Lights in the Sky
One day you are out playing in the snowfields and suddenly this great and wondrous light fills the sky around you, instilling fear in all of those around you and causing your parents to fear for your life. Your mother quickly runs to get you and takes you inside so that the menacing force of the aurora borealis does not sweep down and cut off your head. You of course don't know any better, being only a child, and you consider it to be a strange and beautiful, if somewhat frightening experience.
Since man has lived in the northern hemisphere he has known of the aurora. Many cultures have developed legends and myths such as the one above due to its incredible presence and its constant effect on the lives of tribes of the far north. The vikings thought of them as the reflections of dead maidens and the eskimos of northern Canada and Greenland thought of the aurora as the realm of the dead and that when the lights changed the dead were trying to communicate with the living. There are many other variations of aurora myths such as animal reflections and giant fires however it would seem that the mst prevalent is the association of the aurora with spirits of the dead and with omens, usually bad.
All of this just goes to show how varying an effect the northern lights have had on the many peoples who witness it. Yet it was only in the last few centuries that people really started not only asking "why" but acting upon that question.
The first things that one needs to understand when considering the "why" of the aurora are the earth's atmosphere/magnetic, and the solar wind, and the interaction between the two.
One must first know that the atmosphere of the Earth is made up of many gasses but mostly oxygen and nitrogen and that when the gases are excited/energized they give off energy in the form of visible light, much like a neon tube or a fluorescent light bulb. Secondly one must know that the Earth has a magnetic field. The last thing that one need know about the Earths part in the production of an aurora is the presence of what is commonly referred to as plasma, a vast field of charged particles that circles the Earth. Thus these charged particles can move along the lines of the magnetic field like electricity through copper/silver wiring. It is these charged particles that create the aurora by bombarding the atmospheric gases and causing them to change energy.
Now one must know that the aurora is not always present therefore, something must be causing these charged particles to move along the magnetic lines toward the poles. This force is the solar wind. The solar wind is "a continuous outflow of solar magnetic field and subatomic particles from the solar atmosphere (corona) into the solar system" that is caused by the boiling off of the suns hydrogen which fires the resulting particles into space at extremely high speeds(millions of miles per hour). This firing of particles affects the sun's magnetic field and causes it to fluctuate and is in a sense, dragged along behind the charged particles. Thus the sun is constantly sending high speed electrons at the earth as well as fluctuating fingers of its own magnetic field. This solar wind hits the magnetic field and causes it to compact on the sun side and stretch out on the far side, much like the tail of a comet. This alteration of the magnetic field's shape as well as the extra energy provided by the high speed particles causes the particles of the earth's plasma to move along the magnetic lines of the Earth and come together in a very close, high energy space, causing the gases in that area to become excited and thus create what we know as an aurora.
left- diagram of Earth's magnetic field lines
right- diagram of magnetic field distortion due to solar wind
Now that we know that the aurora is caused by excitation of gases in the upper atmosphere we might wonder why it isn't all one color but different colors under different circumstances. This is due to the fact that different gasses, due to their differing electron structures, respond in different ways to the same amount of energy.
Although the atmosphere is made up of many different gases the two most prevalent are nitrogen and oxygen. Therefore these are the two that have the biggest effect on the color of the aurora. The other gases are present in such small amounts that they have such a small effect on the color that is usually not worth mentioning.
The average solar wind commonly forces the magnetic field within 110 and 200 kilometers above the surface of the earth and at this point one usually sees the colors green, blue/violet or purple, and a pale greenish yellow color with a slight orange or red in the very highest reaches of the bands of color. However there are times of unusually high solar wind activity, specifically due to sunspots or particularly intense explosions on the surface of the sun. During these times the solar wind can be powerful enough to push the magnetic field much closer to the Earths surface and during these times of increased intensity an uncommonly brilliant red can encompass the entirety of the aurora. One other unusual color in the aurora would be a blue that can sometimes be present at the highest reaches of the curtains of light existing with the red that is sometimes there. This is caused by the light from the sun hitting these colors and due to its added spectrum causes a light blue to appear where normally it would be red or green.
left- picture of aurora as seen from outer space
right-Aurora photographed in Ontario during a solar-geomagnetic storm.
The specific sources of the colors are as follows: orange and green and the pale yellow, as well as the high atmosphere red are caused mainly by the excited oxygen atoms, while the varying degrees of blue and purple are caused by nitrogen, as well as the low atmosphere red.
left-spectrum of colors emitted by atoms in Earth's outer atmosphere
right- altitude diagram of auroral color and its source gases
In conclusion, I hope to have successfully conveyed not only why the aurora exists but also made it possible for anyone reading this to relate to the aurora, if not through themselves and their own culture then through knowledge of the ways and customs of others. I find the aurora to be a wonderful example of the unique effects of the sciences and colors of our world, as well as one of the more beautiful ones. I only hope that everyone, including myself, can one day experience the awe inspiring majesty of the aurora, be it via the aurora borealis, the northern lights, or the aurora australisis, the southern lights.
Aurora explained [videorecording] / produced for the Aurora Color Television Project of the Geophysical Institute by the Northern Teleproduction and Communications Research Unit, KUAC-TV and the Dept. of Journalism & Broadcasting, University of Alaska, Fairbanks ; Neal B. Brown, Thomas J. Hallinan, Daniel L. Osborne.