinking the north magnetic pole is a north pole of the Earth magnet. It is the south pole of the Earth magnet, which is why it attracts the north end of a compass.
- Mixing up declination and dip. Declination is a sideways angle (east–west). Dip is an up–down angle.
- Saying the core is a solid magnet. It is too hot. The field comes from moving liquid iron (dynamo).
- Believing the aurora is only in the north. It happens at both poles (aurora australis in the south).