Aurora Borealis - Solar Wind and Magnetism

Aurora Borealis:

(aka. Northern / Southern Lights) are the effect of passing solar wind as they impacts our upper atmosphere.

The process which forms the various colors we see at night are generated by the 'Auroral Mechanism'. Electrons and protons from the solar wind pass through our magnetosphere (50 miles above Earth) where they are trapped due do it's magnetic field. Once they are held in Earth orbit, pre-existing particles collide with the new particles electronically exciting them to make quantum leaps - producing visible light. This reaction can only be stoped through the production of light or further collisions.

Color Spectrum:
Since the electrons and protons are impacting preexisting Earth particles, the colors are defined by the type of atoms they come in contact with. Most aurora's are green and red as a result of oxygen atoms. Nitrogen molecules can produce lower pink's as well as blue/violet colors while ionic nitrogen and helium can cause bright purples. Bright orange flares are sometimes visible during neon collisions. Rare cases of other colors have been seen due to impacted compounds of oxygen/nitrogen/helium although the level of solar wind activity is also a constant variable in the colors and patterns of the aurora.

Theory:
It is important to note that while there are differing theories for the reasons for the aurora's, the three main theories are all based on solar wind and magnetism. Although the 'real' cause may not be clear, the fact that they exist in visible sprectrums references some activity that formed them. Therefore, whatever the cause, the aurora are an excellent example of color produced by effect of an outside source.