Christopher J. Biagi

Co‐location of lightning leader x‐ray and electric field change sources

[2] Although X-ray emission from lightning was long predicted [Wilson, 1925], only recently was the production of X rays in cloud-to-ground lightning confirmed. Moore et al. [2001] first reported the detection of energetic radiation emissions immediately preceding the return stroke of natural cloud-to-ground negative lightning, followed by a similar discovery by Dwyer et al. [2003] for rockettriggered lightning. Dwyer et al. [2004] reported that these emissions were composed of multiple, brief bursts of X rays in the 30–250 keV range, with each burst typically lasting less than 1 ms. Further, they showed that the sources of the X-ray bursts traveled from the cloud toward the ground, supporting the view that the leader front is the source of the X rays. Dwyer et al. [2005] compared X-ray and electric field records simultaneously obtained during the stepped leaders of natural negative cloud-to-ground lightning. The conclusion from this analysis was that the production of X-rays is associated with the electric field changes accompanying the stepping of the leader that initiates the first return stroke. Although an obvious temporal correspondence was observed, uncertainties in measurement time delays and oscilloscope trigger times prevented any accurate determination of the exact temporal relationship between the X-ray bursts and the stepping of the leader. Observations of the similarity in X-ray emissions from natural and triggered lightning imply a common mechanism for different types of negative leaders [Dwyer et al., 2005]. The aforementioned discoveries have had an impact on views of lightning electrical breakdown in air, in that lightning can no longer necessarily be considered a conventional low-energy (eV) discharge, but often involves an electron distribution function that includes a significant high-energy (keV to MeV) component. These recent advancements highlight many unknowns regarding leader propagation, the stepping process, and their association with X rays. Among the most pressing of these issues are the intensity of the X rays at the source, the electric field at the leader front, the directionality and attenuation of the X-ray emissions, and the spatial and temporal relationship between the sources of X rays and leader steps. This paper addresses the issue of independently locating the sources of X-ray emissions and the corresponding leader step electric field changes via time-of-arrival (TOA) measurements, which may allow advancement on many of these issues. Leadersinbothnaturalandtriggeredlightningareconsidered.

Performance characteristics of the NLDN for return strokes and pulses superimposed on steady currents, based on rocket-triggered lightning data acquired in Florida in 2004–2012

We present a detailed evaluation of performance characteristics of the U.S. National Lightning Detection Network (NLDN) using, as ground truth, Florida rocket-triggered lightning data acquired in 2004–2012. The overall data set includes 78 flashes containing both the initial stage and leader/return-stroke sequences and 2 flashes composed of the initial stage only. In these 80 flashes, there are a total of 326 return strokes (directly measured channel-base currents are available for 290 of them) and 173 kiloampere-scale (≥1 kA) superimposed pulses, including 58 initial continuous current pulses and 115 M components. All these events transported negative charge to the ground. The NLDN detected 245 return strokes and 9 superimposed pulses. The resultant NLDN flash detection efficiency is 94%, return-stroke detection efficiency is 75%, and detection efficiency for superimposed pulses is 5% for peak currents ≥1 kA and 32% for peak currents ≥5 kA. For return strokes, the median location error is 334 m and the median value of absolute peak current estimation error is 14%. The percentage of misclassified events is 4%, all of them being return strokes. The median value of absolute event-time mismatch (the difference in times at which the event is reported to occur by the NLDN and recorded at the lightning triggering facility) for return strokes is 2.8 µs. For two out of the nine superimposed pulses detected by the NLDN, we found optical evidence of a reilluminated branch (recoil leader) coming in contact with the existing grounded channel at an altitude of a few hundred meters above ground.

