radiation field pulses associated with the initiation of positive cloud to ground lightning flashes
TRANSCRIPT
Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047–1055www.elsevier.com/locate/jastp
Radiation (eld pulses associated with the initiation of positivecloud to ground lightning *ashes
Chandima Gomesa ;∗, Vernon CooraybaDepartment of Physics, University of Colombo, Colombo 03, Sri Lanka
bInstitute of High Voltage Research, Uppsala University, Sweden
Received 24 March 2003; received in revised form 11 February 2004; accepted 25 March 2004
Abstract
Seventy one electric (eld pulse trains that occurred during millisecond-scale time intervals before positive cloud to groundlightning *ashes were analysed. These pulses are bipolar in nature and somewhat similar in pulse characteristics to thebreakdown pulses preceding negative cloud to ground lightning. However, in the case of these positive *ashes, the pulsecharacteristics of the pulse trains are con(ned in a much wider range of values than those of the pulse trains associated withnegative return strokes. The leading edge of the pulses of the most commonly observed pulse trains that precede positivereturn strokes are relatively smooth, thus, di6erent from their counterparts associated with negative *ashes, in which case afew narrow pulses are superimposed on the rising edge of the bipolar pulses. Considering the initial polarity of pulses, fourtypes of bipolar pulse trains preceding positive return strokes were identi(ed. For each type of pulse trains, statistics of pulsecharacteristics were given. In contrast, in the case of negative ground *ashes, the bipolar pulse trains were almost alwayscomposed of pulses of the same polarity as that of the succeeding return stroke. The possible causes of the observation ofseveral types of pulse trains and the signi(cantly diversi(ed pulse characteristics of the breakdown pulse trains of positive*ashes were discussed. The frequency spectrum of the electric (elds of the most common type of pulse trains was comparedwith the spectrum of the breakdown pulses of negative *ashes and those of negative return strokes. This spectrum of thepreliminary breakdown pulse trains of positive ground *ashes is comparable with that of the preliminary breakdown pulsetrains of negative ground *ashes.c© 2004 Elsevier Ltd. All rights reserved.
Keywords: Positive; Lightning; Radiation; Bipolar; Spectrum
1. Introduction
Bipolar pulse trains that occur before the (rst returnstrokes of cloud to ground (CG) lightning have been ob-served by researchers for many years (Clarence and Malan,1957; Norinder and Knudsen, 1957; Weidman and Krider,1979; Beasley et al., 1982; Brook, 1992; Ogawa, 1993;Gomes et al., 1997; Ushio et al., 1998). In most of thesepublications, analysis were done only on pulse trains pre-ceding negative return strokes. These electric (elds were
∗ Corresponding author. Fax: +94-11-2583810.E-mail address: [email protected] (C. Gomes).
described as due to the breakdown process that takes placeinside the cloud between oppositly charged regions, that (-nally extends a channel towards the ground to make a CG*ash. Hereafter, we term these electric (elds negative break-down pulse (NBP) trains, if they precede negative returnstrokes and positive breakdown pulse (PBP) trains, if theyprecede positive return strokes. The main characteristics ofthe bipolar pulses in these pulse trains that can be found inthe literature are as follows. A train of bipolar pulses oc-curs a few milliseconds to a few tens of milliseconds (sometimes more than 100 ms) before the (rst return stroke in aCG *ash. In most of the cases, bipolar pulses in NBP trainshave an initial polarity similar to that of the return strokesucceeding them. Hereafter, negative polarity is assigned to
1364-6826/$ - see front matter c© 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.jastp.2004.03.015
1048 C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047–1055
the (eld change due to the raising of negative charge awayfrom earth or lowering of positive charge towards earth(i.e. the atmospheric electricity sign convention). Weidmanand Krider (1979) have characterised the NBP trains, byanalysing a large sample of *ashes recorded in Florida. Theinitial polarity of each pulse in a given sequence tends tobe the same. A few fast narrow pulses have been super-imposed on the initial half cycle. The bipolar overshootis smaller in amplitude compared with the initial peak, inmost cases. The frequency spectrum of few pulses of NBPtrains has been given in Weidman et al. (1981), Weidmanand Krider (1986) and Rakov et al. (1996). The time du-ration of the preliminary activity is given by Beasley et al.(1982).
