Science in China Series D: Earth Sciences

© 2007 Science in China Press

Springer-Verlag

www.scichina.com www.springerlink.com Sci China Ser D-Earth Sci | August 2007 | vol. 50 | no. 8 | 1241-1250

Artificially triggered lightning and its characteristic discharge parameters in two severe thunderstorms

QIE XiuShu1,2†, ZHANG QiLin1, ZHOU YunJun1, FENG GuiLi1, ZHANG TingLong1, YANG Jing1, KONG XiangZhen1, XIAO QingFu1 & WU ShuJun3

1 Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China;

2 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; 3 Binzhou Meteorology Bureau, Binzhou 256612, China

The lightning-induced-damages in the mid-latitude regions are usually caused during severe thunder-storms. But the discharge parameters of natural lightning are difficult to be measured. Five lightning flashes have been artificially triggered with the rocket-wire technique during the passage of two severe thunderstorms. The discharge current and close electric field of return stroke in artificially triggered lightning have been obtained in microsecond time resolution by using current measuring systems and electric field change sensors. The results show that the five triggered lightning flashes include 1 to 10 return strokes, and the average return stroke current is 11.9 kA with a maximum of 21.0 kA and a mini-mum of 6.6 kA, similar to the subsequent return strokes in natural lightning. The half peak width of the current waveform is 39 µs, which is much larger than the usual result. The peak current of stroke Ip (kA) and the neutralized charge Q(C) has a relationship of Ip = 18.5Q0.65. The radiation field of return stroke is 5.9 kV·m−1 and 0.39 kV·m−1 at 60 m and 550 m, respectively. The radiation field decreases as r −1.119 with increase of horizontal distance r from the discharge channel. Based on the well-accepted transmission line model, the speed of return stroke is estimated to be about 1.4×108 m·s−1, with a variation range of (1.1―1.6)×108 m·s−1. Because of the similarities of the triggered lightning and natural lightning, the results in this article can be used in the protection design of natural lightning.

artificially triggered lightning, characteristic discharge parameters, current waveform, electric field change in close distance, severe thunderstorm

The serious lightning-induced damages are usually caused by the severe thunderstorm because of the very frequent lightning activities in the mid-latitude region. However, the discharge current and close electric and magnetic fields for natural lightning flashes are very difficult to be measured because of the low probability of striking at a certain object, and the understanding of lightning physical mechanism has been lagged, conse-quently. As a matter of fact, the lightning discharge can be triggered artificially by means of rocket-wire-trailing technique. The first triggered lightning over terrene was accomplished in France in 1973[1]. A common technique for artificially triggering lightning involves launching a

small rocket trailing a thin, grounded copper wire to-ward the charged cloud overhead. This technique for triggered lightning is called “classical” triggering. The lightning will be triggered successfully when the rocket reaches a height of several hundreds of meters in a fa-vorable condition of thunderstorm electricity. Most fre-quently, triggered lightning flashes are negative, that is, they occur when the background electric field is oriented Received April 3, 2006; accepted September 19, 2006 doi: 10.1007/s11430-007-0064-2 †Corresponding author (email: qiex@mail.iap.ac.cn) Supported by the National Natural Science Foundation of China (Grant Nos. 40135010 and 40325013) and the Ministry of Science and Technology of China (Grant No. 2004DEA71070)

1242 QIE XiuShu et al. Sci China Ser D-Earth Sci | August 2007 | vol. 50 | no. 8 | 1241-1250

from the ground to the cloud, and hence negative charge is lowered from the cloud to earth.

