A carrot-sprite observed in 1999
Source: Wyoming InfraRed Observator
Researchers Alejandro Luque and Ute Ebert of the Centrum Wiskunde & Informatica (CWI) in Amsterdam have taken the next step in their study of sprites, which are a type of enormous lightning flashes high above thunderclouds. In new simulations they follow the further evolution of the sprite and provide answers to previously unanswered questions about this phenomenon. Earlier Luque and Ebert had investigated the emergence of sprites. Their results are on the forefront of international research. The researchers published their new findings today in the online version of Geophysical Research Letters.
Observations show two different types of sprites: the thick carrot-sprite (that derives its name from the shape of bunched-up carrots with tops) and the thin c-sprite. They differ in size and intensity. In 2005 high speed movies revealed how these sprites evolve and why they differ so much in shape. Both arise in the conductive nighttime ionosphere at 80-90 km altitude. The c-sprite is a type of column that can shoot thirty kilometers straight downwards. The carrot-sprite shoots not only downwards, but also upwards and outwards producing a shape similar to that of a funnel.
Earlier (Nature Geoscience, Nov. 2009), Luque and Ebert already had shown how both types of sprite emerge. Their computer simulations showed how initially an ionization-screening wave in the ionosphere is triggered by a lightning flash that sometimes is seen as a halo - a diffuse saucer-shaped luminous object and how the sprite shoots out of the halo downwards. In their new simulations Luque and Ebert follow the evolution of the sprite further: how the initial sprite channel, the streamer, extends further downwards, why the brightness of the streamer heads increases exponentially along this path, why the streamer channels behind the glowing channel heads are dark, but become bright again at some distance behind the heads, and from which altitude the streamers in the carrot sprite eventually shoot back upwards. The researchers are the first to simulate emergence and evolution of sprites quantitatively, including the change of air density and electron density with altitude.
The model of Luque and Ebert includes all necessary mechanisms involved in sprite formation and it explains observations. The computer simulations pose an enormous mathematical challenge due to the largely varying length scales that have to be captured. The streamers look like very long fingers growing in a huge outer space, the fingers have a thin electrically charged "skin" and the motion of this thin "skin" must be calculated in a very accurate manner.
The model is also realistic for the ionosphere. The combination of observations and accurate models can give us insight not only in the discharge evolution, but also into the properties of high layers of the atmosphere, and, in particular, of the electron density, and how it is changed by meteorites, gravitational waves, wind shear, cosmic rays, solar radiation and lightning. Earth and Life Sciences is an important theme at CWI. This research is a good example. The research was funded by Technology Foundation STW.
For more high resolution graphics, see:
Temporal evolution of a sprite
(Bron: H.C. Stenbeak-Nielsen en M.G. McHarg, Phys.D:Appl.Phys 41 (2008) 234009)
Simulation of the evolution of a sprite streamer
(Sourc: CWI - Luque and Ebert)