LIGHTNING (below, left) usually carries negative charge from the base of a cloud down to the earth. Sometimes powerful strokes (center) cause the positive charge that had built up near the top of the cloud to disappear abruptly. The large electrical field (gradation in color) created between the cloud top and the ionosphere pulls electrons upward, where they collide with gas molecules. If the electrical field is sufficiently strong and the air sufficiently thin, the electrons will accelerate unimpeded and reach the velocity needed to transfer their kinetic energy to the electronic structure of the molecules with which they collide, raising such molecules to an "excited state." The excited molecules give away their newly acquired energy by the emission of light, causing sprites (below, right). They typically span from 50 to 90 kilometers altitude. 
ionosphere cloud- sprites

Images: Bryan Christie