Earth's Magnetic Field and the Northern Lights, for Kids
The Sun is firing about a million tonnes of charged particles at us every second, at 400 kilometres a second, and has been doing so for four and a half billion years. We are fine because the Earth has a force field. That is not a figure of speech: the planet's magnetic field bends the solar wind around us the way a rock parts a stream, and the northern lights are what the field looks like on the rare occasions it lets something through. Once a child knows that, the aurora stops being a pretty sky and becomes evidence.
Everything below comes from the 3Eyes lesson Earth's Invisible Shield - a 3D interactive where a child turns the Sun up, fires a single particle at the planet and watches the magnetic field catch it and carry it to the pole. The recordings below are taken straight from it; the lesson itself is part of 3Eyes.
The field comes from the core
Earth's magnetic field is made about 3,000 km beneath your feet. The outer core is liquid iron and nickel, hotter than the surface of the Sun, and it is churning - hot metal rising, cooler metal sinking, the whole thing twisted by the planet's spin. Moving molten metal is a moving electric current, and a moving current makes a magnetic field. The Earth is a dynamo, and the field it makes reaches tens of thousands of kilometres out into space.
This is why a compass works: the needle is a small magnet lining up with a very large one. The magnetic pole is not quite at the geographic pole - it currently sits in the Canadian Arctic and drifts about 40 km a year toward Siberia - which is why a compass in Britain points a few degrees west of true north.
The solar wind, and the shape it makes
The Sun leaks. Its outer atmosphere is so hot that particles - mostly protons and electrons - escape at hundreds of kilometres a second in every direction. That stream is the solar wind, and it reaches Earth in about four days.
Charged particles cannot cross magnetic field lines easily; they get turned aside. So when the wind hits Earth's field, the field bends it around the planet, and the wind in turn squashes the field on the Sun-facing side and stretches it into a long tail on the night side. The shape in the video above - blunt in front, streaming out behind - is that push-and-shove, and it is called the magnetosphere. On a quiet day the front edge sits about 11 Earth-widths out toward the Sun.
Why the lights happen at the poles
Most of the solar wind never gets in. But the field lines that wrap the planet all come down to the ground at two places - the north and south magnetic poles - and near the poles the lines are nearly vertical. Particles that do get caught spiral along a field line as if it were a wire, and the wire leads them straight down into the atmosphere at high latitude.
So the aurora is not at the poles because it is cold there. It is at the poles because that is where the field's doors are. The ring of light sits at around 65-70° latitude - over Alaska, northern Canada, Iceland, Norway - and during a big storm the ring widens toward the equator, which is when Scotland, and very occasionally England or the northern United States, get a show.
What decides the colours
The colour is a fingerprint of the gas that was hit and how high up it was.
| Colour | Gas | Height | When you see it |
|---|---|---|---|
| Green | Oxygen | 100 - 150 km | The common one - most auroras |
| Red | Oxygen, thin and high | Above 200 km | Tops of tall curtains; big storms |
| Blue and purple | Nitrogen | Below 100 km | Lower fringes; needs energetic particles |
| Pink | Nitrogen and oxygen mixed | Around 100 km | Fast-moving edges |
Each incoming particle knocks an atom into an excited state, and the atom gives the energy back as a flash of one specific colour, the way a neon sign glows a colour set by its gas. Oxygen's green flash is the brightest, so green dominates. The red only appears where the air is so thin that an oxygen atom can wait the full two seconds it needs before flashing without being bumped first.
When the Sun gets angry
The Sun has storms. A coronal mass ejection throws a billion tonnes of material at once, and if it is aimed at Earth it arrives in a day or two and hits the shield at 800 km/s or more. The magnetosphere is pushed back - during the biggest storms, inside the orbit of the satellites that carry television - and far more particles get in. The aurora reaches the tropics, compasses swing, and currents are driven through power lines and pipelines.
The Carrington Event in 1859 set telegraph wires sparking and lit the sky over Cuba. A storm that size today would damage satellites and could take down power grids for weeks, which is why the Sun is watched around the clock. The lesson's "launch a solar storm" button is a safe way to see what that push looks like.
How to explain it in five minutes
Start with the compass. It points north because the whole planet is a magnet. Ask what could make a planet magnetic - the answer, molten metal moving in the core, is a good one to leave a child with.
Then the wind. The Sun is spraying us with particles all the time. Why are we not cooked? The shield.
Then fire one particle. Watch it get caught and led down to the pole. That single moment explains the location of the aurora better than any map.
Finish with the storm. Turn the Sun up, watch the shield get pushed back. The lights are the shield working, and a big storm is the shield working hard.
Where 3Eyes Fits
The aurora lab above is one of about sixty interactive science lessons built into 3Eyes, which is a parental controls product for family computers - it filters the machine, approves sites rather than blocking them one at a time, and limits YouTube to channels you choose.
The lessons are there because a computer that only ever says no is a difficult thing to defend to a ten-year-old. When the same laptop has sixty science lessons and a tutor on it, the limits stop being the entire relationship.
About two minutes to set up. See what it includes or create an account.
Frequently asked questions
What causes the northern lights? Charged particles from the Sun, caught by Earth's magnetic field and led down into the upper atmosphere near the poles, where they hit oxygen and nitrogen atoms and make them glow. The colour depends on the gas and the height.
Why does the aurora only happen near the poles? Because that is where the magnetic field lines come down to the ground. Caught particles spiral along the lines, and the lines lead to the poles. In a big solar storm the ring of light widens and can be seen much further south.
What is Earth's magnetic field made of? It is not made of anything you can touch - it is produced by electric currents in the liquid iron of the outer core, 3,000 km down, stirred by heat and twisted by the planet's spin. The field is the effect; the moving metal is the cause.
What would happen if Earth had no magnetic field? The solar wind would strip the upper atmosphere over millions of years - which is what appears to have happened to Mars - and the surface would receive far more radiation. Life could probably persist, but not comfortably, and electronics would have a hard time.
Can you see the northern lights in the UK or the US? Occasionally, in northern Scotland and the northern US states, during strong solar storms. The Sun is near the peak of its eleven-year cycle in the mid-2020s, which is why there have been more sightings recently.