Although the dart leaders and following return strokes are similar to those processes in downward negative natural lightning flashes, the initial processes are dis-tinctly different. In order to reproduce the initial proc-esses, including stepped leader and following first return stroke, a so-called “altitude” triggering technique with an ungrounded wire should be used. The rocket usually spools out several tens of meters of insulating Nylon followed by copper wire. When this last wire has been unreeled over a sufficient length, a bidirectional leader initiates from its extremities, i.e. the upward positive leader from the upper end and downward negative leader from the lower end of the triggering wire. As the negative downward leader from the lower end ap-proaches the triggering facility, an upward positive con-necting leader is initiated from the grounded object. Once the two leaders attach each other, a so-called first return stroke is initiated[2]. There is no much information on this first stroke till now. Lalande et al.[3] and Saba et al.[4] reported the direct current measurement of this process. Chen et al.[5] and Zhang et al.[6] analyzed its electric field change and referred it to mini-stroke be-cause it is weak compared with the natural first one.

The overall efficiency of successful triggering is about 60% in the United States[2], and 32% in Brazil[4]. Recently, Rakov et al.[7] reviewed the triggered lightning experiments in the United States from 1993 to 2002. A total of 208 lightning flashes were triggered at Florida in the ten years and 144 of them contained return strokes. The principal results obtained at Florida include the characterization of the close lightning electromagnetic environment[8―10], the first lightning return-stroke speed profiles within 400 m of ground and optical image of upward connecting leader[11,12], and the discovery of X-rays produced by triggered-lightning strokes[13]. Saba et al.[4] observed a very intense altitude triggered light-ning in Brazil, and found that the average peak current for 7 strokes was 33 kA. Pinto et al.[14] found that there is no dependence of the peak current on the surface electric field and the length of triggering wire.

About 50 lightning flashes have been triggered in China since 1989. A distinct difference in discharge processes of triggered lightning flashes has been found in southern and northern China[15―19]. Liu et al.[15,16] measured the discharge current of artificially triggered

lightning during single-cell-storms in Gansu Province, and found that the triggered lightning was characterized by only continuous current stage with a peak current of several hundreds of Ampere and a duration of several hundreds of milliseconds. The lightning flash was trig-gered successfully only in a condition of positive surface electric field because of a special charge structure of thunderstorm in Gansu Province. In the contrary, light-ning was triggered in a condition of negative surface electric field and included several return strokes with large electric current in Jiangxi, Shanghai and Guang-dong Province. Current waveform in microsecond time resolution for a 2-stroke triggered lightning flash once documented in Guangdong in 1999, but the current dur-ing return stroke was saturated[20], and there were no simultaneous electromagnetic field records either.

In the summer of 2005, artificially triggering light-ning experiment, by using the special designed rockets same as in Guangdong 1999, was conducted in Binzhou, Shandong, where the severe thunderstorm passes over quite often. The data of discharge current in microsec-ond time resolution, the corresponding close electric and magnetic fields, and optical images of discharge channel for the triggered lightning flashes have been documented. This paper reports the characteristics of discharge cur-rent and close electric field changes of these flashes.

1 Experiment and instruments

The artificially triggering lightning experiment site is located in Binzhou (117.8°E, 37.7°N) of Shandong Province, middle latitude region of eastern China. The severe thunderstorm passes over this region frequently in the summer season because of the interaction of southern and northern weather systems. Figure 1 shows a picture of Shandong artificially triggering lightning experiment (SHATLE) site in 2005. The lightning cur-rent is measured at the base of the discharge channel by two Rogowski coils with a bandwidth of 300 Hz-1 MHz. The current values up to 2 kA and 100 kA can be meas-ured by two coils, respectively. A 1 mΩ-shunt is used in parallel allowing a peak current of 100 kA to be meas-ured. The signals from the coils and shunt are transmit-ted through optical fibre to an 8-channel oscilloscope in a control room which is 60 m away from the launching site for recording. The three current sensors and E/O converters are put inside a Frandy Cage in the middle of the picture. A copper lightning rod with a height of 5 m

QIE XiuShu et al. Sci China Ser D-Earth Sci | August 2007 | vol. 50 | no. 8 | 1241-1250 1243

is fixed on the middle top of the Cage to attract the lighting flashes. Up to six rockets can be launched dur-ing the same storm event.

Figure 1 A picture of Shandong artificially triggered lightning experi-ment site in 2005.

