A curtain of green light ripples over a Norwegian fjord, folding and brightening as though something alive stood behind it. It has been frightening and delighting people for as long as there have been people that far north.

Not firelight, not a reflection

The old guesses were reasonable enough. The lights must be sunlight glancing off polar ice, or a reflection from the sea, or some cold fire burning in the sky. Every one is wrong. The aurora isn't reflected from anything, and nothing up there is on fire. It is Earth's own air, glowing — lit from within by a rain of particles from the Sun.

And notice what it is not built from: the Sun's light. What triggers an aurora is the Sun's matter — a thin, fast wind of electrons and protons streaming off our star, a thing quite separate from the sunlight that warms your face.

A wind of particles, steered to the poles

The Sun blows off this stream of charged particles constantly, the solar wind, and when it's stormy it flings out great gusts of them. Reaching Earth, they run into our planet's magnetic field — the same field that swings a compass needle. A charged particle can't cross magnetic field lines easily; it gets caught and guided along them, and Earth's lines plunge into the atmosphere near the magnetic poles. So the particles are funnelled down in two vast rings around the poles. That's why you chase the aurora toward the Arctic and Antarctic, and why it hangs in shifting curtains that trace an invisible field. (The full choreography — how the particles are stored far out and then hurled down during a "substorm" — is genuinely intricate; the short, true version is that the field steers them poleward.)

Why the colours are so pure

Down they come, slamming into oxygen and nitrogen a hundred kilometres up and higher. Each collision knocks an atom's electrons to a higher rung; when they fall back, they let the energy go as a photon of one exact colour. This is nothing like the broad glow of a hot star, which pours out every colour at once. The aurora shines in a few pure lines, each the fingerprint of a particular atom — the sky turned into an enormous neon sign.

The commonest colour, that eerie green, comes from oxygen, and it hangs on a quirk. The green light is slow: an excited oxygen atom dawdles nearly a full second before releasing its photon. Down low, where the air is dense, it gets jostled and surrenders its energy in silence before it can shine. Only high up, where an atom can wait undisturbed, does the green escape. Rarer still is the deep red from oxygen higher yet, slower again, glowing only where the air thins almost to nothing.

The northern lights are the Sun's breath made visible — its particles, snared in Earth's magnetic net, striking our own air until it glows in the colours of the atoms it's made of.

In September 1859 a colossal solar storm lit auroras as far south as the Caribbean and set telegraph wires sparking in their operators' hands. People who had never seen the lights woke to a sky on fire, with no idea the Sun had reached out and touched their instruments.