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avalanche will wander back to where the avalanche began and can produce new seeds for a second generation of avalanches. If the electric field region is large enough, the number of second-generation avalanches will exceed the number of first-generation avalanches and the number of avalanches itself grows exponentially. This avalanche of avalanches can produce extremely large populations of energetic electrons. This process eventually leads to the decay of the electric field below the level at which feedback is possible and therefore acts as a limit to the large-scale electric field strength.
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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
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RREA avalanches generally move opposite the direction of the electric field. As such, after the avalanches leave the electric field region, frictional forces dominate, the electrons lose energy, and the process stops. There is the possibility, however, that photons or positrons produced by the
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Dwyer, J. R., Smith, D. M., Uman, M. A., Saleh, Z., Grefenstette, B. W, Hazelton, B. J, et al. (2010). Estimation of the fluence of high-energy electron bursts produced by thunderclouds and the resulting radiation doses received in aircraft. Journal of
Geophysical Research, 115(D9), D09206.
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The dynamic friction function, shown in the Figure, takes into account only energy losses due to inelastic collisions and has a minimum of ~216 keV/cm at electron energy of ~1.23 MeV. More useful thresholds, however, must include also the effects due to electron momentum loss due to elastic
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collisions. In that case, an analytical estimate gives the runaway threshold of ~282 keV/cm, which occurs at the electron energy of ~7 MeV. This result approximately agrees with numbers obtained from Monte Carlo simulations, of ~284 keV/cm and 10 MeV, respectively.
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Lehtinen, N. G., Bell, T. F., & Inan, U. S. (1999). Monte Carlo simulation of runaway MeV electron breakdown with application to red sprites and terrestrial gamma ray flashes. Journal of
Geophysical Research, 104(A11), 24699-24712.
119:. In very strong electric fields, stronger than the maximum frictional force experienced by electrons, even low-energy ("cold" or "thermal") electrons can accelerate to relativistic energies, a process dubbed "thermal runaway."
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Colman, J. J., Roussel-Dupré, R. a, & Triplett, L. (2010). Temporally self-similar electron distribution functions in atmospheric breakdown: The thermal runaway regime. Journal of
Geophysical Research, 115, 1-17.
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of the electron rises. These higher-energy electrons thus see less frictional force as their velocity increases. In the presence of the same electric field, these electrons will continue accelerating, "running away".
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The RREA mechanism above only describes the growth of the avalanche. An initial energetic electron is needed to start the process. In ambient air, such energetic electrons typically come from
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Carlson, B. E., Lehtinen, N. G., & Inan, U. S. (2008). Runaway relativistic electron avalanche seeding in the Earth's atmosphere. Journal of
Geophysical Research, 113(A10), A10307.
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Babich, L. P.; Donskoy, E. N.; Il'Kaev, R. I.; Kutsyk, I. M.; Roussel-Dupre, R. A. (2004). "Fundamental parameters of a relativistic runaway electron avalanche in air".
83:. For low-energy electrons, faster drift velocities result in more interactions with surrounding particles. These interactions create a form of
144:. Large RREA events in thunderstorms may also contribute rare but large radiation doses to commercial airline flights. The American physicist
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Betz, H. D., Schumann, U., & Laroche, P. (Eds.). (2009). Lightning: Principles, Instruments and
Applications. Springer Verlag, ch. 15.
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The large population of energetic electrons produced in RREA will produce a correspondingly large population of energetic photons by
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When an electric field is applied to a material, free electrons will drift slowly through the material as described by the
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Dynamic friction of free electrons in air compared to an applied electric field showing the runaway electron energy range
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Gurevich, A. V., & Zybin, K. P. (2005). Runaway
Breakdown and the Mysteries of Lightning. Physics Today, 58(5), 37.
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Dwyer, J. R. (2003). A fundamental limit on electric fields in air. Geophysical
Research Letters, 30(20), 2055.
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A comparison between Monte Carlo simulations of runaway breakdown and terrestrial gamma-ray flash observations
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driven through a material (typically air) by an electric field. RREA has been hypothesized to be related to
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that slow the electrons down. Thus, for low-energy cases, the electron velocities tend to stabilize.
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development. RREA is unique as it can occur at electric fields an order of magnitude lower than the
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RREA simulation showing electrons (black), photons (blue), and positrons (red)
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140:. These photons are proposed as the source of
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285:10.1029/2003GL017781
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311:2004PlPhR..30..616B
61:dielectric strength
36:of a population of
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47:initiation,
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437:Categories
390:Current TV
240:Atmosphere
156:References
443:Lightning
67:Mechanism
45:lightning
41:electrons
458:Electron
421:April 9,
395:April 9,
123:Feedback
85:friction
32:) is an
307:Bibcode
111:Seeding
55:, and
57:spark
423:2012
397:2012
30:RREA
369:doi
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258:hdl
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