Two observation sites were established during the

experiment. Site 1 (the control room) was located 60 m away from the launching site, while Site 2 (the main observation site) was located 550 m away. Field mill, slow antenna and fast antenna were installed at both sites. The filed mill with a measuring range of 0±50 kV·m−1 was used to monitor continuously the electric field of thunderstorm and reflect roughly the electric field change of lightning discharge. Its output signals were digitized by an A/D converter with an 8-bit resolu-tion and recorded at a sampling rate of 10 Hz. Slow and fast antenna, with a bandwidth of 2 MHz and 5 MHz and a time constant of 6 s and 2 ms, respectively, were used to measure the fine electric field changes of light-ning discharge in a high time resolution. The signals of electric field changes were digitized with a 2.5 MHz sampling rate and 16-bit amplitude resolution. The sig-nals at two sites were recorded in computers and syn-chronized by GPS in a time resolution of 50 ns. A mag-netic field sensor was also installed at Site 1. In addition, an 8-channel oscilloscope, working at a sampling rate of 1 MHz and a memory of 4 Mb·ch−1, was located in Site 1 to record simultaneously the current, magnetic field and electric field changes. A high speed digital camera with 1 ms time resolution and spectrum measuring sys-tem were installed at Site 2.

Five lightning flashes were successfully triggered during the passage of two severe thunderstorms on June 28 and August 1―2, 2005. A total of 9 rockets were

launched, and the overall efficiency of successfully triggering is 55.6%. Five triggered lightning flashes are named 0501, 0502, 0503, 0504, and 0505, respectively. The first 2 occurred on June 28, and late 3 occurred on August 2. Among them, the 0501, 0502 and 0503 used classical triggering method, and 0504 and 0505 used altitude triggering method. Five discharges were all triggered in a surface electric field environment con-trolled by negative charge overhead, and were negative discharges. The flash 0503 struck at the lightning rod. Its current and corresponding electric field changes at 60 m and 550 m were obtained. The other three did not strike at the lightning rod, and only the electric and magnetic fields were documented.

2 The characteristics of radar echo and surface electric field of the severe storms

Doppler radar at Qihe, Shandong, 130 km away from the lightning triggering site, indicated that the two severe thunderstorms on June 28 and August 1―2 were severe mesoscale convective system, and moved from south-west to northeast. The horizontal ranges of two storms were all greater than 200 km in their mature stage. The storm on June 28 lasted about 4.5 hours and its center passed over the site, while that on August 1―2 lasted longer than 10 hours and its north edge passed over the site. The storms were weakening when they passed over the triggering site. The maximum radar echo was about 50 dBz over the site during the period of lightning trig-gering, and the corresponding cloud top of two storms was 12 km and 11 km high, respectively.

Figure 2 shows two radar echo pictures for the storm on August 1―2 in different developing stages. At 16:00 (Beijing Time) of August 1, several small convective cells appeared in the middle-south of Hebei Province, and merged into a severe convective system when it was moving northeastwards. At 20:30, the system entered Shandong Province, and the echo area increased and featured obviously with severe mesoscale convective system. The strong convective echo was in a shape of bow, followed by a large-scale stable precipitation area (see Figure 2(a)). The strongest radar echo was at 23:29 with the maximum intensity of 60 dBz and a cloud top of over 15 km. The field mill started to show obvious electric field and lightning activities at 23:12. At 00:00 on August 2, the north edge of the storm arrived at the

1244 QIE XiuShu et al. Sci China Ser D-Earth Sci | August 2007 | vol. 50 | no. 8 | 1241-1250

Figure 2 Two radar echo pictures for the storm on August 1―2. (a) During the mature stage; (b) during the period of passing over the site. The center corresponds to the radar location at Qihe. The interval between two neighbor circles is 50 km, and “+” indicates the lightning triggering site.

triggering site (see Figure 2(b)). Referred to the evolu-tion of surface electric field, three lightning flashes were successfully triggered at 00:04, 00:19 and 00:24 during the passage of the storm, named 0503, 0504 and 0505. The maximum echo just over the site was 45―50 dBZ during the lightning triggering, and the strongest echo of the storm was in the southeast of the site.

