Observation of the Whole Discharge Process During a Multi-stroke Triggered Lightning by Continuous Interferometer

Characteristics of a Negative Cloud-to-Ground Lightning Discharge Based on Locations of VHF Radiation Sources

Electromagnetic Environment Associated with Lightning Strikes to Tall Strike Objects

High‐time‐resolved mapping results of two rare reactivation processes following a negative return stroke are discovered and analyzed. The reactivation processes can be roughly divided into two different stages. At first, positive discharges occurred for tens of milliseconds in a limited space near a decayed negative leader channel. The discharges produced no detectable electric field change. Then, negative discharges started and propagated along the decayed downward negative leader channels at a speed exceeding 106 m/s for a few microseconds and produced negative electric field changes. The analysis reveals that the reactivation processes are physically distinct from recoil leaders and side discharges regarding propagation behaviors, electromagnetic characteristics, and time scales. The possible mechanisms of the processes are discussed. The observation suggests that the reactivation processes of the negative leaders may lead to subsequent return strokes.

Lightning Mapping Array (LMA) data are used to compare the propagation paths of seven rocket‐triggered lightning flashes to the inferred charge structure of the thunderstorms in which they were triggered. This is the first LMA study of Florida thunderstorm charge structure. Three sequentially (within 16 min) triggered lightning flashes, whose initial stages were the subject of Hill et al. (2013), are reexamined by comparing the complete flashes to the preceding natural lightning to demonstrate that the three rocket‐triggered flashes propagated through an inferred negative charge region that decreased from about 6.8 to about 4.4 km altitude as the thunderstorm dissipated. Two other flashes were also sequentially triggered (within 9 min) in a thunderstorm that contained a convectively intense region ahead of a stratiform region, with similar observed results. Finally, two unique cases of triggered lightning flashes are presented. In the first case, the in‐cloud portion of the triggered lightning flash, after ascending to and turning horizontal at 5.3 km altitude, just above the 0°C level, was observed to very clearly resemble the geometry of the in‐cloud portion of the preceding natural lightning discharges. In the second case, a flash was triggered relatively early in the storm's lifecycle that did not turn horizontal near the 0°C level, as is usually the case for triggered lightning in dissipating storms, but ascended to nearly 7.5 km altitude before exhibiting extensive horizontal branching.

The physical processes of leader in one triggered lightning flash using the altitude‐triggering technique are studied. The main characteristics involved in the triggered lightning are inferred from data of simultaneous electric field change measurements at two stations and highly time‐resolved optical signals. The results reveal the characteristics of leader propagation in triggered lightning. When rocket attached thin steel wire is ascending to the height of several hundred meters, a bi‐directional leader is initiated at top and bottom end of the wire. When the negative leader propagating downward approaches to the ground, a positive connecting leader is initiated from the ground and propagates upwardly. The mean speed of the negative and connecting positive leader is around 0.86×105 m/s. A mini‐return stroke is produced with the connection to the ground of the downward negative leader. The upward positive leader initiated from the top end of wire propagates at a speed of about 1.1×105 m/s.

Fast electric field changes from subsequent return strokes of natural and triggered lightning with propagation paths almost entirely over water are compared. Data were collected at the Kennedy Space Center, Florida, during the summer of 1987. Comparisons have been made of the average shape, the risetime, and the spectrum of the electric field changes. To a first approximation, the waveforms are very similar; however, the electric field changes from the triggered flashes tend to rise to peak faster and decay faster than do their counterparts in natural cloud‐to‐ground flashes.

The artificially triggered lightning test was used and a 4-element AWS was simulated near the lightning channel to analyze and study the induced effects caused by the lightning and the key lightning protection techniques for AWS. This paper analyzes the induced overvoltage characteristics and SPD residue voltage characteristics caused by triggered lightning with multiple return strokes on the transmission line of an AWS, as well as the relationship with the triggered lightning. The results show that: (a) The return strokes of the near triggered lightning generate bipolar induced overvoltages on the overhead line, with peaks up to more than 10 kV. The overvoltages can be divided into the main peak phase and the subsequent overvoltage phase, which last about 100µs and 4ms respectively on average. The period of the subsequent overvoltages is related to the fluctuation of the current of the triggered lightning; (b) Overvoltages of 2kV or above can be induced at the initial continuous current phase. They last a long period of time and concentrate about 12ms, with the mean voltage being 332.5V. Its potential damage to the transmission line cannot be neglected.

