Beginner · 1:47
Earth's magnetic shield
How Earth holds off the wind from the Sun, and what a solar storm does
Each is simulated from real physics and measurements; anything no eye could see, like magnetic fields and radio waves, is drawn and labelled as drawn.
Below are the 9 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.
Open Cosmic Scale then the Phenomena tab
1. Earth is a magnet
Molten iron churning in Earth's outer core makes a magnetic field. At the ground it is about a hundred times weaker than a fridge magnet, yet it reaches far into space. Its poles lie about 9° from the axis Earth spins on.
Almost halfway to Earth's centre lies the outer core: an ocean of liquid iron, thousands of degrees hot. Heat rising from below keeps it churning, and Earth's spin twists the flows into spirals. Moving iron carries electric currents, electric currents make a magnetic field, and the field keeps the currents flowing: a natural dynamo that has run for billions of years.
The lines here show the field's shape: a compass needle lines up along them. They leave Earth in the southern hemisphere and come back in near the north, so the pole a compass points to is, strictly speaking, a magnetic south pole.
The magnetic poles wander, and every few hundred thousand years, on average, the field flips: north and south swap places. The last flip was about 780,000 years ago. Lava records the field's direction as it cools, so old rocks keep a diary of every flip.
2. The wind from the Sun
The Sun blows out a wind of charged particles at about 400 km/s, carrying its own magnetic field. It takes some four days to cross the 149.6 million km to us. Earth's field turns it aside.
The Sun's outer atmosphere, the corona, is hundreds of times hotter than its visible surface, so hot that the Sun's gravity cannot hold on to it. It streams away into space in every direction as the solar wind. Eugene Parker predicted it in 1958, and spacecraft confirmed it a few years later.
The wind is plasma: gas so hot that its atoms are broken into charged particles, mostly protons and electrons. It is extremely thin, only a few particles in a space the size of a sugar cube, emptier than the best vacuum made in a laboratory. Yet it never stops, and it carries the Sun's magnetic field out across the whole solar system.
Charged particles cannot easily cross magnetic field lines; they spiral around them instead. So Earth's field works as a shield, steering the wind around the planet. Mars has no such global field, and over billions of years the solar wind has stripped away much of its air.
3. The shield
The wind brakes sharply at the bow shock, 86,704 km out, then flows round the magnetopause, the edge of Earth's field, 66,695 km from Earth. Behind Earth it stretches the field into a tail.
The solar wind travels faster than any wave that could run ahead through it to warn it of the obstacle. So it cannot ease its way around Earth: it slows abruptly and heats up at the bow shock, like the wave piled up in front of a moving boat.
Past the shock, the slowed wind flows around the magnetopause, where its push and the push of Earth's field balance. That is the edge of the magnetosphere, Earth's magnetic bubble: inside, Earth's field is in charge; outside, the Sun's wind.
The bubble is squashed on the side facing the Sun and stretched out far behind, like a windsock. Its size changes with the wind: when the wind blows harder, the magnetopause is pushed closer to Earth. In space both boundaries are invisible; here they are drawn as glowing sheets.
4. A tail past the Moon
The tail is more than a hundred Earth widths long, reaching far past the Moon's orbit, 384,400 km away. Each month, around full moon, the Moon passes through it.
On the night side, the solar wind drags Earth's field out into a long tail, the magnetotail. No one knows exactly how long it is. Spacecraft have found it far beyond the Moon, and it may stretch more than 15 times as far as the Moon is.
Down the middle of the tail runs the plasma sheet, a layer of hot charged particles. Above and below it the field lines point in opposite directions, stretched like rubber bands and storing energy from the solar wind. When they snap back, that energy lights up the auroras, as a later step shows.
The Moon spends about six days each month inside the tail, from three days before full moon to three days after. There it is sheltered from the solar wind, and Japan's Kaguya spacecraft found oxygen from Earth's upper air landing on it.
5. The radiation belts
Some particles are trapped, spiralling along the field and bouncing between the poles: the Van Allen belts, found by Explorer 1 in 1958. The inner belt lies 1,000 km to 12,000 km up, the outer 13,000 km to 60,000 km.
A charged particle spirals around a magnetic field line. Near the poles, where the lines crowd together and the field grows stronger, the spiral tightens until the particle is turned back, as if by a mirror. So it bounces from pole to pole in a second or less, while drifting all the way around Earth, and it can stay trapped for years.
The inner belt holds mostly protons of enormous energy, and it hardly changes. The outer belt is mostly electrons and changes all the time: a solar storm can empty it or fill it within hours.
