Rajeev Thottappillil

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.

Parameters of triggered-lightning flashes in Florida and Alabama

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. Additionally, 16-mm cinematic records with 3- or 5-ms resolution were obtained for all flashes, and streak camera records were obtained for three of the Florida flashes. The 17 flashes analyzed here contained 69 strokes, all lowering negative charge from cloud to ground. Statistics on interstroke interval, no-current interstroke interval, total stroke duration, total stroke charge, total stroke action integral (∫ i2dt), return stroke current wave front characteristics, time to half peak value, and return stroke peak current are presented. Return stroke current pulses, characterized by rise times of the order of a few microseconds or less and peak values in the range of 4 to 38 kA, were found not to occur until after any preceding current at the bottom of the lightning channel fell below the noise level of less than 2 A. Current pulses associated with M components, characterized by slower rise times (typically tens to hundreds of microseconds) and peak values generally smaller than those of the return stroke pulses, occurred during established channel current flow of some tens to some hundreds of amperes. A relatively strong positive correlation was found between return stroke current average rate of rise and current peak. There was essentially no correlation between return stroke current peak and 10–90% rise time or between return stroke peak and the width of the current waveform at half of its peak value. Parameters of the lightning flashes triggered in Florida and Alabama are similar to each other but are different from those of triggered lightning recorded in New Mexico during the 1981 Thunderstorm Research International Program. Continuing currents that follow return stroke current peaks and last for more than 10 ms exhibit a variety of wave shapes that we have subdivided into four categories. All such continuing currents appear to start with a current pulse presumably associated with an M component. A brief summary of lightning parameters important for lightning protection, in a form convenient for practical use, is presented in an appendix.

Distribution of charge along the lightning channel: Relation to remote electric and magnetic fields and to return‐stroke models

We derive exact expressions for remote electric and magnetic fields as a function of the time- and height-varying charge density on the lightning channel for both leader and return-stroke processes. Further, we determine the charge density distributions for six return-stroke models. The charge density during the return-stroke process is expressed as the sum of two components, one component being associated with the return-stroke charge transferred through a given channel section and the other component with the charge deposited by the return stroke on this channel section. After the return-stroke process has been completed, the total charge density on the channel is equal to the deposited charge density component. The charge density distribution along the channel corresponding to the original transmission line (TL) model has only a transferred charge density component so that the charge density is everywhere zero after the wave has traversed the channel. For the Bruce-Golde (BG) model there is no transferred, only a deposited, charge density component. The total charge density distribution for the version of the modified transmission line model that is characterized by an exponential current decay with height (MTLE) is unrealistically skewed toward the bottom of the channel, as evidenced by field calculations using this distribution that yield (1) a large electric field ramp at ranges of the order of some tens of meters not observed in the measured electric fields from triggered-lightning return strokes and (2) a ratio of leader-to-return-stroke electric field at far distances that is about 3 times larger than typically observed. The BG model, the traveling current source (TCS) model, the version of the modified transmission line model that is characterized by a linear current decay with height (MTLL), and the Diendorfer-Uman (DU) model appear to be consistent with the available experimental data on very close electric fields from triggered-lightning return strokes and predict a distant leader-to-return-stroke electric field ratio not far from unity, in keeping with the observations. In the TCS and DU models the distribution of total charge density along the channel during the return-stroke process is influenced by the inherent assumption that the current reflection coefficient at ground is equal to zero, the latter condition being invalid for the case of a lightning strike to a well-grounded object where an appreciable reflection is expected from ground.

