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On the initiation of lightning in thunderclouds

Profile image of Suren SoghomonyanSuren Soghomonyan

2017, Scientific reports

https://doi.org/10.1038/S41598-017-01288-0
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Abstract

The relationship of lightning and elementary particle fluxes in the thunderclouds is not fully understood to date. Using the particle beams (the so-called Thunderstorm Ground Enhancements - TGEs) as a probe we investigate the characteristics of the interrelated atmospheric processes. The well-known effect of the TGE dynamics is the abrupt termination of the particle flux by the lightning flash. With new precise electronics, we can see that particle flux decline occurred simultaneously with the rearranging of the charge centers in the cloud. The analysis of the TGE energy spectra before and after the lightning demonstrates that the high-energy part of the TGE energy spectra disappeared just after lightning. The decline of particle flux coincides on millisecond time scale with first atmospheric discharges and we can conclude that Relativistic Runaway Electron Avalanches (RREA) in the thundercloud assist initiation of the negative cloud to ground lightning. Thus, RREA can provide enoug...

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References (28)

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On the initiation of lightning in thunderclouds OPEN
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The relationship of lightning and elementary particle fluxes in the thunderclouds is not fully understood to date. Using the particle beams (the so-called Thunderstorm Ground Enhancements – TGEs) as a probe we investigate the characteristics of the interrelated atmospheric processes. The well-known effect of the TGE dynamics is the abrupt termination of the particle flux by the lightning flash. With new precise electronics, we can see that particle flux decline occurred simultaneously with the rearranging of the charge centers in the cloud. The analysis of the TGE energy spectra before and after the lightning demonstrates that the high-energy part of the TGE energy spectra disappeared just after lightning. The decline of particle flux coincides on millisecond time scale with first atmospheric discharges and we can conclude that Relativistic Runaway Electron Avalanches (RREA) in the thundercloud assist initiation of the negative cloud to ground lightning. Thus, RREA can provide enough ionization to play a significant role in the unleashing of the lightning flash. Among top unanswered questions in lightning research 1 state as number one: " By what physical mechanism or mechanisms is lightning initiated in the thundercloud? " and-number two: " What physical mechanisms govern the propagation of the different types of lightning leaders? ". They also mentioned that " The problem of how lightning is initiated inside thunderclouds is not only one of the biggest unsolved problems in lightning physics; it is also probably one of the biggest mysteries in the atmospheric sciences ". One of the candidates related to initiation and propagation of lightning is considered to be energetic runaway electrons. Electron acceleration in the thunderstorm atmospheres was first recognized by CTR Wilson 2 ; then Gurevich et al. 3 introduced the electron runaway concept (named Runaway Breakdown-RB, now mostly referred as Relativistic Electron Runaway Avalanche – RREA); in 2003 J. Dwyer 4 developed the feedback model of intracloud electron-gamma ray avalanches exponentially enhancing electron number. Recent observations of hundreds of the Thunderstorm ground enhancements (TGE, an abrupt enhancement of the secondary cosmic rays measured on the Earth's surface in correlation with thunderstorms) on Aragats provide an extensive source for the development of models of particle acceleration and multiplication in thunderclouds 5, 6. The electric field strength and spatial extent required for the RB/RREA development was measured during balloon flights in thunderstorm atmospheres at New Mexico. A 1.87 kV/cm field extended 1 km downwards from the height of 5.77 km would give an RREA multiplication factor of about 650 7. In situ measurements of the RREA by the network of particle detectors on Aragats allow retrieving the RREA parameters and developing a TGF initiation model 8, 9. Estimated multiplication factor was ~330, e-folding length ~250–300 m and maximum energy of RREA electrons in the cloud-40–50 MeV. The strength of the uniform vertically downward field of 1.5 km elon-gation expected to be 1.8–2.0 kV/cm. For the both observed cases, RREA electron flux will significantly increase the electrical conductivity in the cloud and possibly would not only introduce an additional leakage current but also can assist propagation of the lightning leader. In this study, we analyze a special kind of TGEs, i.e. TGEs abruptly terminated by lightning flashes. To our knowledge, first reports on the particle flux abruptly terminated by lightning flash come from measurements made on board of NASA STORM Hazard Project F-106 aircraft. The X-ray flux is sometimes seen to increase prior to observed lightning discharge and then return to background level 10. The balloon flights near Norman, Oklahoma in the spring of 1995 reveals an increase in X-ray intensity of 2 orders of magnitude lasting for approximately 1 min. The X-ray intensity returned to background level at the time of a lightning flash that reduced the electric field strength measured at the balloon 11. The Baksan group reported the first TGEs of this kind 12. They

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Thunderstorm ground enhancements—Model and relation to lightning flashes
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