The structure of X‐ray emissions from triggered lightning leaders measured by a pinhole‐type X‐ray camera

Abstract

Key Points:• Investigation into the structure of X-ray emissions was completed • Maximum X-ray source region radii were between 2 and 3 m • Leaders displayed consistency with both compact and diffuse electron sources Of those five leaders, one dart-stepped leader and two chaotic dart leaders are the focus of this paper. These three leaders displayed unique X-ray emission patterns: a chaotic dart leader displayed a diffuse structure (i.e., a wide lateral "spraying" distribution of X-rays), and a dart-stepped leader and a chaotic dart leader exhibited compact emission (i.e., a narrow lateral distribution of strong X-ray emission). These two distinct X-ray emission patterns (compact and diffuse) illustrate the variability of lightning leaders. Using Monte Carlo simulations, we show that the diffuse X-ray source must originate from a diffuse source of energetic electrons or possibly emission from several sources. The compact X-ray sources originate from compact electron sources, and the X-ray source region radius and electric charge contained within the X-ray source region were between 2 and 3 m and on the order of 10-4 C, respectively. For the leaders under investigation, the X-ray source region average currents were determined to be on the order of 102 A.

Modeling of X‐ray Images and Energy Spectra Produced by Stepping Lightning Leaders

“…This quantity is presented separately in Figure b. We emphasize that this trend is in line with XCAM observations during triggered lightning discharges (e.g., Dwyer et al, , Figure 8; Schaal et al, , Figure 13). Second, photons originating from lightning leaders at high altitudes are captured as a bright spot in X‐ray images.…”

Observations of corona in triggered dart‐stepped leaders

“…[] find that for a leader tip potential of 5 and 10 MV, X‐ray bursts, as observed from the ground, are associated with streamer zone radii of 2 to 4 m, respectively, similar to the experimental results reported by Schaal et al . []. Images of corona streamers made in the laboratory [e.g., Bazelyan and Raizer , , p. 150] are likely more sensitive than ours taken 200–300 m from the lightning channel base, so laboratory corona may well appear longer than our lightning corona, for a given voltage.…”

“…Our measurements of optical streamer zone length raise the following questions: (1) Is it reasonable for the X‐ray source region be 2 to 4 m long, as claimed by Schaal et al . [] and Xu et al . [], in view of our observed 9 m optical length and the observation by Hill [] that the X‐ray sources are typically 20 m below the causative dE/dt sources?.…”

“…Schaal et al . [], using an X‐ray pin‐hole‐type camera to image triggered lightning leaders near ground, suggested that the X‐ray source region for subsequent leaders that occurred in a preexisting channel, inferred to be below the leader tip, had a radius of 2 to 3 m. Xu et al . [] found that X‐ray bursts produced by 5 MV to 10 MV leaders would theoretically produce X‐ray burst spectra consistent with the experimental results reported by Schaal et al .…”

“…[] found that X‐ray bursts produced by 5 MV to 10 MV leaders would theoretically produce X‐ray burst spectra consistent with the experimental results reported by Schaal et al . []. These potential drops in the leader tip region involve streamer zone radii of 2 to 4 m.…”