Negative leader step mechanisms observed in altitude triggered lightning

We present 63 high-speed video frames (108 kilo-frames per second (kfps), 9.26 µs per frame) showing the development of the downward negative stepped leader in the initial stage of an altitude-triggered flash. The downward negative stepped leader initiated from the bottom of the triggering wire at a height of about 128 m above ground and, 553 µs later, it struck a lightning rod located at a distance of about 50 m from the launch tower. During the leaders development, electric field derivative pulses were detected associated with leader stepping. The interpulse intervals ranged from 3 to 27 µs with a mean value of 13 µs. Distinct segments of luminosity were observed ahead of the main leader channel that appear similar to space leaders were observed in the high-speed video frames. A total of eight luminous segments were observed that were 1 m to 6 m in length and were centered at distances from the main leader channel ranging from 3 m to 8 m. The new leader steps that appeared in the frames following the luminous segments were 5 m to 8 m in length. Two of the observed segments apparently never connected to the leader channel and thus failed to produce a new leader step.

NLDN responses to rocket-triggered lightning at Camp Blanding, Florida, in 2004–2009

We evaluated performance characteristics of the U.S. National Lightning Detection Network (NLDN) using rocket-triggered lightning data acquired in 2004–2009 at Camp Blanding, Florida. A total of 37 negative flashes, that contained leader/return stroke sequences (a total of 139) were triggered during these years. For all the return strokes, locations of channel terminations on ground were known exactly, and for 122 of them currents were measured directly using non-inductive shunts. The NLDN recorded 105 Camp Blanding strokes in 34 flashes. The resultant flash and stroke detection efficiencies were 92% and 76%, respectively. The median absolute location error was 308 m. The median NLDN-estimated peak current error was -6.1%, while the median of absolute value of current estimation error was 13%. The results are applicable to negative subsequent strokes in natural lightning.

Characteristics of the initial rising portion of near and far lightning return stroke electric field waveforms

We examine the shapes and relative magnitude of slow fronts and fast transitions in electric field waveforms of first return strokes in negative cloud-to-ground lightning recorded simultaneously at near and far distances from the lightning channel. The near and far field-measuring stations are located at Camp Blanding and in Gainesville, Florida, respectively, separated by a distance of about 45 km. A total of five return strokes had been recorded in 2007–2008, four of which were analyzed in detail (one was not suitable for analysis due to saturation of electric field waveform at the far station). Field waveform characteristics, including overall zero-to-peak and 10-to-90% risetimes, duration of slow front, fast transition 10-to-90% risetime, and magnitude of slow front relative to the peak, were found to be similar to those reported from other studies, in which the field propagation path was over ground (as opposed to sea water). It is shown, via modeling, that the slow front in electric field waveforms at far distances is primarily due to the radiation field component, while at near distances it is composed of more or less equal contributions from all three components of electric field. For both measured and model-predicted waveforms, the durations of the slow front appear to be similar at near and far distances from the lightning channel.

Fine structure of electric field waveforms recorded at near and far distances from the lightning channel

We examine the shapes and relative magnitude of slow fronts and fast transitions in electric field waveforms of first return strokes in negative cloud-to-ground lightning recorded simultaneously at near and far distances from the lightning channel. The near and far field-measuring stations are located at Camp Blanding and in Gainesville, Florida, respectively, separated by a distance of about 45 km. A total of five return strokes had been recorded in 2007–2008, four of which were analyzed in detail (one was not suitable for analysis due to saturation of electric field waveform at the far station). The AM and GM zero-to-peak risetimes for four strokes were 7.2 µs and 6.6 µs, respectively, at the near station, and 7.0 µs and 6.5 µs, respectively, at the far station. The AM and GM 10-to-90% risetimes were 4.9 µs and 4.6 µs, respectively, at the near station, and 4.0 µs and 3.6 µs, respectively, at the far station. Three of the four first strokes exhibited the two distinct phases, the slow front and fast transition, in the initial rising portion of their electric field waveforms. For two return strokes the amplitude of the slow front was 49% of the peak at the near station and 44% of the peak at the far station, while for one it was 43% and 18% of the peak at the near and far stations, respectively. The AM and GM 10-to-90% risetimes of the fast transition for the three strokes were both 0.6 µs at the near station versus 0.9 µs at the far station. It is shown, via modeling, that the slow front in electric field waveforms at far distances is primarily due to the radiation field component, while at near distances it is composed of more or less equal contributions from all three components of electric field. For both measured and model-predicted waveforms, the durations of the slow front appear to be similar at near and far distances from the lightning channel.