D. M. Jordan

A ground level gamma-ray burst observed in association with rocket-triggered lightning

energetic radiation was observed at much earlier times, up to 160 ms before the return strokes. Because for such times, the dart leader tip must have been about 1000 m above the ground, it cannot be ruled out that for these events a gamma-ray (>1 MeV) component also originated from the cloud. [3] In this paper, we report an unusual event that occurred during the last rocket-triggered flash of the 2003 season. For this flash, an intense burst of MeV gamma-rays was observed from a distance of 650 m from the lightning channel, not in association with the dart leader or return stroke, but in association with a large current pulse (11 kA) occurring during the initial-stage (during the initial continuous current), about 20 ms after the vaporization of the triggering wire. In triggered lightning, the initial-stage is characterized by a steady current, preceding the return strokes, with superimposed pulses up to several kA in amplitude [Wang et al., 1999]. Considering the large distance of the detectors and the high energy of the gamma-rays, it is plausible that the burst originated in the cloud processes, perhaps many thousands of meters above the ground. This result may greatly facilitate the study of runaway breakdown of air inside thunderclouds [Gurevich et al., 1992], since it implies that observations of this phenomenon from the ground at sea level may be practical.

Observed dart leader speed in natural and triggered lightning

From a data base consisting of (1) correlated optical and electric field measurements for 11 natural lightning strokes in Florida, (2) correlated optical and current measurements for 32 artificially initiated (triggered) lightning strokes in New Mexico, and (3) correlated optical and current measurements for 36 triggered lightning strokes in Florida, dart leader speed is examined as a function of the following return stroke initial electric field peak, of the following return stroke current peak, and of the duration of the previous interstroke interval (excluding the duration of continuing current, if present). Return stroke current peaks in both New Mexico and Florida triggered lightning were converted to electric field peaks via the field-current regression equation obtained by Willett et al. (1989), while the electric field peaks in Florida natural lightning were converted to current peaks using the current-field regression equation derived by Rakov et al. (1992), both formulas being based on the same Florida triggered lightning measurements. For each of the three data sets, dart leader speed and the following return stroke field peak or current peak are positively correlated. The relations between leader speed and field or current peak for Florida triggered and Florida natural lightning are similar, possibly indicating a similarity between at least some features of dart leaders and return strokes in natural and in triggered lightning at the same geographic location. On the other hand, leaders in New Mexico triggered lightning are, for the same value of return stroke field or current peak, about twice as fast as those in both triggered and natural lightning in Florida, the difference being likely associated with the relatively short preceding interstroke intervals in New Mexico triggered lightning. For all triggered and natural lightning data taken together, there is a weak but statistically significant tendency for lower leader speed to be associated with a longer previous interstroke interval. However, neither the New Mexico triggered lightning data nor the Florida triggered lightning data, when taken separately, show this tendency.

Lightning induced disturbances in buried cables - part II: experiment and model validation

This paper presents experimental results obtained at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida during the summers of 2002 and 2003. Currents induced by triggered and natural lightning events were measured at the terminations of a buried power cable, in the cable shield, and in the inner cable conductor. Measurements of the horizontal component of the magnetic field above the ground surface for both natural and triggered lightning are also presented. For distant natural lightning events, locations of ground strike points were determined using the U.S. National Lightning Detection Network (NLDN). Based on the theoretical developments presented in Part I of this paper , the field-to-buried cable coupling equations are solved in both the time domain and in the frequency domain. The obtained experimental results are then used to validate the numerical simulations provided by the relevant developed codes.

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.

Distribution of Currents in the Lightning Protective System of a Residential Building—Part I: Triggered-Lightning Experiments

We present the results of structural lightning protective system (LPS) tests conducted in 2004 and 2005 at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL. Lightning was triggered using the rocket-and- wire technique, and its current was directly injected into the LPS. The test configurations in 2004 and 2005 differed in the lightning current injection point, number of down conductors, grounding system at the test house, and the use of surge protective devices. The primary objective was to examine the division of the injected lightning current between the grounding system of the test house, and remote ground accessible via the neutral of the power-supply cable. In 2004, the mean value of the peak current entering the electrical circuit neutral in search of its way to remote ground was about 22% of the injected lightning current peak, while in 2005, it was about 59%. For comparison, more than 80% of the injected peak current was observed to enter the electrical circuit neutral in similar 1997 tests at the ICLRT in which a different test house with a different (poorer) grounding system was used (Rakov et al. 2002 [1]). An attempt to model the 2004 and 2005 experiments is presented in a companion paper.

