On the mechanism of X-ray production by dart leaders of lightning flashes (original) (raw)
2010, Journal of Atmospheric and Solar-Terrestrial Physics
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High-speed X-ray images of triggered lightning dart leaders
Journal of Geophysical Research, 2011
We present the first high-time resolution two-dimensional images of X-ray emissions from lightning. The images were recorded at a rate of 10 million per second using a new pinhole-type camera, located 44 m from rocket-and-wire-triggered lightning. We report observations of two dart leaders, one in each of two lightning flashes triggered during the summer of 2010 in north-central Florida. In both events, as the dart leader approached the ground, the X-ray source was also seen to descend along the previous lightning channel. For the second event, the X-ray source exhibited a downward speed of 4.5 × 10 7 m/s, in agreement with independent dE/dt time-of-arrival (TOA) measurements of the speed of the dart leader front, demonstrating that the dart leader front was the source of the X-ray emission. The camera also recorded bursts of MeV gamma rays originating from the dart leader and/or the ground attachment process of the leader. Overall, these results provide new insight into the production of energetic radiation and the propagation and attachment of lightning, all of which remain poorly understood.
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On the electric field at the tip of dart leaders in lightning flashes
Journal of Atmospheric and Solar-terrestrial Physics - J ATMOS SOL-TERR PHYS, 2009
The results obtained in this study show that as the dart leader tip passes a given point on the defunct return stroke channel the electric field increases within a fraction of a microsecond to values larger than the critical electric field necessary for the initiation of cold electron runaway in low-density air comprising the channel. These results are in support of the hypothesis that cold runaway electron breakdown may play a role in the emission of X-ray bursts by dart leaders. The calculations also show that the peak power dissipated by a typical dart leader is about 300–500MW/m and the energy dissipated within the first 10μs or so is about 500–600J/m. Furthermore, the minimum resistance and the maximum radius of the core of a typical dart leader are estimated to be about 3Ω/m and 0.003m, respectively.
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