The recent discovery of long recovery, early VLF scattering events (LOREs) indicates that the electric field changes from lightning discharges are capable of producing long‐lasting disturbances (up to tens of minutes) in the upper mesosphere and lower ionosphere. Comparison of lightning mapping array, broadband (up to 10 MHz) electric field, and VLF (∼300 Hz to 42 kHz) magnetic field measurements shows that the field changes produced by initial breakdown (IB) processes and the following leaders in natural, cloud‐to‐ground lightning discharges are detectable in VLF magnetic field measurements at long distances. IB radiation has been detected in VLF for lightning discharges occurring up to 2630 km away from the VLF observing station. Radio atmospherics associated with 52 LOREs, 51 regular recovery events, and 3098 flashes detected by National Lightning Detection Network and/or GLD360 were examined for IB radiation occurring up to 15 ms before the return stroke. Our analysis reveals that in contrast to regular recovery early VLF events, LOREs are strongly associated with lightning discharges which exhibit an intense IB process and a fast first leader (typical duration <4 ms). These experimental results demonstrate that initial breakdown and leader processes are indicators of discharge properties highly relevant to the total energy transfer between lightning discharges and the middle/upper atmosphere.

An improved return stroke model that is both physically oriented and has a relatively straightforward mathematical basis is proposed. The current at the channel base is specified, and a time‐dependent discharging of the charge stored on the leader channel determines the channel current as a function of height and time. The discharging process is separated into (1) the exponential discharge of the leader head and leader core with a relatively short time constant, less than 1 μs, which we call the “breakdown” time constant, and (2) the exponential discharge of the charge stored around the leader core with a longer time constant, of the order of microseconds. If a typical measured channel‐base current is assumed and if the discharge time constants are properly chosen, electric and magnetic field wave shapes calculated with the model exhibit all the significant characteristics of measured fields. From a comparison of calculated and measured field wave shapes, we find a ratio of the breakdown time constant to the channel‐base current rise time between 1 and 5. Comparison of typical characteristics of field wave shapes from natural and from artificially initiated (triggered) lightning indicates a faster discharging process for triggered lightning. Depending on the breakdown time constant, the return stroke speed determined using the well‐known formula for the transmission‐line model, with inputs being the peak electric field and peak current from the present model, are in the range from about 50 percent to 90 percent of the return stroke speed assumed in the present model. The corresponding transmission‐line model speeds determined from the peak derivatives of the electric field and current are in the range from about 140 percent to 160 percent of the assumed return stroke speed. These results supply some indication of why transmission‐line model speeds determined from the ratio of measured peak current and field derivatives in triggered lightning are greater than the speeds determined from the ratio of measured peak currents and fields. For a given channel‐base current, the initial peak electric field and field derivative derived from the model increases as the height above ground of the strike point increases. The new model can therefore explain the differences in the data obtained from the triggered lightning studies at Kennedy Space Center in 1985 and in 1987 as being due to the different height of the triggering structures in those two years. If natural lightning strikes an elevated object, the increase of the initial electric field and field derivative can result in an additional substantial error in determining the peak current and peak current derivative from the transmission‐line model.

We have directly measured the time derivative of the electric field of triggered lightning strokes at distances of 10, 14, and 30 m. The data were taken in 1998 at the International Center for Lightning Research and Testing at Camp Blanding, Florida. We compare our results with those of similar triggered lightning measurements made previously at the Kennedy Space Center at distances of 50 m and 5 km and in France at 50 m. We also compare our electric field derivative waveforms with previous measurements at the Kennedy Space Center of natural lightning strokes over the Atlantic Ocean at distances of the order of tens of kilometers and with overland natural lightning data obtained at 0.7 to 14 km in Germany. Our return stroke electric field derivative peak values normalized (assuming the inverse distance dependence valid for radiation fields) to 100 km are similar to all previous measurements for both natural and triggered lightning at distances from 50 m to about 50 km, all being several tens of volts per meter per microsecond, with the exception of the German overland peak derivative values which are an order of magnitude lower. Our 10‐ to 30‐m field derivative zero‐to‐peak risetimes are typically 50 to 100 ns (minimum of 30 ns and maximum of 180 ns), and widths at half‐peak value are typically 100 to 200 ns. There is essentially no difference between our electric field derivative waveshapes measured simultaneously at 10 m and at 30 m, with the closer waveform being about a factor of 2 greater in amplitude. Fourier analysis of our electric field derivative waveforms indicates that the primary frequency content of the waveforms is below about 20 MHz. Our close return stroke field derivative waveforms differ from those of Leteinturier et al. (1990) recorded 50 m from triggered lightning at the Kennedy Space Center in 1985 in that their derivative waveforms typically decrease rapidly after the peak and exhibit zero crossings and in that their waveforms tend to have multiple peaks, while our derivative waveforms are generally single peaked and decay more gradually to zero after the peak, with no zero crossings. We argue that the differences between their waveforms and ours are related to the relatively large rocket‐launching structure used at the Kennedy Space Center in 1985.

