An aurora is a natural light display in the night sky, seen mostly near the Arctic and Antarctic. It appears as shifting ribbons, curtains and arcs of green, red and violet light.
Auroras are most common in a ring around each magnetic pole. During strong solar activity they can be seen much farther from the poles, sometimes at mid latitudes.
Causes
Auroras occur when charged particles from the Sun, carried by the solar wind, are steered by Earth's magnetic field toward the poles. There they collide with oxygen and nitrogen high in the atmosphere, and the excited atoms release the energy as light.
Colours
Green, the most common colour, comes from oxygen about 100 km up. Red appears higher, above 200 km. Nitrogen adds blue and purple fringes along the lower edge of the display.
Observing
The best views come on clear, dark nights, far from city lights. Activity follows the 11-year solar cycle and peaks near solar maximum, with the strongest displays often around the March and September equinoxes.
In culture
Auroras feature in the folklore of many northern peoples, who described them as spirits, torches or messengers. Today they draw travellers to places such as Iceland, northern Norway and Alaska.
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