Explorer 1, the first American satellite, carried a radiation counter built by James Van Allen's team. High up it counted nothing at all, which made no sense until Explorer 3 showed why: the radiation was so intense that it swamped the counter.
With the sound on, you are hearing the belts: radio waves recorded by NASA's Van Allen Probes. Their frequencies are the same as those of sounds we can hear, so a speaker can play them directly. Waves like these speed some electrons up and knock others down into the air.
6. A solar storm
Sometimes the Sun hurls billions of tonnes of gas at once: a coronal mass ejection, at up to 3,000 km/s. On 8 and 9 May 2024 a giant group of sunspots sent five of them towards Earth.
Sunspots are darker, cooler patches where the Sun's magnetic field breaks through its surface, tangled and twisted. When a twisted field snaps into a simpler shape, it lets go of enormous energy: a flash of light and X-rays called a flare, and often a cloud of gas thrown into space with its own magnetic field, a coronal mass ejection.
The flare's light reaches Earth in about eight minutes; a cloud takes one to four days. Telescopes watching the Sun give a few days' warning, but how strong a cloud really is only shows when it passes spacecraft stationed four times farther away than the Moon, less than an hour before it reaches us.
In May 2024 the sunspot group AR 3664 was more than 15 times as wide as Earth, big enough to see through eclipse glasses without a telescope. The Sun was near the peak of its 11-year cycle, and on 8 and 9 May the group launched cloud after cloud towards Earth.
7. Squeezed
At 19:12 UT on 10 May the magnetopause was measured just 32,110 km from Earth, inside the orbit of geostationary satellites. The cloud's field pointed south, against Earth's: the two joined, and Earth's field was torn open on the day side and piled into the tail.
The storm clouds struck much harder than the usual wind: faster, denser, and with a stronger magnetic field. They pushed the magnetopause, normally about 66,695 km out, to less than half that distance. For a while, geostationary satellites over the day side were outside Earth's shield, in the solar wind itself.
What matters most is which way a cloud's own field points. On the side facing the Sun, Earth's field points north. When a cloud's field points south, the two are opposite and can join, the way opposite poles of magnets snap together. The shield opens: Earth's field lines are hooked onto the wind, which drags them over the poles into the tail, pouring energy in.
A cloud whose field points north mostly slides past, and even a fast one does little harm. That is why forecasters cannot tell how strong a storm will be until the cloud reaches the spacecraft waiting for it.
8. Northern and southern lights
In the tail, stretched field lines snap back and hurl particles down along the field towards the poles. 90 km to 400 km up they make oxygen glow green, and red higher up, and nitrogen purple.
The energy piled into the tail does not stay there. The stretched field lines snap back towards Earth like a catapult and fling electrons down along the field. The lines come down near the poles, so the electrons strike the upper air in a ring around each magnetic pole: the auroral oval glowing here.
When an electron hits an atom of oxygen or a molecule of nitrogen, it gives it extra energy, which comes back out as light of one exact colour, the way a neon sign glows. Oxygen gives green. Higher up, where the air is thinnest, it gives red instead: that red light takes almost two minutes to come out, and lower down a collision would steal the energy first. Nitrogen glows blue and purple.
Auroras in the north and south shine at the same time, almost mirror images of each other. Other planets with magnetic fields have them too: Jupiter's auroras are bigger than the whole Earth.
9. The storm of May 2024
It was the strongest storm since 2003. Auroras were seen as far from the poles as Mexico, India and Namibia, and in June NASA's CIRBE satellite found two new belts of particles it had left between the Van Allen belts.
As the storm poured in energy, the rings of light spread far towards the equator, and millions of people saw an aurora for the first time. Phone cameras caught more than eyes did: they pick up faint reds and pinks that our eyes barely see at night.
Space storms reach down to our technology too. GPS positions drifted, so tractors steered by satellite wandered off their rows during spring planting in North America. The heated upper air swelled and dragged on low satellites, and thousands of them had to fire their engines to hold their orbits.
The storm also left something new behind. In June, NASA's CIRBE, a satellite the size of a shoebox, found two new belts between the Van Allen belts, one of them with protons in it, a make-up never seen before. The electron belt faded after about three months; the other lasted far longer.
Still, it was not the strongest storm on record. In 1859 the Carrington Event lit up auroras as far as the tropics and made telegraph wires spark. A storm like that today could do far more damage, which is why the Sun is watched around the clock.