PBP trains were previously studied by Gomes et al. (1997)and Ushio et al. (1998). The sample size (5 pulse trains) ofGomes et al. (1997) was not statistically suIcient to makeany conclusions quantitatively, on the characteristics of ei-ther individual pulses or pulse trains. Ushio et al. (1998)have analysed 19 PBP trains pertinent to winter thunderstorms. They have conducted their measurements at theHokuriku coast in Japan. The main results of the analysisof Gomes et al. (1997) and Ushio et al. (1998) are given inthe discussion of this paper.
The main objective of the present study is to investigatethe nature and temporal characteristics of the PBP trains,of summer thunder storms in temperate regions. This infor-mation will be very useful in understanding the initiatingmechanism of positive ground *ashes. We employ a sta-tistically signi(cant sample, obtained during several frontalthunder storms in Sweden. The results are compared withthe characteristics of PBP trains recorded in Japan, by Ushioet al. (1998), the only other study on PBP trains. We alsocompare our results with the characteristics of NBP trainsrecorded in Sweden and in elsewhere. Based on our observa-tions, we discuss possible causes of the PBP trains. We alsogive the frequency spectrum of a set of PBP trains pertinentto distant located ground *ashes. As the study of Ushio etal. (1998) is based on winter lightning and the sample sizeof the study of Gomes et al. (1997) is small, this is the (rstdetailed study on PBP trains, pertinent to summer thunderstorms.
2. Experiment
The measurements were conducted in Uppsala (lati-tude 59.8N and longitude 17.6E), Sweden during severalfrontal thunder storms in the periods of June, 1993 andJune–August 1996. The measuring station is situated about70 km inland of the Baltic sea. Only the positive *asheswith breakdown pulse activity were recorded. Hence, thedata sets are selective. The approximate range of distancesto the lightning strike location from the measuring sta-tion was 50–200 km as recorded by the LLP system inSweden.
The vertical electric (elds were sensed by a *at plate an-tenna of which the capacitance to ground is 58 pF and thephysical height is 1:88 m. After passing through a bu6erampli(er (an operational ampli(er and a RC circuit which,together, acts as an active integrator), the signal was directlyfed to a transient recorder, through a properly terminatedand double screened 50 J coaxial cable of few meters. Therise time of the system is determined by the gain bandwidthproduct and the slew rate of the operational ampli(ers usedin the bu6er circuit. These parameters were measured bysimulating the antenna by a 58 pF capacitor and applying asquare wave pulse to the system. The 10–90% rise time ofthe output of the antenna system for the step input voltagewas less than 20 ns (according to the manufacturer’s infor-mation, the risetime response of the operational ampli(er is10 ns). The decay time constant of the system is determinedby the time constant of the RC circuit connected at the inputstage of the operational ampli(er. The calculated decay timeconstant was about 20 ms. We checked the validity of thisestimation by applying a step voltage input to the system.We found that the decay time does not deviate very muchfrom 20 ms, during the operation. This time constant wasmuch longer than the duration of the radiation pulses (fewto few tens of microseconds) observed in this study. Notethat, it is not our intention to investigate the static (eld char-acteristics of PBP trains, which may be several times longerthan the decay time constant of our measuring system. Dueto the same reason given above, we did not (lter the 50 Hznoise of the power system that has been interfered with ourrecorded waveforms. This measuring system was recentlycalibrated, by placing the *at plate antenna in a known (eld(in between a large wire mesh electrode, raised to a highpotential, by connecting to a Max generator and a groundplane). However, we do not discuss the absolute amplitudesof pulses of the entire sample, as several *ashes were notlocated.
The recording system consists of a LeCroy (1993 se-ries) transient recorder with 1-Megabyte memory. The tran-sient recorder was operated in the pre-trigger mode. Thewaveforms were continuously and selectively (only positiveground *ashes with PBP trains) recorded for 500 ms froma single negative trigger (sign convention as described inthe Introduction). The sampling period was 50 ns and thepre-trigger delay time was set either 100 or 200 ms. Mostof the (eld traces recorded were triggered by the PBP train.Seventy-one positive CG *ashes with PBP trains are anal-ysed in this paper. In addition to these 71 cases, in one ofthe records, we have found that a pulse train very similar toa PBP train (which will be described in the next section),which does not precede a return stroke. We give a descrip-tion of this *ash, too. The 16 breakdown pulse trains ofnegative *ashes, which have been used to calculate the fre-quency spectrum, for the comparison, were recorded at thesame location with the same antenna and recording systemstwo of these negative *ashes were recorded in 1996 whilethe other 14 were recorded in 1998.