Comparison of lightning return-stroke models

Five return-stroke models, each allowing the use of measured channel-base current and return-stroke speed as inputs for the computation of channel current distribution and remote electric field, are compared and evaluated using 18 sets of three simultaneously measured triggered lightning features: channel-base current, return-stroke speed, and electric field at a distance of about 5 km from the channel base. The experimental data were acquired during a triggered lightning experiment at the NASA Kennedy Space Center, Florida, in 1987 and were reported in part by Willett et al. (1989) and Leteinturier et al. (1991). The models compared are the transmission line (TL) model, the modified transmission line (MTL) model, the traveling current source (TCS) model, the Diendorfer-Uman (DU) model, and the modified Diendorfer-Uman (MDU) model. The TL, MTL, DU, and MDU models each predict the measured initial electric field peaks with an error whose mean absolute value is about 20%; the TCS model has a mean absolute error about twice that value. For the prediction of overall measured field wave shape, none of the models is clearly preferred, although for the model parameters assumed, the MDU model gave the best wave shape match. Most of the return strokes that exhibited very narrow sharp initial peaks in the measured electric field waveforms had a maximum rate of rise of channel-base current closer to the peak of the measured channel-base current waveform than did return strokes not exhibiting these sharp field peaks. The calculated fields from the TL and the MTL models do not have narrow sharp peaks similar to those found in many of the measured fields, while the fields calculated from the TCS, DU, and MDU models had somewhat similar peaks in most of the cases where those peaks were found in the measured fields. On the basis of the comparison of the five models, we recommend the TL model for calculating the peak channel-base current from the measured initial peak electric field because the TL model provides a similar or better result from a simpler mathematical relation.

Initial stage in lightning initiated from tall objects and in rocket‐triggered lightning

We examine the characteristics of the initial stage (IS) in object-initiated lightning derived from current measurements on the Gaisberg tower (100 m, Austria), the Peissenberg tower (160 m, Germany), and the Fukui chimney (200 m, Japan) and their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. For rocket-triggered lightning the geometric mean (GM) values of the three overall characteristics of the initial stage, duration, charge transfer, and average current, are similar to their counterparts for the Gaisberg tower flashes and the Peissenberg tower flashes, while the Fukui chimney flashes are characterized by a shorter GM IS duration and a larger average current. The GM IS charge transfer for the Fukui chimney flashes is similar to that in the other three data sets. The GM values of the action integral differ considerably among the four data sets, with the Fukui action integral being the largest. The observed differences in the IS duration between the Fukui data set and all other data considered here are probably related to the differences in the lower current limits, while the differences in the action integral cannot be explained by the instrumental effects only. There appear to be two types of initial stage in upward lightning. The first type exhibits pulsations (ringing) during the initial portion of the IS, and the second type does not. The occurrence of these types of IS appears to depend on geographical location. The characteristics of pulses superimposed on the initial continuous current (ICC pulses) in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of 2 but differ more significantly from their counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.

Review of lightning properties from electric field and TV observations

From analysis of simultaneous electric field and TV records of 76 negative cloud-to-ground lightning flashes in Florida, various lightning properties have been determined and several new facets of ...

An improved transmission-line model of grounding system

This paper presents a time-domain transmission line model of grounding system, which includes the mutual electromagnetic coupling between the parts of the grounding structure and the influence of air-earth interface. The model can be used to simulate the transient behavior of the grounding system under lightning strike. The simulation results are in good agreement with that of the model based on the solution of full Maxwells equations. The influence of different parameters, such as the soil relative permittivity /spl epsi//sub /spl tau//, the soil resistivity /spl rho/, and the conductivity and diameter of the conductor, on the transient voltage distribution of the grounding system is investigated. It shows that, among the parameters investigated here, the soil resistivity is the most important parameter that affects the transient response of bare buried conductors. The soil permittivity has very little influence on the transient response of the grounding system when the grounding system is buried in the soil with low resistivity, but have moderate influence in the soil with extremely high resistivity. The conductivity of the conductor and skin effect have practically no influence on the peak transient voltage of the grounding system. Increase in conductor diameter tends to decrease the peak transient voltage. The model presented in this paper is simple, but sufficiently accurate and can be used easily in engineering practice. Since the model is in the time domain, it could be easily coupled to the other time-domain models of nonlinear surge-protection components.

