What creates Earth's magnetic field?


The Earth's core works like a giant bicycle dynamo in reverse. Vishnu Varma reports. 


Northern Lights By Full Moon: the Aurora Borealis in Thingvellir National Park (UNESCO World Heritage Site), Iceland.
Barcroft/Contributor/Getty

Travelling to see the Northern or Southern lights has made its way into almost everyone’s bucket list. But unknown to most, these beautiful displays of light are caused by dangerous cosmic rays that have been deflected by our Earth’s magnetic field.

Magnetic fields around planets behave in the same way as a bar magnet. But at high temperatures, metals lose their magnetic properties. So it’s clear that Earth’s hot iron core isn’t what creates the magnetic field around our planet.

Labelled cross section of the earth interior view.
Diane Labombarbe

Instead, Earth’s magnetic field is caused by a dynamo effect.

The effect works in the same way as a dynamo light on a bicycle. Magnets in the dynamo start spinning when the bicycle is pedalled, creating an electric current. The electricity is then used to turn on the light.

This process also works in reverse. If you have a rotating electric current, it will create a magnetic field.

On Earth, flowing of liquid metal in the outer core of the planet generates electric currents. The rotation of Earth on its axis causes these electric currents to form a magnetic field which extends around the planet.

The magnetic field is extremely important to sustaining life on Earth. Without it, we would be exposed to high amounts of radiation from the Sun and our atmosphere would be free to leak into space.

This is likely what happened to the atmosphere on Mars. As Mars doesn’t have flowing liquid metal in its core, it doesn’t produce the same dynamo effect. This left the planet with a very weak magnetic field, allowing for its atmosphere to be stripped away by solar winds, leaving it uninhabitable.

Terrestrial magnetic field, similar to magnetic field of bar magnet tilted 11 degrees from spin axis of Earth.
DEA/D'ARCO EDITORI/Getty

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Vishnu Varma R Vejayan is a physics student from Queen Mary University of London with an interest in scientific writing and research in physics. He interned at Cosmos in early 2017.
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