Few natural sights have inspired as many legends as the aurora borealis, the shifting sheets of green, red and violet light that appear in the skies of the far north. Inuit hunters once believed the lights were the torches of spirits guiding the dead across the heavens, while Scandinavian sailors thought they were reflections of the armour of warrior maidens. Only in the last century have scientists been able to offer a convincing physical explanation.
The source of the aurora lies ninety-three million miles away, on the surface of the Sun. Eruptions on the Sun regularly hurl streams of charged particles, known as the solar wind, into space. When these particles approach Earth, most are deflected by the planet's magnetic field. A small fraction, however, is funneled toward the polar regions, where the magnetic field lines dip down into the atmosphere.
As the particles collide with oxygen and nitrogen atoms high above the ground, they transfer energy that is later released as light. The colour of the glow depends on which gas is involved and at what altitude: oxygen at high altitudes produces the familiar green, while rarer red displays occur when oxygen atoms are struck far higher up. Nitrogen is responsible for the blues and violets that sometimes edge the curtains of light.
Because the aurora is driven by solar activity, it becomes more frequent and more intense when the Sun is passing through one of its eleven-year peaks of turbulence. Modern forecasters use satellites to predict strong displays days in advance, but even with this warning the sight of a sky suddenly ablaze still catches observers by surprise.