Nobuyuki Takagi

Observed leader and return-stroke propagation characteristics in the bottom 400 m of a rocket-triggered lightning channel

Using a high-speed digital optical system, we determined the propagation characteristics of two leader/return-stroke sequences in the bottom 400 m of the channel of two lightning flashes triggered at Camp Blanding, Florida. One sequence involved a dart leader and the other a dart-stepped leader. The time resolution of the measuring system was 100 ns, and the spatial resolution was about 30 m. The leaders exhibit an increasing speed in propagating downward over the bottom some hundreds of meters, while the return strokes show a decreasing speed when propagating upward over the same distance. Twelve dart-stepped leader luminosity pulses observed in the bottom 200 m of the channel have been analyzed in detail. The luminosity pulses associated with steps have a 10-90% risetime ranging from 0.3 to 0.8 μs with a mean value of 0.5 μs and a half-peak width ranging from 0.9 to 1.9 μs with a mean of 1.3 μs. The interpulse interval ranges from 1.7 to 7.2 μs with a mean value of 4.6 μs. The step luminosity pulses apparently originate in the process of step formation, which is unresolved with our limited spatial resolution of 30 m, and propagate upward over distances from several tens of meters to more than 200 m, beyond which they are undetectable. This finding represents the first experimental evidence that the luminosity pulses associated with the steps of a downward moving leader propagate upward. The upward propagation speeds of the step luminosity pulses range from 1.9×10 7 to 1.0×10 8 m/s with a mean value of 6.7×10 7 m/s. In particular, the last seven pronounced light pulses immediately prior to the return stroke pulse exhibit more or less similar upward speeds, near 8×10 7 m/s, very close to the return-stroke speed over the same portion of the channel. On the basis of this result, we infer that the propagation speed of a pulse traveling along the leader-conditioned channel is primarily determined by the channel characteristics rather than the pulse magnitude. An inspection of four selected step luminosity pulses shows that the pulse peak decreases significantly as the pulse propagates in the upward direction, to about 10% of the original value within the first 50 m. The return-stroke speeds within the bottom 60 m or so of the channel are 1.3×10 8 and 1.5×10 8 m/s for the two events analyzed, with a potential error of less than 20%.

Attachment process in rocket-triggered lightning strokes

In order to study the lightning attachment process, we have obtained highly resolved (about 100 ns time resolution and about 3.6 m spatial resolution) optical images, electric field measurements, and channel-base current recordings for two dart leader/return-stroke sequences in two lightning flashes triggered using the rocket-and-wire technique at Camp Blanding, Florida. One of these two sequences exhibited an optically discernible upward-propagating discharge that occurred in response to the approaching downward-moving dart leader and connected to this descending leader. This observation provides the first direct evidence of the occurrence of upward connecting discharges in triggered lightning strokes, these strokes being similar to subsequent strokes in natural lightning. The observed upward connecting discharge had a light intensity one order of magnitude lower than its associated downward dart leader, a length of 7–11 m, and a duration of several hundred nanoseconds. The speed of the upward connecting discharge was estimated to be about 2 × 107 m/s, which is comparable to that of the downward dart leader. In both dart leader/return-stroke sequences studied, the return stroke was inferred to start at the point of junction between the downward dart leader and the upward connecting discharge and to propagate in both upward and downward directions. This latter inference provides indirect evidence of the occurrence of upward connecting discharges in both dart leader/return-stroke sequences even though one of these sequences did not have a discernible optical image of such a discharge. The length of the upward connecting discharges (observed in one case and inferred from the height of the return-stroke starting point in the other case) is greater for the event that is characterized by the larger leader electric field change and the higher return-stroke peak current. For the two dart leader/return-stroke sequences studied, the upward connecting discharge lengths are estimated to be 7–11 m and 4–7 m, with the corresponding return-stroke peak currents being 21 kA and 12 kA, and the corresponding leader electric field changes 30 m from the rocket launcher being 56 kV/m and 43 kV/m. Additionally, we note that the downward dart leader light pulse generally exhibits little variation in its 10–90% risetime and peak value over some tens of meters above the return-stroke starting point, while the following return-stroke light pulse shows an appreciable increase in risetime and a decrease in peak value while traversing the same section of the lightning channel. Our findings regarding (1) the initially bidirectional development of return-stroke process and (2) the relatively strong attenuation of the upward moving return-stroke light (and by inference current) pulse over the first some tens of meters of the channel may have important implications for return-stroke modeling.

Spatial and temporal properties of optical radiation produced by stepped leaders

The relative light intensities as a function of height and time for two negative downward stepped leaders, A and B, recorded by a high-speed digital 16 × 16 photodiode array photographic system, are studied. For leader A it is found that the light waveform for each segment of the leader channel starts with a series of sharp light pulses followed by several slow-rising and longer-lasting light surges, with both the light pulses and surges superimposed on a continuous luminosity slope that has a long rising front followed by an almost constant light level. Analysis indicates that each light pulse involves a step process; it originates at the leader tip and appears to propagate upward, with the pulse amplitude suffering little degradation within the first several tens of meters to 200 m from the leader tip up (bright tip length) but with a severe attenuation above. The light surges are observed to be almost constant in amplitude above the bright tip, and for one of them an upward propagation speed of the order of 108 m/s is inferred. From appearances of the light pulses it is determined that the leader A has an overall velocity of 4.5–11.2×105 m/s, a step interval of 5–50 μs, and a step length of 7.9–19.8 m. For leader B the step light pulses are found to propagate from the leader tip back up at a speed of 0.14–1.7×108 m/s, and the overall leader velocity, the step interval, and the step length are determined to be about 4.9–5.8×105 m/s, 18–21 μs, and 8.5 m, respectively. In addition, on the basis of the light waveforms of the leader A it is inferred that the current of a stepped leader may consist of two parts: an impulsive current within the bright tip and a continuing current above it. After propagating along the bright tip up, because of increasing resistance and capacitance of the leader channel the impulsive current rapidly transforms into part of the continuing current.

