The typical “northern lights,” or aurora borealis, are caused by collisions between fast-moving electrons and the oxygen and nitrogen in Earth’s upper atmosphere. The electrons – which come from the magnetosphere, the region of space controlled by Earth’s magnetic field – transfer energy to the oxygen and nitrogen gases, making them “excited”. As they “calm down” and return to their normal state, they emit photons, small bursts of energy in the form of light.
When a large number of these collisions occur, the oxygen and nitrogen can emit enough light for the eye to detect. This ghostly light will produce the dance of colors in the night sky we call the aurora. Most of the light comes from altitudes between 60 and 200 miles. Since the aurora is much dimmer than sunlight, it cannot be seen from the ground in the daytime.
The color of the aurora depends on which gas – oxygen or nitrogen – is being excited by the electrons, and on how excited it becomes. Oxygen emits either a greenish-yellow light (the most familiar color of the aurora) or a red light; nitrogen generally gives off a blue light. The blending of these colors can also produce purples, pinks and white. The oxygen and nitrogen also emit ultraviolet light, which can be detected by special cameras on satellites but not by the human eye.
Scientists are still trying to answer this question. The shape of the aurora depends on the source of the electrons in the magnetosphere and on the processes that cause the electrons to precipitate into the atmosphere. Dramatically different shapes can be seen over the course of a single night.