Electric field waveforms at horizontal distances from the triggered lightning channel attachment point ranging from 0.1 to 1.6 m have been measured with Pockels sensors at the International Center for Lightning Research and Testing at Camp Blanding, Florida. The measuring system had a dynamic range from 20 kV/m to 5 MV/m and a bandwidth from 50 Hz to 1 MHz. The corresponding currents at the channel base and electric fields at 5, 15, and 30 m from the lightning channel were also measured using a current viewing resistor and flat‐plate antennas, respectively. Very close vertical electric fields for 36 strokes in nine triggered lightning flashes were obtained using Pockels sensors. For 8 out of the 36 strokes, horizontal electric fields were also measured using Pockels sensors. Electric field waveforms appear as pulses, with the leading edge of the pulse being due to the leader and the trailing edge due to the return stroke. Of the 36 vertical electric field waveforms, six were more or less V‐shaped, while 30 exhibited a considerably slower variation during the return‐stroke stage than during the leader stage. Vertical electric field pulse peaks are in the range from 176 kV/m to 1.5 MV/m (the median is 577 kV/m), and horizontal electric field pulse peaks are in the range from 495 kV/m to 1.2 MV/m (the median is 821 kV/m). On‐site calibration results show that these electric fields measured using Pockels sensors may be underestimated by 40% or so due to the insufficient upper frequency response of 1 MHz of the measuring system. Additionally, vertical electric fields due to M components were measured and compared to electric fields produced by leader/return stroke sequences. For 8 out of 10 M components having channel‐base peak currents greater than 500 A, vertical electric fields at 0.1 to 1.6 m were below 20 kV/m, the lower measurement limit. For the remaining 2 of 10 M components, whose current peaks were between 2.3 and 3.2 kA, vertical electric field peaks were about 100 and 48 kV/m at a distance of 0.1 m from the attachment point, apparently unaffected by the upper frequency response of the measuring system. The vertical electric field measured very close to the lightning channel tends to increase with an increase in the previous no‐current interval, that is, in the time elapsed from the cessation of current of the preceding stroke (or of the initial‐stage current).

With the measurements in SHandong Triggering Lightning Experiment and Guangdong Comprehensive Observation Experiment on Lightning Discharge in China, we examine the electromagnetic signals associated with the upward positive leaders during the initial stage of negative triggered lightning. The magnetic field (B field) signals measured at close range (<100 m) for both sites can be divided into two categories (i.e., impulsive and ripple pulses) according to the discernibility of separation between individual pulses. The impulsive pulses are well simulated by using the transmission line model, which suggests that these pulses are generated by leader current pulses propagating downward along the steel wire. Because the length of extended leader channel ahead of the wire is not negligible during the stage of ripple pulses, the waveform of impulsive current pulses is changed after traveling through the high impedance leader channel. Taking the filtered current pulse as the input variable, the waveform of ripple pulse can be simulated properly, which indicates that ripple pulses are caused by the attenuation of impulsive current along prolonging leader channel. In addition, the paper analyzes the fast electric field (E field) changes measured at 60‐m range from the launching site during the initial stage by using the transmission line model and shows that the polarity of E field change at a given range is determined by the inception height of upward leader, namely the surface E field change caused by the individual charge transfer of initial upward leader also involves the problem of reverse distance as present for a vertical dipole.

Estimation of peak return stroke currents, current time derivatives and return stroke velocities from measured fields

Based on simultaneous observation of currents and electric field changes, the differences have been identified between flashes with and without return strokes in rocket‐triggered lightning. The triggered lightning events without return stroke have an obvious no‐current period up to some milliseconds before the current flow is reestablished, then a relatively large amplitude (1.2 kA or so) and fast risetime (0.7 μs or less) pulse similar to those of the return stroke current are reestablished between the upward positive leader and the ground. The triggered lightning events with return strokes have no no‐current period in the initial current variation, and the current pulse of wire destruction and plasma channel reestablishment process is similar to the M‐component pulse, featuring a small amplitude (0.17 kA or so) and slow risetime (57 μs or more). The stronger initial stage is usually followed by return strokes.

Characteristics of a Negative Cloud-to-Ground Lightning Discharge Based on Locations of VHF Radiation Sources