Figure 3 shows the evolution of surface electric field during the storm was passing over the site on August 1―2. The pulses in the waveforms are caused by lightning discharges. The maximum flash rate inferred from these pulses is about 20 fl·min−1 (the detection range of field

Figure 3 Evolution of surface electric field during the storm was passing over the site on August 1―2.

mill is just about 20 km, and the total flash rate of the storm should be much higher). The three triggered light-ning flashes are marked by arrows, and the corresponding surface electric fields are −4―−8 kV·m−1.

3 Characteristics of current waveform and electric field change of the triggered lightning flashes

Table 1 shows the basic features of the 5 artificially triggered lightning flashes. Each flash included 1 to 10 return strokes with flash 0503 having a maximum of 10 strokes and flash 0505 a minimum of 1 stroke. The left panel in Figure 4 shows the still picture of flash 0503 taken at Site 1 at a distance of 60 m. The picture shows that the discharge channel is blown from left to right because of the horizontal wind, just like that taken by streaking camera. The smooth bright channel on the left results from the explosion of wire by the upward leader current, and the 10 channels correspond to 10 return strokes. The right panel in Figure 4 shows the still pic-ture of flash 0504 taken at a distance of 550 m. This flash used the altitude triggering method with an 80 m-Nylon below the bottom of the wire, and the dis-charge channel near the ground is in and out as a result. The flash included 5 return stokes. The flashes 0501 and 0502 used the classical method, and included 7 and 5 strokes, respectively. The flash 0505 used the altitude triggering method, and included 1 stroke.

QIE XiuShu et al. Sci China Ser D-Earth Sci | August 2007 | vol. 50 | no. 8 | 1241-1250 1245

Table 1 Basic features of the 5 artificially triggered lightning flashes

No. Triggered time

Triggering method

Flash polarity

Stroke number

Lasting time(ms)

0501 22:29:25 June 28

classical − 7 837.6

0502 22:33:30 June 28

classical − 5 732.0

0503 00:04:02 August 2

classical − 10 1120.0

0504 00:19:40 August 2

altitude (80 m-nylon) − 5 750.4

0505 00:24:12 August 2

altitude (80 m-nylon) − 1 518.0

Figure 4 Pictures of two triggered lightning flashes. Left: Flash 0503 taken at 60 m; right: Flash 0504 taken at 550 m.

The average lasting time of 5 triggered flashes is 791

ms with a maximum of 1120 ms and a minimum of 518 ms. The average lasting time of natural negative CG flash is usually several hundred milliseconds. The last- ing time and return stroke number of 5 triggered flashes are similar to the natural ones[21]. There are no obvious differences either in lasting time and stroke number be- tween two triggering methods based on Table 1.

3.1 The characteristics of current waveform of triggered lightning flash

The flash 0503 struck at the lightning rod, and the cur- rent waveforms of 10 strokes have been well recorded.

Figure 5 shows the current waveform of 7th stroke with a median current value. The peak current Ip of this stroke was 18.1 kA. The time between half peak value thpw was 30 μs, and the rising time from 30%―90% peak t30%―90% was less than 1.2 μs.

Table 2 shows the statistic results of current wave-form for 10 strokes. The geometric mean value is 11.9 kA (the average value is 12.6 kA) with a maximum of 21.0 kA and a minimum of 6.6 kA. The result is in good agreement with those of triggered flashes by Rakov et al.[7], Fisher et al.[22] and Pinto et al.[14], and of natural flashes by Anderson et al.[23] and Berger et al.[24]. The half-peak width value thpw is 39 µs, which is 4 times lar-ger than the typical values by Rakov et al.[7], and 30 times larger than that by Saba et al.[4] for altitude trig-gered lightning. Saba et al. suggest that the higher peak current usually indicates a shorter duration of the strokes. Although the peak current in our case is similar to others, the duration is longer. Thus the destructivity might be larger due to its long effect on the object. The rise time from 30% to 90% peek t30%―90% in our case is 2 times higher than others. The possible reason is that we used a lower sample rate of 1 μs, and the value has been over-estimated.

