G. H. Schnetzer

New insights into lightning processes gained from triggered-lightning experiments in Florida and Alabama

Analyses of electric and magnetic fields measured at distances from tens to hundreds of meters from the ground strike point of triggered lightning at Camp Blanding, Florida, and at 10 and 20 m at Fort McClellan, Alabama, in conjunction with currents measured at the lightning channel base and with optical observations, allow us to make new inferences on several aspects of the lightning discharge and additionally to verify the recently published “two-wave” mechanism of the lightning M component. At very close ranges (a few tens of meters or less) the time rate of change of the final portion of the dart leader electric field can be comparable to that of the return stroke. The variation of the close dart leader electric field change with distance is somewhat slower than the inverse proportionality predicted by the uniformly charged leader model, perhaps because of a decrease of leader charge density with decreasing height associated with an incomplete development of the corona sheath at the bottom of the channel. There is a positive linear correlation between the leader electric field change at close range and the succeeding return stroke current peak at the channel base. The formation of each step of a dart-stepped leader is associated with a charge of a few millicoulombs and a current of a few kiloamperes. In an altitude-triggered lightning the downward negative leader of the bidirectional leader system and the resulting return stroke serve to provide a relatively low-impedance connection between the upward moving positive leader tip and the ground, the processes that follow likely being similar to those in classical triggered lightning. Lightning appears to be able to reduce, via breakdown processes in the soil and on the ground surface, the grounding impedance which it initially encounters at the strike point, so at the time of channel-base current peak the reduced grounding impedance is always much lower than the equivalent impedance of the channel. At close ranges the measured M-component magnetic fields have waveshapes that are similar to those of the channel-base currents, whereas the measured M-component electric fields have waveforms that appear to be the time derivatives of the channel-base current waveforms, in further confirmation of the “two-wave” M-component mechanism.

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.

Characterization of the initial stage of negative rocket‐triggered lightning

We performed a statistical study on the initial stage (IS) of negative rocket-triggered lightning using 37 channel-base current recordings obtained during the summer of 1994 at Fort McClellan, Alabama, and during the summers of 1996 and 1997 at Camp Blanding, Florida. The IS can be viewed as composed of an upward positive leader (UPL) followed by an initial continuous current ( ICC ). The IS has a geometric mean (GM) duration of 279 ms and lowers a GM charge of 27 C to the ground. The average IS current in an individual lightning discharge varies from a minimum of 27 A to a maximum of 316 A with a GM value of 96 A for the entire sample of 37 discharges. We examined the current variation at the beginning of the IS in 24 flashes. In 22 out of 24 cases this initial current variation (ICV) includes a current drop, probably associated with the disintegration of the copper triggering wire and the subsequent current reestablishment. The GM time interval between the onset of the initial stage and the abrupt decrease in current is 8.6 ms, and the GM current level just prior to the current decrease is 312 A, a value about 3 times the GM value of average current for the whole IS, 96 A. Before this abrupt current decrease, a GM charge of 0.8 C has been lowered to ground with a corresponding GM action integral of 110 A2 s. The abrupt current decrease takes typically several hundred microseconds and is followed, immediately or after a time interval up to several hundred microseconds, by a pulse with a typical peak of about 1 kA and a typical risetime of less than 100 μs. The ICC usually includes impulsive processes that resemble the M processes observed during the continuing currents that follow return strokes in both natural and triggered lightning. We present statistics for the following parameters of current pulses superimposed on the ICC: magnitude, risetime, half-peak width, duration, charge transferred, preceding continuous current level, interpulse interval, and time interval between the onset of the IS and the first ICC pulse. The observed characteristics of ICC pulses varied significantly among the three data sets. For all data combined, the characteristics of the ICC pulses are similar to those of the M-component current pulses studied by Thottappillil et al [1995]. This latter finding suggests that ICC impulsive processes are of the same nature as M processes.

Properties of M components from currents measured at triggered lightning channel base

Channel base currents from triggered lightning were measured at the NASA Kennedy Space Center, Florida, during summer 1990 and at Fort McClellan, Alabama, during summer 1991. An analysis of the return stroke data and overall continuing current data has been published by Fisher et al. [1993]. Here an analysis is given of the impulsive processes, called M components, that occur during the continuing current following return strokes. The 14 flashes analyzed contain 37 leader-return stroke sequences and 158 M components, both processes lowering negative charge from cloud to ground. Statistics are presented for the following M current pulse parameters: magnitude, rise time, duration, half-peak width, preceding continuing current level, M interval, elapsed time since the return stroke, and charge transferred by the M current pulse. A typical M component in triggered lightning is characterized by a more or less symmetrical current pulse having an amplitude of 100–200 A (2 orders of magnitude lower than that for a typical return stroke [Fisher et al., 1993]), a 10–90% rise time of 300–500 μs (3 orders of magnitude larger than that for a typical return stroke [Fisher et al., 1993]), and a charge transfer to ground of the order of 0.1 to 0.2 C (1 order of magnitude smaller than that for a typical subsequent return stroke pulse [Berger et al., 1975]). About one third of M components transferred charge greater than the minimum charge reported by Berger et al. [1975] for subsequent leader-return stroke sequences. No correlation was found between either the M charge or the magnitude of the M component current (the two are moderately correlated) and any other parameter considered. M current pulses occurring soon after the return stroke tend to have shorter rise times, shorter durations, and shorter M intervals than those which occur later. M current pulses were observed to be superimposed on continuing currents greater than 30 A or so, with one exception out of 140 cases, wherein the continuing current level was measured to be about 20 A. The first M component virtually always (one exception out of 34 cases) occurred within 4 ms of the return stroke. This relatively short separation time between return stroke and the first M component, coupled with the observation of Fisher et al. [1993] that continuing currents lasting longer than 10 ms never occur without M current pulses, implies that the M component is a necessary feature of the continuing current mode of charge transfer to ground.