A high speed optical imaging system for studying lightning attachment process

We have developed a high speed optical imaging system for studying lightning attachment process. The system has the feature of wide dynamic range, long recording length, high time resolution and data collection efficiency. We have installed three of such systems at The International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida to observe the attachment process of both natural and rocket triggered lightning. We have obtained some preliminary results which demonstrate that the system is a useful tool for studying lightning attachment process.

Lightning attachment processes of three natural lightning discharges

Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the attachment process for six strokes in three natural lightning flashes. All strokes initiate at a height above ground and propagate bidirectionally from that height, similar to the return strokes of artificially initiated (triggered) lightning previously reported by Wang et al. (2013, 2014). Though the data are quite limited, these natural return strokes suggest a correlation between larger peak current and greater initiation height. Initiation heights determined here span 12–60 m with a typical uncertainty of less than 10 m. The initial upward return stroke luminosity speeds range from (0.8 ± 0.2) to (2.0 ± 0.4) × 108 m/s. Two first return strokes downward luminosity speeds are assessed as (1.6 ± 0.3) × 107 m/s and (1.4 ± 0.3) × 108 m/s. One of the first return strokes appeared to be initiated with a stepping pulse discharge of its leader as an inseparable part of the return stroke.

Expansion of the luminous region of the lightning return stroke channel

We developed a high-speed line-scanning camera (HSLSC) for measuring the radial variation of optical intensity of lightning channels. With this system we succeeded in obtaining a few examples of data for natural lightning. From these data we found that the luminous region of return stroke channels may expand with a velocity of more than 10 5 m/s during the initial stage and then reach the maximum diameter of several tens of meters after about 100 μs from the initiation of the return stroke and finally decrease with time. This suggests the occurrence of lateral discharges along the lightning channel during the propagation of return strokes.

Development of a Space-charge-sensing System

A system for remotely measuring the distribution of air space charge in real time is developed. The system consists of a loudspeaker and an electric field antenna. By propagating a burst of directional sound wave from the speaker, a modulation in the space charge and, therefore, an electric field change at ground is produced. The distribution of the space charge density is derived from the E-field change which can be measured by the E-field antenna. The developed system has been confirmed by both laboratory and field experiments.

Luminosity progression in dart-stepped leader step formation

Using a high-speed optical imaging system, we have observed the step formation bright pulse discharges occurring at the tip of dart-stepped leaders of rocket triggered lightning with a time resolution of 0.1 µs and a spatial resolution of about 1.4 m. Each of the step formation pulse discharges appeared to initiate at a location immediately below the bottom of its previous pulse discharge and to propagate in bidirectional (upward and downward) waves with a speed on the order of 107 m/s. The downward waves of the pulse discharges tended to slow down significantly after they propagated a distance of about 2 m. Based on the results observed in this study and those published in literatures, we propose a conceptual view of leader step formation.

Characteristics of electric field change preceding negative first return stroke produced by preliminary breakdown

Understanding the initiation process of preliminary breakdown during negative CG lightning discharges to the ground is important to design a lightning protection system. We observed electric field change preceding negative cloud to ground lightning flashes related to preliminary breakdown in tropical region, Indonesia. The characteristics of electric field change of negative first return stroke produced by preliminary breakdown process were studied. The results showed that arithmetic mean and geometric mean of pre-return stroke separation is 57 ms and 32 ms, respectively. Other statistical parameters also are shown in this paper.

Lightning Mapping With an Array of Fast Antennas

Fast Antenna Lightning Mapping Array (FALMA), a low-frequency lightning mapping system comprising an array of fast antennas, was developed and established in Gifu, Japan, during the summer of 2017. Location results of two hybrid flashes and a cloud-to-ground flash comprising 11 return strokes (RSs) are described in detail in this paper. Results show that concurrent branches of stepped leaders can be readily resolved, and K changes and dart leaders with speeds up to 2.4 × 10 m/s are also well imaged. These results demonstrate that FALMA can reconstruct three-dimensional structures of lightning flashes with great details. Location accuracy of FALMA is estimated by comparing the located striking points of successive RSs in cloud-to-ground flashes. Results show that distances between successive RSs are mainly below 25 m, indicating exceptionally high location accuracy of FALMA. Plain Language Summary We have developed a lightning mapping system called Fast Antenna Lightning Mapping Array (FALMA). FALMA detects low-frequency radio waves produced by lightning discharges at multiple sites and can reconstruct 3-D structures of lightning flashes. Three lightning flashes are analyzed in detail in this paper, and animations of seven cases are provided in the supporting information. These results demonstrate that FALMA can show development of lightning discharges in great details and accuracy. Types and polarities of lightning discharges can also be determined by the recorded electric field change waveforms. FALMA is expected to become a unique tool for lightning and thunderstorm researches. Research studies of several topics are already underway based on the observation data in the summer of 2017.