The geometric mean value of neutralized charge for 10 return strokes is 0.5 C (average value is 0.58 C), and the total neutralized charge by the flash is 5.8 C. Figure 6 shows the relationship between peak current Ip (kA) and the neutralized charge Q (C). The two parameters follow a relationship below: Ip=18.5Q0.65. (1) Curve 1 in Figure 6 corresponds to the result from this study, and curve 2 is from Berger et al.[24] (Ip=10.6Q0.7). Our result is obviously biased from Berger’s. The reason is that Berger et al. used a total charge transported to

Figure 5 Current waveform of 7th stroke in flash 0503. (a) Whole waveform; (b) time expanded waveform.

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Table 2 Characteristic parameters of current waveform for 10 strokes of flash 0503

Reference Location Sample Type Ip (kA) σ log

Q (C ) σ log

t30%―90% (µs) σ log

tHPW (µs) σ log

This study Binzhou 10 ATF 11.9 0.2

0.5 0.2

0.8 0.1

39.0 0.1

Rakov et al.[7] Florida 64 ATF 14.5 0.2

− −

0.2 0.29

10.5 0.3

Fisher et al.[22] Alabama 37 ATF 11.0 −

− −

0.25 −

17.0 −

Florida 8 ATF 15.0 −

− −

0.46 −

23.0 −

Pinto et al.[14] Southern Brazil 9 ATF 17.0 −

− −

− −

− −

Southern Brazil 12 SNF 10 −

− −

− −

− −

Anderson & Erikssion[23] Sweden 114 SNF 12.0 0.2

− −

0.4 0.4

− −

Berger et al.[24] Sweden 135 SNF 12.0 0.2

0.95 −

− −

− −

The results are geometric mean values, and σ log is the standard deviation of the logarithm (base 10) of the parameter. ATF: Artificially triggered flash; SNF: subsequent stroke of natural flash.

Figure 6 The relationship between the peak current Ip (kA) and the cor-responding neutralized charge Q (C). Curve 1 is from this study, and curve 2 is from Berger et al.[24]. ground in 1 ms from the start of return stoke. In fact, the return stroke usually finishes in several to several tens of microseconds, Berger et al. might consider part of the neutralized charge during the continuing current stage into return stroke.

3.2 The signatures of electric field change at differ-ent distances

Figure 7 shows the discharge current at the base of the discharge channel and corresponding electric field changes at 60 m and 550 m during the second dart stepped leader and return stroke stages. The electric field changes in the left panel are from slow antenna. The electric field changes at 60 m and 550 m produced by the leader-stroke sequence are characterized by a V-shaped structure, with the bottom of V-shape corre-sponding to the end of leader process and the start of

return stroke. The electric field change EL is produced by the downward dart leader, and is mainly composed of static electric field. The geometric mean value of EL for 10 strokes is 17.8 kV·m−1 and 1.2 kV·m−1 (the arith-metic average value is 18.5 kV·m−1 and 1.24 kV·m−1 at 60 m and 550 m, respectively. The ER is produced by the return stroke, and its mean value is 16.7 kV·m−1 and 1.65 kV·m−1 (the average value is 18.0 kV·m−1 and 1.59 kV·m−1) at 60 m and 550 m, respectively. The electric field changes caused by dart leader and return stroke are similar at 60 m, indicating that the return stroke nearly neutralized all the charges deposited in the channel by the dart leader. Based on the source point charge model[25], the linear charge density deposited by the dart leader is estimated to be 6.3×10−5 C·m−1, and the left linear charge density after return stroke is 1.7×10−6 C·m−1.