Lightning attachment processes of an “anomalous” triggered lightning discharge

Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the process for stepped, dart, and dart-stepped leaders in an anomalous rocket-triggered lightning flash that terminated on a 10 m grounded utility pole. The initiation of the first return stroke was found to occur at a height of 23 ± 3 m above the top of the utility pole and was associated with three “slow front” dE/dt pulses. A time of 1.5 µs later, a fast rise in luminosity at 18 ± 2 m was associated with a “fast transition” dE/dt pulse. The first return stroke propagated bidirectionally from its initiation height, as did subsequent return strokes from their initiation heights of 8 ± 1 m to 16 ± 2 m above the top of the utility pole. The initial upward speed of the first return stroke was 1.4 × 108 m/s, while its initial downward speed was 2.2 × 107 m/s. The channel bottom luminosity of the first return stroke rose more slowly to a two or more times larger amplitude than that of the subsequent stroke luminosities. In contrast, the National Lightning Detection Network-derived first-return-stroke peak current is smaller than that of the second and the third strokes, and our electric field records at 45 km show similar behavior for the initial field peaks of the first and subsequent strokes.

Luminosity characteristics of dart leaders and return strokes in natural lightning

Streak-camera photographs were obtained in daylight for 23 subsequent strokes in five Florida negative cloud-to-ground flashes. Out of the 23 return-stroke streaked images, only 11 were accompanied by leader streaked images, while all 23 leaders were identified in corresponding electric field records. Thus, 12 subsequent leaders (one of which created a new channel to ground) failed to produce luminosity above the daylight background level. The brightest three dart-leader/return-stroke sequences from two flashes have been examined for relative light intensity as a function of time and height. Dartleader light waveforms appear as sharp pulses with 20-to-80% risetimes of about 0.5–1 μs and widths of 2–6 μs followed by a more or less constant light level (plateau). The plateau continues until it is overridden by the return-stroke light waveform, suggesting that a steady leader current flows through any channel section behind the downward moving leader tip before the return-stroke front has passed that channel section. Return-stroke light pulses near ground have 20-to-80% risetimes of about 1–2 μs and amplitudes a factor of 2 to 3 greater than those of the dart-leader light pulses. As opposed to the return-stroke light pulses that suffer appreciable degradation during the upward propagation of the return-stroke front, the dart-leader light pulses preserve their shape, and the pulse amplitude is either more or less constant or increases as the leader approaches ground. The average electric field intensity across the dart-leader front, whose length is inferred from measured light-pulse risetimes and propagation speed to be of the order of 10 m, should be at least an order of magnitude greater than the average electric field intensity across the return-stroke front, whose length is inferred to be of the order of 100 m.

Luminosity characteristics of lightning M components

A high-speed streak photograph of a natural cloud-to-ground lightning return stroke followed by two M components is analyzed. As opposed to the return stroke light pulse whose amplitude and waveshape vary markedly with height, the amplitude and waveshape of one M component light pulse is essentially invariant with height between the cloud base (about 1 km) and ground, while the other M component has a relatively constant light waveshape and a light amplitude that varies somewhat with height. The two M component light pulses, both occurring within about 0.6 ms of the return stroke pulse, exhibit a more or less symmetrical waveshape with a risetime and falltime of the order of many tens of microseconds. For one of the two M components a downward direction of propagation and a corresponding speed of the order of 108 m/s are inferred.

Dart‐stepped‐leader step formation in triggered lightning

Dart-stepped-leader step formation in triggered lightning is documented with high-speed video recorded at 648 kiloframes per second (1.16 µs exposure time, 380 ns dead time) and linear streak film with a temporal resolution of about 1 µs. Locally luminous points and segments of channel both separate and below the main descending leader tip were recorded on the high-speed video. Bidirectional leaders were imaged initiating at the locally luminous points below the main channel tip, points that remain stationary during the interstep process. The average speed of five bidirectional leaders was 8.4 × 105 m/s upward and 4.8 × 105 m/s downward, assuming 1.5 µs between successive images. The main dart-stepped-leader channel tip moved downward between steps. Leader steps extended below the bottom of the previous bidirectional leader. Processes that can be seen between steps on high-speed video are generally below the noise threshold of the streak film, which shows primarily the newly formed steps.

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.