A sprite is an electrical discharge similar to lightning, but it occurs in the upper atmosphere (50-90 kilometers – 31-56 miles – in altitude), above large thunderstorms. Sprites were first photographed in 1989.
These large flashes of light, which are triggered in almost all cases by positive lightning discharge between the thundercloud and ground, can span tens of kilometers of altitude.
High-speed videos in recent years show that they always start in the ionosphere (sometimes out of a wide saucer-shaped halo) and first shoot downward in the form of long ionized filaments called streamers; sometimes they shoot up again later.
The downward streamers are bright at their growing tips, and the channel is first dark and becomes bright again after some distance. While observations continue, many features of sprites remain unexplained.
To shed light on some of the characteristics of sprites, Luque and Ebert develop simulations that, unlike previous models, included variations in air density along the length of sprite streamers.
Their results help to explain some previously unexplained observations associated with sprites, such as light emission from the streamer trail, the emergence of negatively charged upward propagating streamers, the increase in light emissions from the streamer head with air density, and changes in streamer speed.
Title: Sprites in varying air density: Charge conservation, glowing negative trails and changing velocity, A. Luque: CWI, Amsterdam, Netherlands and Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain; U. Ebert: CWI, Amsterdam, Netherlands and Department of Physics, Eindhoven University of Technology, Eindhoven, Netherlands. Geophysical Research Letters (GRL) paper DOI:10.1029/2009GL041982, 2010