C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047–1055 1049
3. Results
Most of the positive return strokes considered in this studywere preceded by pulse trains with well structured bipolarpulses. Four types of PBP trains were identi(ed in the anal-ysis: 57 *ashes contained PBP trains with pulses of negativeinitial polarity (henceforth termed type a). Six PBP trainshad two distinct regions. The (rst region consists of pulseswith positive initial polarity and the second with pulses ofnegative initial polarity (henceforth termed type b). FivePBP trains contain pulses of positive initial polarity (hence-forth termed type c). In 3 cases, the PBP trains consist ofpulses with irregular initial polarity and an erratic structure(henceforth termed type d). The labelling of pulse types asa, b, c, and d has been done by considering the descend-ing order of the number of pulse trains that belongs to eachcategory.
Fig. 1 shows a type a PBP train, which is themost commontype of pulse trains that precede positive CG *ashes. The*ash was struck approximately 80 km from the site. As itcan be seen in Fig. 1b, the leading edge of the initial halfcycle of the pulses is relatively smooth compared to that ofthe pulses in the other types of PBP trains (described below)
0.1 0.3 0.5 0.7 0.9 1.1Time (ms)
-1.75
-1.25
-0.75
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Ele
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ld(V
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0.25 0.3 0.35 0.4 0.45 0.5 0.55Time (ms)
-1.75
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Ele
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ld (
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Fig. 1. (a) Flash No. 960724.05 A part of a type a PBP train.(b) Same (eld in an expanded time scale. A positive (eld changede*ects upwards.
and NBP trains. Fig. 2 depicts the distribution of the pulsecharacteristics of type a PBP trains. The duration of the pulsetrain was de(ned as the time between the regions of pulseactivity at the beginning and at the end of the pulse trainthat have amplitudes of 10% of the maximum amplitude. Inall cases, this value was above the background noise level.However, as the pulse amplitude attenuates with distance,the pulse train duration given in this study may be a lowerestimation for the distant *ashes than that for the nearby*ashes. T1 and T2 are approximate duration of the (rst halfcycle and the second half cycle, respectively, of an individ-ual bipolar pulse. Thus, T1 + T2 is approximately equal tothe total pulse width. Pulse separation is the time intervalbetween the crests of two adjacent bipolar pulses. Pulseswith amplitude less than 10% of the maximum amplitudeare neglected in estimating the pulse separation, as they arehard to be distinguished from the background noise. In eachpulse train, 5 adjacent pulses were selected from the mostactive region (usually the (rst (ve pulses), to calculate themean values of T1, T2 and the pulse separation. Apart frombeing convenient in the analysis, there is no signi(cance inlimiting the number of pulses to 5. The criteria used to ob-tain T1, T2 and the pulse separation, may give higher valuesfor each of these parameters, compared to the case where allthe pulses of a PBP train are taken into account in the sta-tistical analysis, as narrow and low amplitude pulses can beomitted. PBP-RS separation is the time duration between thehighly active region of the pulse train and the return stroke.T1 and T2 have the same mean value of 19 �s. The meanpulse width is 38 �s. The pulse separation, the pulse trainduration and the PBP-RS separation have the mean values of96 �s, 3 and 56 ms, respectively. All the above mean valuesdi6er considerably from the mode value of the correspond-ing parameters. In cases of the pulse separation, the pulseduration and the PBP-RS separation, one may see that thedistributions are double peaked. The arrows in the charts ofFig. 2 are pointed to the columns, to which the mean valuesof the corresponding parameters of several studies belong.
Fig. 3 illustrates a type b PBP trains. The (rst region haspulses of positive initial polarity and the second region haspulses of negative initial polarity. The ground *ash of thePBP train in Fig. 3 has not been located. The two regionswith pulses of opposite polarity are given in an expandedtime scale. Pulses in the (rst region are very similar to thoseof NBP trains, in pro(le. A few narrow pulses are superim-posed on the leading edge of the (rst half, which is a typicalcharacteristic of pulses in NBP trains. The pulse pro(le ofthe second region is somewhat similar to that of type a PBPtrains. However, in several type b pulse trains, one or twonarrow unipolar spikes are superimposed on the (rst half offew pulses. Typically, the magnitude of the pulses, in the(rst region, is larger than that in the second region (Fig. 3).
