Like celestial halos, elves are circles of light that appear some 50 miles or
more above thunderstorms. Triggered by lightning flashes far below, these
ephemeral discs spread out radially across the bottom of the ionosphere in the
briefest instant, expanding up to hundreds of miles in diameter in less than a
millisecond. Experts believe elves are caused by lightning processes that
accelerate electrons in the lower ionosphere.