Named for the Roman goddess of dawn, the aurora is a mysterious and unpredictable display of light in the night sky. The aurora borealis and aurora australis-the northern lights and southern lights-are common occurrences at high northern and southern latitudes.The typical aurora is caused by collisions between fast-moving electrons from space with the oxygen and nitrogen in Earth's upper atmosphere. The electrons-which come from the Earth's magnetosphere, the region of space controlled by Earth's magnetic field-transfer their energy to the oxygen and nitrogen atoms and molecules, making them "excited". As the gases return to their normal state, they emit photons, small bursts of energy in the form of light. When a large number of electrons come from the magnetosphere to bombard the atmosphere, the oxygen and nitrogen can emit enough light for the eye to detect, giving us beautiful auroral displays. This ghostly light originates at altitudes of 100 to more than 400 km.

The colour of the aurora depends on which gas-oxygen or nitrogen-is being excited by the electrons, and on how excited it becomes. The colour also depends upon how fast the electrons are moving, or how much energy they have at the time of their collisions. High energy electrons cause oxygen to emit green light (the most familiar colour of the aurora), while low energy electrons cause red light. Nitrogen generally gives off blue light. The blending of these colours can also lead to purples, pinks, and whites. The oxygen and nitrogen also emit ultraviolet light, which can be detected by special cameras on satellites.

The different shapes of auroras are a mystery that scientists are still trying to unravel. The shape seems to depend on where in the magnetosphere the electrons originate, what causes them to gain their energy, and why they dive into the atmosphere.


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