In the figures of fast electric field changes (right panel), Er corresponds to the peak radiation field of return stroke. Table 3 shows part of the waveform pa-rameters of the radiation field. The geometric mean value of electric field change is 5.9 kV·m−1 (arithmetic average value is 5.4 kV·m−1) at 60 m, and that is 0.39 kV·m−1 (0.45 kV·m−1) at 550 m. Based on the electric fields at two distances, the relationship between the peak electric field change Er (kV·m−1) and distance r (m) can be drawn as Er = 528r −1.119. (2)

If the ground is regarded as an ideal conductor, the radiation field should decrease with distance r as r −1.

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Figure 7 Discharge current at the base of the discharge channel and corresponding electric field changes during the second dart stepped leader and return stroke processes at 60 m and 550 m. The left panels are in 1 ms time scale, and the right panels are in 0.09 ms time scale.

Table 3 Characteristic parameters of radiation field of return stroke

R (m) ΔEr (kV·m−1)

σlog t30%―90% (μs)

σlog tHPW (μs)

σ log

5.9 0.4 2.5 60

0.12 0.01 0.17 0.39 0.5 38.8

550 0.2 0.4 0.58

The results are geometric mean values, and σ log is the standard devia-tion of the logarithm (base 10) of the parameter. Our result here decreases faster. This is because the real ground has a limited conductivity. According to Cooray et al.[26], the ground conductivity is 10−2―10−3 S·m−1. In addition, the real ground surface is not even, and at-tenuation effect during the transmission of radiation field should not be neglected.

To illuminate the similarities of the intensity of artifi-

Table 4 Discharge current of return stroke in triggered and natural light-ning[14]

Type of return stroke Sample Peak current(kA)

Downward first stroke 29 45 Downward subsequent stroke 59 18 Natural lightning

Upward stroke 12 10 Classical triggered stroke 9 17

Triggered lightningAltitude triggered stroke 7 33

cially triggered lightning and natural lightning, Table 4 shows a summary of discharge current obtained in the same area of southern Brazil for different types of return strokes[14]. One can see that the first-stroke in downward natural flashes is the most intense with a value of 45 kA, followed by the strokes in the altitude triggered flash with a value of 33 kA. Peak currents of subsequent

1248 QIE XiuShu et al. Sci China Ser D-Earth Sci | August 2007 | vol. 50 | no. 8 | 1241-1250

strokes of downward natural flashes and strokes in clas-sical triggered flashes are of the same order. Upward stroke of natural flash is less intense than the other strokes. There are no distinct differences between the triggered and natural flash in intensity on average, and the discharge parameters of the triggered lightning can be used in the protection design of natural lightning.

4 Return stroke velocity and estimation of peak current of return stroke

The propagation speed is an important parameter for the understanding of return stroke development, and is a key point for return stroke model as well. Idone and Or-ville[27] obtained a speed of 1.2×108 m·s−1 for natural subsequent stroke using a streaking camera. Wang et al.[12] obtained the stroke speed of artificially triggered lightning using ALPS with a time resolution of 0.1 µs and a spatial resolution of 60 m. He found that it was 1.3×108 m·s−1 and 1.5×108 m·s−1 for two strokes below 60 m. To estimate the stroke speed in the artificially trig-gered lightning, the radiation field and discharge current are assumed to follow the well-accepted transmission line model[28―30]. The peak radiation field Er and peak current Ip follow the relationship below:

20

,2r P

vE Ic rπε

= (3)

where c is the speed of light, v is the speed of stroke, r is the horizontal distance between observation site and the discharge channel. The speed of return stroke is es-timated to be (1.1―1.6)×108 m·s−1 with an average value of 1.4×108 m·s−1 by using the peak current and peak radiation field at 60 m of flash 0503, which is in agreement with that by Idone and Orville[27] and Wang et al.[12].

According to the stroke speed above, the peak current can be estimated for other four triggered lightning flashes by using formula (3). Figure 8 shows the current distribution for 22 return strokes including 10 measured and 12 estimated values. The geometric mean value is 11.0 kA (average value is 11.7 kA). This result is quite similar to that of natural subsequent strokes by Anderson and Eriksson[23] and Berger et al.[24].

