Elves or 'Emission of Light and Very Low Frequency' occurs at the base of the ionosphere. Their duration is less than 1/1000s. They occur above both positive or negative polarity clouds.

'Summary of Elve origin

Elves expand rapidly to horizontal disk of up to 500km in diamater. Elves most likely result when an EMP (energetic electromagnetic pulse) propagates through the ionosphere. Though they can be accompanied by sprites, there origin is different (EMP)

Elves appear to be similar to halos, but do occur at higher altitudes (above 95 kilometers), at the bottom of the ionosphere.