Crown flash is a rare but observed phenomenon involving "The brightening of a thunderhead crown followed by the appearance of aurora-like streamers emanating into the clear atmosphere". The current hypothesis is that sunlight is reflecting off or refracting through tiny ice crystals above the crown of a cumulonimbus cloud. These ice crystals are aligned by the strong electro-magnetic effects around the cloud, so the effect may appear as a tall streamer or pillar of light. When the electro-magnetic field is disturbed by lightning flashes within the cloud, the ice crystals are re-orientated causing the light pattern to shift very rapidly and appear to 'dance' in a strikingly mechanical fashion. The effect may also sometimes known as a "leaping sundog". As with sundogs, the observer would have to be in a specific position to see the effect, which is not a self-generated light such as seen in a lightning strike, but rather a changing reflection/refraction of the sunlight.
Mentioned in Nature in 1971 and in a letter to Nature slightly earlier in the same year, this phenomenon is regarded as rare and not well documented. Recently several YouTube videos have emerged that appear to document this phenomenon.