Why are auroras seen mostly near the poles?

Near the North and South Poles, the night sky sometimes fills with moving curtains of coloured light. These light shows are called auroras. Near the North Pole they are known as the aurora borealis, or northern lights; near the South Pole they are the aurora australis, or southern lights.

They begin at the Sun, which is always sending energy and tiny particles out into space. Most of these never reach us, because the Earth's magnetic field acts as a shield. During a solar storm, however, the Sun can throw out huge bubbles of electrified gas. Some of the particles become trapped in the Earth's magnetic field and travel along it towards the poles. There they rush down into the atmosphere and react with the gases in the air, which give off light.

The colour depends on the gas. Oxygen gives off green and red light, while nitrogen glows blue and purple.

Auroras are not found only on Earth. Jupiter and Saturn have them too: Saturn's appear as red swirls around its south pole, and Jupiter's have been photographed in ultraviolet light. Scientists think that any planet with an atmosphere and a magnetic field probably has auroras of its own.