The Northern Lights, or Aurora Borealis, are glowing bands, circles and streams of colored lights that sometimes appear in the northern latitudes. The southern hemisphere has similar light shows that are called Aurora Australis or the Southern Lights. Both Northern and Southern Lights are referred to collectively as Aurora Polaris. The lights can span the visible, infrared and ultraviolet spectrum, and vary in intensity, duration and extent. They can last a few minutes or all night; Northern Lights can also occur during the day, but sunlight makes them invisible.
The Northern Lights are caused by activity on the sun. Strong magnetic activity is continually taking place on the surface of the sun, and electrons and ions are constantly being thrown out into space. This 'plasma', called the solar wind, is ejected in all directions. It is only when a strong solar wind blows in our direction that Northern Lights occur.
Because electrons and ions are charged particles, they are influenced by the magnetic field of the earth, which sweeps them up as they approach and funnels them toward both poles. The particles spiral down the 'cone' of the magnetic field until they hit the atmosphere, where they interact with atmospheric gases to cause the lights in the sky.
The stronger the 'storm' on the surface of the sun, the more particles are ejected into space and potentially into our atmosphere. The more particles, the farther south the particles reach before being consumed in a reaction with atmospheric gases. Auroras are visible in the Arctic and Antarctic nightly whenever there is solar storms, and are routinely seen in northern countries with high latitudes. The farther south you go, the less likely you are to see Northern Lights, with the aerial display being visible at the equator only once every century or two.
The colors of the Northern Lights are indicative of how high in the atmosphere the reaction is taking place. Red lights indicate particles reacting at higher altitudes than green, for example. Since the solar wind is always bathing the earth with particles, there is always some upper atmospheric reactions taking place. It's only when the solar wind is particularly intense that the reaction is bright enough to be seen with the naked eye.