Presented by

Ethan Ross & Tyler Goss

Up to about 1915 auroral heights they where the most unknown question. The precise, quantitative measurements of the auroral heights were carried out by Carl Størmer between 1910 and 1940, using the parallax method. Two assistants- between 50 to 100 kilometers apart - took pictures of the same aurora at the same time. From the star patterns, they could read the angular distance and thus calculate the height.

But even with such speeds (over a million kilometer per hour), it takes these plasma clouds roughly three days to reach our planet. When they are closing in on Earth, they are captured by Earth's magnetic field and are guided towards Earth's two magnetic poles; the geomagnetic south pole and the geomagnetic north pole.

On their way down towards the geomagnetic poles, the solar

particles are stopped by Earth's atmosphere, which acts as an effective shield against these deadly particles.

When the solar particles are stopped by the atmosphere, they collide with the atmospheric gases present, and the collision energy between the solar particle and the gas molecule is emitted as a photon - a light particle. And when you have many such collisions, you have an aurora - lights that may seem to move across the sky.

Since people in olden times did not understand what northern lights were, they often created mythology and superstition to explain the dancing spirits or fighting hordes in the sky. Auroras were commonly associated with dancing in Norway; inhabitants believed that northern lights were old maids, dancing and waving.

VikingsDuring the Viking period, northern lights were referred

to as reflections from dead maidens. Among the Eskimos in Greenland and northern Canada, the aurora was the realm of the dead, and when the lights changed rapidly, it meant that dead friends were trying to contact their living relatives.

Native AmericansMany native Americans believed that they could

conjure up ghosts and spirits by whistling to the lights. It was a common belief that the northern lights were the reflections in the sky of huge fires in the distant north, or that the mighty God himself lighted up the dark and cold parts of the world.

DanishOne romantic conception found in Danish folklore is

that these lights were due to a throng of swans flying so far to the north that they were caught in the ice. Each time they flapped their wings, they created reflections which created the northern lights.

Based on more than 20.000 parallactic auroral pictures, aforementioned Størmer and his assistants accurately calculated the average height of the auroras. Most of the night-time auroras within the auroral zones are found between 90 and 150 kilometers. A few auroras may extend up to roughly 500 kilometers. Average altitude is between 100 and 120 kilometers.

Altitude affects auroral colour.The strong, green light originates at altitudes of 120 to 180 kilometers. Red northern lights occur at even higher altitudes, while blue and violet occur mostly below 120 kilometers. When the sun is "stormy", red colour occur at altitudes between 90 to 100 kilometers. Entirely red northern lights are sometimes seen, particularly at lower latitudes, and are often mistaken for a fire on the horizon. 

The main point in the auroral theory is that electrically charged particles excite the atmospheric gases, that is, the electrons begin to circle the nucleus in a different orbit because of an excess of energy. The excited particle is unstable and will give up its excess energy by emitting light. This is what is called aurora.

Sound waves are pressure waves which travel about 340 meters per second in air at ground level. At altitudes between 80 and 500 kilometers, where the aurora occurs, we have a near-vacuum, so it is not possible for sound waves to propagate. But since there are a lot of reports of crackling sounds during an aurora display, there may be some sort of physics behind it.

Some can last for hours, days, weeks or months depending on the intensity and size of the solar flare.

http://www.northern-lights.no/english/

http://www.wikipedia.org/

http://www.northern-lights.no/english/what/altitudes.shtml