ELECTROMAGNETIC PULSES given off by strong lightning discharges create elves. Such a pulse, which is in essence an intense burst of radio static, propagates at the speed of light in all directions away from a lightning bolt. When the upward-going part of the pulse (spherical shells) reaches a critical height in the atmosphere (about 75 to 100 kilometers), the electrical field it carries accelerates electrons with great efficiency. These electrons strike air molecules, knocking them into an excited state that allows the release of light. This mechanism generates expanding rings of light along the intersection of the spherical pulse with the critical layer. This intersection widens so quickly (in fact, faster than the speed of light) that these expanding rings appear as flattened disks. 

Image: Bryan Christie

Image: Stephen B. Mende