The aurora in this photo has two colors of light, green and purple. Oxygen atoms give off the green light. The purple light comes from nitrogen molecules.
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Image courtesy of Jan Curtis.
Colors of the Aurora
The aurora (Northern Lights and Southern Lights) are caused by collisions between energetic particles in Earth's magnetosphere and atoms or molecules in the upper atmosphere. Particles, mostly electrons and protons, are accelerated to high speeds and energies as they spiral along Earth's magnetic field lines. The particles bounce back and forth between Earth's magnetic poles, where magnetic field lines dip into the planet's upper atmosphere. Some of the energetic particles collide with atoms and molecules of oxygen and nitrogen in the upper atmosphere near the poles.
These collisions add energy to the atoms, which in turn shed the excess energy by emitting light. The brilliant displays of auroral lights have characteristic colors which depend on the type of atoms or molecules which are giving off the light. Oxygen atoms emit green light that is seen in many auroral displays. Oxygen atoms above 150 km (93 miles) in altitude can also give off red light. Energized nitrogen molecules emit red, blue, and violet light.
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