(See description below.)
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Auroras occur because Earth's magnetic field interacts with the solar wind, a tenuous mix of charged particles blowing away from the sun. This wind from the sun sweeps by Earth in the interplanetary magnetic field which is produced by the sun. We are protected from the solar wind's direct effects by Earth's comet-shaped magnetosphere, where the Earth's magnetic field is distorted by the interplanetary magnetic field and the solar wind. The electrical energy generated by the charged particles blowing across the Earth's magnetic field send charged particles down into the Earth's upper atmosphere.
Auroral light is similar to light from color television. In the picture tube, a beam of electrons controlled by electric and magnetic fields strikes the screen, making it glow in different colors, according to the type of chemicals (phosphors) that coat the screen. Auroral light is the from the air glowing as charged particles, particularly electrons, rain down along the Earth's magnetic field lines. The color of the aurora depends on the type of atom or molecule struck by the charged particles.
Each atmospheric gas glows with a particular color, depending on its electrical state (ionized or neutral) and on the energy of the particle that hits the atmospheric gas. High-altitude oxygen, about 200 miles up, is the source of the rare, all-red auroras. Oxygen at lower altitudes, about 60 miles up, produces a brilliant yellow-green, the brightest and most common auroral color. Ionized nitrogen molecules produce blue light; neutral nitrogen glows red. The nitrogens create the purplish-red lower borders and ripple edges of the aurora.