- Runaway breakdown
Runaway breakdown is a theory of
lightning initiation proposed byAlex Gurevich in1992 .Electron s in air have amean free path of ~1cm. Fast electrons which move at a large fraction of thespeed of light have a mean free path up to 100 times longer. Given the long free paths, anelectric field can accelerate these electrons to energies far higher than that of initially static electrons. If they strike air molecules, more relativistic electrons will be released, creating an avalanche multiplication of "runaway" electrons.Conventional
electron avalanche s leading to theelectrical breakdown of air at 1-atm pressure will take place atfield strength s above 30 kV/cm. Since high-energy electrons have much longer free trajectories, far weaker fields can triggeravalanche breakdown , but only if a source of high-energy electrons is present to start the "runaway" process. This phenomenon was not commonly seen in the laboratory since the breakdown regions can occur only on a scale of fifty meters or more.In the upper atmosphere,
cosmic ray s striking air molecules withinthunderstorm s can supply the relativistic electrons which trigger a breakdown in "runaway" mode. The breakdown region is a conductive plasma many tens of meters long, and it can supply the "seed" which triggers a lightning flash.External links
* [http://www.newscientist.com/channel/fundamentals/mg18624981.200 How cosmic rays trigger lightning strikes]
* [http://www.aip.org/pt/vol-58/iss-5/p37.shtml Runaway Breakdown and the Mysteries of Lightning - Physics Today May 2005]
* [http://www.pbs.org/wgbh/nova/sciencenow/3214/02.html Nova Science Now segment on Lightning - Aired on PBS October 18, 2005]
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