An engineering model for transient analysis of grounding system under lightning strikes: nonuniform transmission-line approach

A nonuniform transmission line approach is adopted in this paper for modeling the transient behavior of different types of grounding systems under lightning strikes in time domain by solving Telegraphers equations based on finite-difference time-domain (FDTD) technique. Electromagnetic couplings between different parts of the grounding wires are included using effective per-unit length parameters (l, c, and g), which are space and time dependent. The present model can predict both the effective length and the transient voltage of grounding electrodes accurately, while, an uniform transmission line approach with electrode length dependent per-unit length parameters fails to predict the same. Unlike the circuit theory approach , the present model is capable of predicting accurately the surge propagation delay in the large grounding system. The simulation results for buried horizontal wires and grounding grids based on the present model are in good agreement with that of the circuit and electromagnetic field approaches , . From an engineering point of view, the model presented in this paper is sufficiently accurate, time efficient, and easy to apply.

Lightning subsequent-stroke electric field peak greater than the first stroke peak and multiple ground terminations

For 46 multiple-stroke flashes in which each stroke ground termination was located using a TV camera network and thunder ranging, 15 flashes (33%) had one or more subsequent return strokes whose initial electric field peak normalized to 100 km was greater than the first-stroke field peak of the flash. In 9 of these 15 flashes the subsequent strokes with field peaks greater than the first stroke followed the same channel as the first stroke; in five flashes the subsequent strokes with the greater peaks followed a different channel to ground; and in one flash the subsequent strokes with the greater peaks occurred both in the first-stroke channel and in a different channel. The interstroke intervals immediately preceding the 13 larger subsequent strokes that followed the first-stroke channel had a geometric mean (GM) duration of 98 ms, 1.7 times greater than the GM of 57 ms for all 199 interstroke intervals (46 flashes) without any selection. Eight of the 13 larger subsequent strokes for which leader durations were measurable had a GM leader duration of 0.55 ms, 3.3 times smaller than the GM of 1.8 ms for 117 subsequent leaders with measurable duration in a previously formed channel of the 46 multiple-stroke flashes. For the six larger subsequent strokes that created a new channel to ground, the preceding interstroke interval had a GM of 130 ms, and the leader duration had a GM of 15 ms. No subsequent stroke with peak field exceeding the first in any category had a preceding interstroke interval less than 35 ms. Analysis of direct current measurements from Switzerland shows that subsequent-stroke currents exhibit many features similar to those of Florida subsequent-stroke electric fields. In 22 Florida single-stroke and multiple-stroke ground flashes the distances between multiple channel terminations in a given flash (33 measurements) ranged from 0.3 km to 7.3 km, with a GM of 1.7 km.

Initial‐stage pulses in upward lightning: Leader/return stroke versus M‐component mode of charge transfer to ground

[1] Weanalyzedhigh-speedvideoimagesandcorresponding current records for eight upward lightning flashes initiated by the Peissenberg tower (160 m) in Germany. These flashes contained a total of 33 measurable initial stage (IS) current pulses, which are superimposed on steady IS currents. Seven IS pulses had relatively short ( 8 ms) risetimes. Six (86%) of seven IS current pulses with shorter risetimes each developed in a newly-illuminated branch, and 25 (96%) of 26 IS pulses with longer risetimes occurred in already luminous (current-carrying) channels. These results support the hypothesis that longer risetimes are indicative of the M-component mode of charge transfer to ground, while shorter risetimes are associated with the leader/return stroke mode. Similar results were obtained for M-component pulses thataresuperimposedoncontinuingcurrentsfollowingreturnstroke pulses. Citation: Flache, D., V. A. Rakov, F. Heidler, W. Zischank, and R. Thottappillil (2008), Initial-stage pulses in upward lightning: Leader/return stroke versus M-component mode of charge transfer to ground, Geophys. Res. Lett., 35, L13812, doi:10.1029/2008GL034148.