An aurora is a flickering light mostly caused by the sun's radiation, usually found near the poles. They come in red, green and occasionally blue, and can sometimes resemble fire. In fact, the Roman Emperor Tiberius thought a city was on fire, so he sent fire engines to that city. The city on fire was actually a city against a backdrop of a red aurora.
An aurora can happen in the Arctic, around the North Pole (Aurora borealis - Dawn of the North - there it is also called the Northern Lights) or in the Antarctic around the South Pole (Aurora australis - Dawn of the South). An aurora can often be seen from long distances and stretching in the sky many hundreds of kilometers or miles.
Auroras can only be seen at night because their light is not as strong as the light of day. For instance, faint stars can be seen through the aurora. However they do happen during the day as well.
Aurora occurs when the Sun sends off matter we call particles to the empty space. These particles are charged and contain energy, which means they contribute to electricity. These particles flying in space are called "solar wind". Sometimes the solar wind hits Earth. Earth has a protection shield of energy around it. This is called the "magnetic field" and forms an elongated sphere around the Earth called the "magnetosphere". The magnetic field wards off most of the solar wind. At high-latitude areas (polar areas), the magnetic field is less powerful, and cannot protect Earth from the solar wind. There the particles of the solar wind and coming from the magnetosphere may hit the particles of the air (Earth's atmosphere). When they hit, the atmosphere is heated and excited and the excess energy gets away, a phenomenon which we see as moving lights in the sky above 100 km altitude typically. An aurora can also occur following a solar event called a coronal mass ejection (CME), when the charged particles rip through the electromagnetic field because of their power.
Auroral phenomena have been observed on other planets than Earth that have a magnetic field, such as Jupiter, Saturn and more recently Mars. It is believed to be a widespread phenomenon in the Solar System and beyond.
Many legends are associated with the aurora, in all countries where this phenomenon regularly occurs.
Source : http://simple.wikipedia.org/wiki/Aurora
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