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relativistic runaway electron avalanche
A relativistic runaway electron avalanche (RREA) is an avalanche growth of a population of relativistic electrons driven through a material (typically air) by an electric field. RREA has been hypothesized to be related to lightning initiation,〔 terrestrial gamma-ray flashes,〔 sprite lightning,〔 and to spark development.〔 RREA is unique as it can occur at electric fields an order of magnitude lower than the dielectric strength of the material.
==Mechanism==
When an electric field is applied to a material, free electrons will drift slowly through the material as described by the electron mobility. For low-energy electrons, faster drift implies higher friction so the drift speed tends to stabilize. For electrons with energy above about 100 eV, however, higher speeds imply lower friction. An electron with a sufficiently high energy therefore may be accelerated by an electric field to even higher and higher energies, encountering less and less friction as it accelerates. Such an electron is described as a "runaway."
As runaway electrons gain energy from an electric field, they occasionally collide with atoms in the material, knocking off secondary electrons. If the secondary electrons also have high enough energy to run away, they too accelerate to high energies, produce further secondary electrons, etc. As such, the total number of energetic electrons grows exponentially in an avalanche.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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