The electric currents that enter the atmosphere several hundred kilometers above the ground are invisible, until they slam into atoms of the atmosphere. The chief constituents of the atmosphere are nitrogen (78 percent) and Oxygen ( 22 percent). The green light comes from the oxygen atoms being stimulated to emit light as the electrons collide with them and cause the oxygenic electrons to take quantum jumps. When the atoms de-excite, they emit light at a single frequency which we see as green. At 400 - 1000 kilometers above the ground, auroras first become visible but by about 100 kilometers, the atmosphere is too dense for the atoms to give of enough light between collisions among themselves. At lower altitudes, nitrogen emits with a faint blue color and a pinkish color. Because the nitrogen atoms emit light faster, the lower parts of the auroral draperies seem to move faster than the upper, slower parts controlled by the more sluggish emission of the oxygen atoms.