When the excited electrons finally relinquish that extra energy upon returning to their initial, lower energy state, they release photons of visible light. The particular colours we see in an auroral display depends on the specific atmospheric gas struck, its electrical state at the time of collision, and the energy of the extraterrestrial particle involved in the collision. Each atmospheric gas glows with specific colors. Nitrogen molecules in an ionized state will produce blue light while electrically neutral nitrogen molecules create purplish-red auroral colours. The mix of these emissions from different atoms and at different altitudes produce the great variability of auroral colours.
Auroras come in many shapes and sizes as well as a variety of colours.
A display may change colours or pulse in intensity as the ever-changing geomagnetic field is disturbed by the solar wind and additional solar particles stream into the atmosphere. Aurora may appear to hang static in the sky, or they may dance and flutter, or even appear to explode upward along magnetic field lines. Auroral arcs or curtains typically cover large areas of sky. They are usually an evenly curved arc of light with a smooth lower boundary, which may extend from horizon to horizon. Arcs may dance and turn, rippling like a curtain responding to a light breeze.