A field guide to the sky
On a cold, clear night far from any city, the sky begins to move. Sheets of green light ripple overhead, folding and unfolding like a curtain in a slow wind.
That glow is the atmosphere itself, lit up where the Sun has struck it. Here is how particles from ninety-three million miles away end up painting the polar sky.
§1 The idea
A neon sign works on one simple trick. Run an electric current through a sealed tube of gas, and the current hands energy to the atoms inside. The atoms cannot hold that energy for long, so they let it go as light. The colour is fixed by the gas: neon glows red-orange, argon glows blue. Change the gas and you change the colour.
The aurora is the same trick at the scale of a planet. The tube is the thin upper air a hundred kilometres above your head. The current is a stream of charged particles from the Sun. And the colour, once again, is set by the gas that gets hit: oxygen and nitrogen, each glowing its own signature shade. Keep that one picture in mind and the rest of the aurora falls into place.
Each gas, one signature colour
557.7 nm · ~100–150 km
The everyday aurora. Atomic oxygen at middle altitude glows the familiar vivid green.
630.0 nm · >200 km
The same atom, far higher, where the air is thin, emits deep red instead.
427.8 nm · <100 km
Molecular nitrogen, low and energetic, edges the curtains with blue, violet and pink.
§2 The source
The Sun does not simply shine. It also leaks. A steady stream of charged particles, mostly electrons and protons, boils off its outer atmosphere and blows outward across the solar system in every direction. This is the solar wind.
At Earth's distance the solar wind moves at roughly 400 kilometres per second, and it never stops. Most of the time it is a light breeze. But the Sun is a violent star. Flares and vast eruptions called coronal mass ejections fling billions of tonnes of charged gas into space at once. When one of those gusts reaches Earth, two or three days later, the aurora flares up and pushes far beyond its usual haunts.
The particles arrive carrying energy, but they cannot simply rain straight down. Something stands between the Sun's wind and the air we breathe: the planet's own magnetic field.
§3 The funnel
Earth sits inside a magnetic bubble, the magnetosphere, generated by molten iron churning in its core. That bubble deflects most of the solar wind around the planet like water parting at the bow of a ship. But magnetic field lines are not a solid wall. They dive into the atmosphere at the north and south magnetic poles, and charged particles, forced to travel along those lines, are funnelled down toward both poles at once.
The result is not a cap over the pole but a ring around it, the auroral oval, sitting near 65 to 70 degrees of magnetic latitude. When the solar wind is calm the ring is thin and tight. When a storm hits, the ring brightens and swells outward toward the equator, and people who never see the aurora suddenly do. Drive the storm below and watch the oval reach south.
CITIES NOW UNDER THE OVAL
§4 The colours
When a particle from the funnel finally strikes an atom, it hands over energy and the atom glows. Which colour comes out depends on two things that travel together: how deep the particle drives before it hits something, and which gas is waiting at that depth. Energy sets the depth, depth sets the gas, and the gas sets the colour.
§5 The curtains
The aurora almost never appears as a flat wash. It hangs in tall, rippling curtains with sharp vertical folds. That shape is the magnetic field made visible. Particles can only slide along field lines, not across them, so each glowing streak traces one nearly vertical line of force. A whole sheet of neighbouring lines becomes a curtain, and its lower edge marks the altitude where the particles finally stop.
The dancing is a matter of supply. The stream of particles is gusty and uneven, surging and fading from second to second as currents shift in the magnetosphere far overhead. Where the flow strengthens, the curtain brightens and sharpens; where it eases, the curtain dims. Because that changes faster than the eye can settle, the whole display seems to shiver and flow.
The aurora is warm, or close enough to touch, or makes a crackling sound as it moves.
It glows 100 kilometres up in air so thin it is nearly a vacuum. It carries almost no heat, and it is far higher than any cloud or aircraft.
§6 Chasing them
You now have every ingredient. Catching the lights in person is a matter of stacking the odds: get under the oval, get the sky dark, and get the Sun cooperating.
§7 Test yourself
No score is kept anywhere but this page. Pick an answer to see whether it holds, and why.