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.  Auroral light is caused by charged particles, particularly electrons, raining down along the Earth's magnetic field lines and striking air molecules.   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 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.  Nitrogen creates the rare purplish-red lower borders and ripple edges of the aurora.  Auroral displays vary from night to night and during a single night.   Appearing within arcs are upward-reaching striations aligned with the magnetic field, giving the impression of curtains of light. Ripples and curls dance along the arc curtains and pulsating patches of light may appear in the morning hours.