Thick sparks flash faster through the air than thin ones - at least, when positive discharges are concerned.
Furthermore, positive sparks are usually faster than negative ones, even though researchers expected exactly the opposite. These effects were first calculated by Alejandro Luque, Valeria Ratushnaya and Ute Ebert of the Centrum Wiskunde & Informatica (CWI) in Amsterdam. The researchers published their findings in the special issue on Streamers, Sprites and Lightning of the Journal of Physics D: Applied Physics that appeared on 7 December 2008. The simulations confirm results of recent experiments at Eindhoven University of Technology.
In nature lightning strokes can be both positive and negative. About 90% of the lightning strokes in nature carry negative charge from cloud to earth. Although lightning is a common phenomenon, many puzzles remain. For instance, in the laboratory it is much harder to generate negative discharges than positive ones,.
‘Lightning Professor' Ute Ebert: "It is fascinating to investigate how these complex processes work, from mile-long lightning bolts to small sparks appearing in energy efficient discharge lamps. At CWI we now have a theoretical model allowing us to calculate properties of positive and negative discharges, like velocity or thickness. Before a lightning stroke flashes, its path is paved by many streamer channels. The velocity of positive streamers seems to be proportional the square of their thickness. Positive streamers have sharper ti[s enabling them to penetrate the air better than negative ones."
The researchers are now examining whether this effect might explain why only the relatively rare positive lightning causes so called sprite discharges: huge lightning discharges high above the clouds.
The online version of the special issue can be found on: http://www.iop.org/EJ/toc/0022-3727/41/23. Editors are Ute Ebert and Dave Sentman (Fairbanks, Alaska). The experiments on positive and negative discharges are described there as well. Picture: TU/e.