5 Conclusion and discussion

The surface electric field has been used as a main refer-

Figure 8 The distribution of peak current for 22 triggered subsequent strokes. ence of thunderstorm electric intensity in the experiment of lightning triggering. However, the comparison of electric fields aloft and on surface shows that this refer-ence could be unreliable[31,32]. Under a condition of se-vere thunderstorm, corona ions are released from natural points on the ground, and a screening layer with a thickness of several hundreds meters forms near the ground. The surface electric field, affected greatly by this layer, recovers the lower value very quickly after a lightning flash, and then stabilises between 4― 10 kV·m−1 until the next flash. So the surface E-field is not a reliable indicator for triggered lightning. To increase the successful triggering efficiency, it is very important to infer the intrinsic electrification of the storm. Mean-while, the triggering efficiency is affected by the high flash rate during the severe storm. Our experiment dur-ing two severe storms shows an average triggering effi-ciency of 55.6%, indicating that the triggering can be successful when the triggering time is chosen carefully. The five triggered lightning flashes occurred during the weakening period of the storm, and among them, the 3 flashes occurred in the edge of the most intense echo. It will be interesting to study the relationship between successful shooting and developing stage and intensity of storm when having enough cases.

The serious lightning-induced damage usually caused by the severe thunderstorm because of the very frequent lightning activities in the severe storms. The under-standing of discharge parameters is significant for the lightning protection. Especially, the discharge current and close electric and magnetic fields are very important for understanding of lightning mechanism and evalua-tion of lightning-induced effects in various electric cir-

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cuits and systems. Although the flash rate can be as high as several tens flash per second, the possibility of light-ning striking at a certain object is quite low. The artifi-cially triggered lightning provides a very effective tool for direct measurement of these parameters. Although the whole current waveform is measured only in one triggered flash, the waveform for 10 strokes is similar to the natural subsequent strokes. The 5 triggered flashes included 1 to 10 strokes. The average peak current was 11.9 kA with a variation range of 6.6 kA to 21.0 kA. The half peak width was 39 µs, which is much larger than the usual value in other places, and indicating the de-structivity might be larger due to its long effect on the object. The peak current Ip (kA) and neutralized charge Q(C) has a relationship of Ip = 18.5Q0.65.

The statistic analysis on 22 strokes in 5 triggered flashes shows that the radiation field produced by return stroke was 5.9 kV·m−1 and 0.39 kV·m−1 at 60 m and 550 m, respectively. The radiation field decreases with dis-tance r as r−1.119. The estimated speed of return stroke is 1.4×108 m·s−1 with a variation of (1.1―1.6)×108 m·s−1 based on the transmission line model. The geometric mean value of current for 22 return strokes, including 10 measured and 12 estimated values, is 11.7 kA.

Because of the unpredictability of striking location and occurring time of natural flash, the discharge current

is very difficult to be measured. Traditionally, the most important database of lightning current is that published by Berger et al.[24], which was obtained at an instru-mented tower on Monte San Salvatore, Switzerland. Recently, Measurements at Morro do Cachimbo Station in Southeastern Brazil provided complementary data in the tropical region[14]. Triggered lightning experiments will enlarge the database with information regarding subsequent strokes because of the similarities of current waveform and radiation field between artificial and natural lightning. This study enriches the understanding of lightning discharge in the severe thunderstorm and in the middle latitude region. With the simultaneous meas-urement of electric and magnetic field at different dis-tances, triggered lightning will contribute greatly to the understanding of lightning physics, the designing of lightning protection and the establishment of lightning standard. The triggering technology and measuring in-struments will be modified continuously in the future to increase the efficiency of successfully triggering and the precision of striking at the ground facilities, thus to ac-cumulate more data.

The authors would like to thank Liu Xinsheng, Zhang Yijun and Guo Changming for their fruitful discussions. Special thanks to the Key Labo-ratory of high voltage and Isolation of Wuhan University for the technical support in calibration of the current sensors.

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