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Sky on fire, quietly

What Are the Northern Lights?

The Sun throws tiny particles at us. Our air lights up where they land.

1 The Sun flingsparticles2 Earth'smagnetismsteers them3 The sky glows
A neon sign 100 km tall, powered by the Sun.

  1. Step 1 of 5

    The Sun is constantly leaking.

    The Sun Earth, in thestreamA million tonnes of particles leave the Sun every second

    As well as light, the Sun throws out a thin stream of electrically charged particles in every direction. It is called the solar wind.

  2. Step 2 of 5

    Earth is a giant magnet, and that saves us.

    Without the shield Particles hit thesurfaceWith the shield Most are steered aroundEarth

    Molten metal churning in Earth's core makes a magnetic field that wraps around the planet. It deflects most of those particles safely around us.

  3. Step 3 of 5

    But the shield funnels some to the poles.

    Field lines divein at the poles High air above theArcticWhich is why you go north to see them

    A magnetic field has two ends. Near the north and south poles the field lines dive into the planet, and they take trapped particles down with them.

  4. Step 4 of 5

    The particles smash into air and it glows.

    A curtain of light One atom hit, one flashof colour

    About 100 km up, each particle slams into a gas atom and knocks it into an excited state. When the atom settles back down it spits out a flash of coloured light.

  5. Step 5 of 5

    Which gas you hit picks the colour.

    Red — oxygen, 300 km uphighestGreen — oxygen, 120 kmmostcommonPurple — nitrogen, 100 kmloweredge

    Oxygen high up glows green, and red when higher still. Nitrogen glows blue and purple. The curtain's colours are a readout of what is up there.

    The south pole gets the same show at the same moment. There it is called the aurora australis.

The short version

The northern lights are Earth's air glowing where the Sun's particles crash into it.

Try it yourself

Look at a neon sign. Same physics: energy hits gas, gas glows, and the gas decides the colour.

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