The close lightning electromagnetic environment: Dart‐leader electric field change versus distance

Net electric field changes due to dart leaders in triggered lightning from experiments conducted in 1997, 1998, and 1999 at the International Center for Lightning Research and Testing at Camp Blanding, Florida, are analyzed and compared with similar data obtained in 1993 at Camp Blanding and at Fort McClellan, Alabama. In 1997–1999 the fields were measured at 2–10 stations with distances from the lightning channel ranging from 10 to 621 m, while in 1993 the fields were measured at three distances, 30, 50, and 110 m, in Florida and at two distances, about 10 and 20 m, in Alabama. With a few exceptions, the 1997–1999 data indicate that the distance dependence of the leader electric field change is close to an inverse proportionality (r−1), in contrast to the 1993 data in which a somewhat weaker distance dependence was observed. The typically observed r−1 dependence is consistent with a uniform distribution of leader charge along the bottom kilometer or so of the channel.

Time derivative of the electric field 10, 14, and 30 m from triggered lightning strokes

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.

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.

Test of the transmission line model and the traveling current source model with triggered lightning return strokes at very close range

We test the two simplest and most conceptually different return stroke models, the transmission line model (TLM) and the traveling current source model (TCSM), by comparing the first microsecond of model-predicted electric and magnetic field wave forms and field derivative wave forms at 15 m and 30 m with the corresponding measured wave forms from triggered lightning return strokes. In the TLM the return stroke process is modeled as a current wave injected at the base of the lightning channel and propagating upward along the channel with neither attenuation nor dispersion and at an assumed constant speed. In the TCSM the return stroke process is modeled as a current source traveling upward at an assumed constant speed and injecting a current wave into the channel, which then propagates downward at the speed of light and is absorbed at ground without reflection. The electric and magnetic fields were calculated from Maxwells equations given the measured current or current derivative at the channel base, an assumed return stroke speed, and the temporal and spatial distribution of the channel current specified by the return stroke model. Electric and magnetic fields and their derivatives were measured 15 m and 30 m from rocket-triggered lightning during the summer of 2001 at the International Center for Lightning Research and Testing at Camp Blanding, Florida. We present data from a five-stroke flash, S0105, and compare the measured fields and field derivatives with the model-predicted ones for three assumed lightning return stroke speeds, v = 1 x 10 8 m/s, v = 2 x 10 8 m/s, and v = 2.99 × 10 8 m/s (essentially the speed of light). The results presented show that the TLM works reasonably well in predicting the measured electric and magnetic fields (field derivatives) at 15 m and 30 m if return stroke speeds during the first microsecond are chosen to be between 1 × 10 8 m/s and 2 x 10 8 m/s (near 2 x 10 8 m/s). In general, the TLM works better in predicting the measured field derivatives than in predicting the measured fields. The TCSM does not adequately predict either the measured electric fields or the measured electric and magnetic field derivatives at 15 and 30 m during the first microsecond or so.

Testing of Lightning Protective System of a Residential Structure: Comparison of Data Obtained in Rocket-Triggered Lightning and Current Surge Generator Experiments

We present a comparison of data obtained during testing of lightning protective system of a residential structure in rocket-triggered lightning experiment at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida, and current surge generator experiment at Rzeszow University of Technology in Poland. Three different configurations of LPS were tested in Poland with the dc grounding resistances of the entire system 4.09 Omega (LPS 1a), 1.65 Omega (LPS 1b), and 2.88 Omega (LPS 2). For LPS 1a with three ground rods the value of the peak current entering the electrical circuit neutral was about 56% of the injected current peak, and for LPS 1b with two additional ground rods, each connected by a buried horizontal conductor 5 m long, was about 16%. For LPS 2 with five ground rods interconnected by a buried loop conductor this ratio was 21%. The current waveshapes in the ground rods differed from the injected current waveshapes and the current waveshapes in other parts of the test system, especially, for poorer LPS 1a. The surge-generator results are consistent with those of triggered-lightning experiments at Camp Blanding, Florida (DeCarlo et al., 2006 [2]).

Direct Lightning Strikes to Test Power Distribution Lines—Part I: Experiment and Overall Results

The interaction of rocket-triggered lightning with two unenergized power distribution lines of about 800 m length was studied at the International Center for Lightning Research and Testing in Florida. A horizontally configured line was tested in 2000, and a vertically configured line in 2001, 2002, and 2003. The horizontally and vertically configured lines were equipped with six and four arrester stations, respectively, and, additionally, in 2003, the vertical line with a pole-mounted transformer. During the 2000, 2001, and 2002 experiments, arresters were frequently rendered inoperable by disconnector operation during triggered lightning strokes, but there was no disconnector operation during the 2003 experiment when the transformer was on the line. In all four years, there were commonly flashovers from the struck phase-conductor to the closest phase-conductor not subjected to direct lightning current injection. The self-consistency of measurements is assessed via comparison of the injected lightning current with: 1) the total current flowing to Earth through the multiple line groundings and 2) the total phase-to-neutral current flowing through the line arresters and line terminations. This paper is part one of two related papers.