Table 1 delineates the statistics of PBP trains with tworegions of pulses with opposite polarity. The last row showsthe mean of the characteristics, of the six cases. The sepa-ration between the two pulse regions is de(ned as the time
1050 C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047–1055
0-5
6-10
11-1
5
16-2
0
21-2
5
26-3
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31-3
5
36-4
0
T1: Width of the first half cycle (µs)
0
2
4
6
8
10
12
14
16N
umbe
r of
PB
pul
se tr
ains
n= 57X= 19 ∝ sS= 9 ∝ s
N1
P1
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ber
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ulse
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n= 57X= 19 ∝ sS= 9 ∝ s
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ulse
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n= 57X= 38 ∝ sS= 16 ∝ s
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31-4
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ulse
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ns
n= 57X= 96 ∝ sS= 56 ∝ s
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ulse
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ns
n= 57X= 3.0 msS= 1.8 ms
N1
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n= 57X= 56 ms S= 48 ms
P1N2
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(a) (d)
(b) (e)
(c) (f)
Fig. 2. Distributions of pulse characteristics of type a PBP trains. (a) Width of the initial half cycle of pulses (T1). (b) Width of the secondhalf cycle of pulses (T2). (c) Width of pulses (T1 + T2). (d) Separation between individual pulses. (e) Pulse train duration. (f) Separationbetween the pulse train and the succeeding return stroke. The arrows indicate the mean values of characteristics reported in several studies.P1: Present study of type a PBP trains; P2: Ushio et al. (1998) study of PBP trains; N1: Gomes et al. (1997) study of NBP trains andN2: Weidman and Krider (1979) study of NBP trains. The other abbreviations are, n: Number of pulse trains; X : Mean value; S: Standarddeviation.
duration between the most active places of the two regions.In several cases, there exists a low active period in betweenthe two pulse regions. In the (rst region T1, T2, T1 + T2 and
Pulse separation have the mean values of 13, 13, 27 and62 �s, respectively. In the second region, the above valuesare 12, 14, 26 and 38 �s, respectively. The mean pulse train
C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047–1055 1051
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Fig. 3. Flash No. 960724.04 A type b PBP train. (a) The entirepulse train. 1: First region with pulses of positive polarity, 2:Second region with pulses of negative polarity. (b) A part of the(rst region and (c) a part of the second region, in expanded timescales. A positive (eld change de*ects upwards.
duration of the (rst region and the second region are 1.3 and2:8 ms, respectively. The mean PBP-RS separations for thetwo regions are 81 and 77 ms, respectively. The pulse char-acteristics of both regions are similar to those of NBP trains(except for the polarity reversal in the second region).
Fig. 4 shows a part of a type c PBP train. Table 2 showsthe pulse characteristics of this type of PBP trains. The pulse
characteristics, T1, T2, T1 + T2 and Pulse separation havethe mean values of 16, 15, 31 and 51 �s, respectively. Thepulse train duration and the PBP-RS separation have meanvalues of 2.1 and 44 ms, respectively. Pulse characteristicsand pulse pro(le of this type of PBP trains is comparablewith those of NBP trains.
All 3 PBP trains with pulses of irregular polarity continueupto the return stroke. Fig. 5 epitomises one such case. Inall 3 cases, this irregular pulse activity is visible for a fewmilliseconds, even after the return stroke. However, Cooray(1984) has observed that in a number of positive returnstrokes, a pulse burst appeared immediately after the returnstroke, in which case, there was no pulse activity immedi-ately prior to the return stroke. Hence, one cannot con(-dently conclude, that the pulses, which appear after the re-turn stroke, in the 3 type d PBP trains, are continuations ofthe respective PBP trains. The time from the beginning ofthe PBP train to the return stroke, in these 3 cases are 33,27, and 22 ms. The width of these pulses is in the rangeof 5–15 �s, while the pulse separation is in the range of20–80 �s. Thus, in pulse train duration, pulse width andpulse separation, this type of PBP trains are considerably dif-ferent from chaotic pulse trains, that occur mainly in associ-ation with negative subsequent strokes (Gomes et al., 1998).
Fig. 6 (curve 1 and 2) shows the frequency spectrumof the electric (elds of 23 type a PBP trains normalisedto a distance of 50 km. The *ashes of these pulse trainshave occurred at a distance of 50–150 km. Thus, the higherfrequency part of the spectrum may be reduced due to thepropagation e6ects. The spectrum is calculated between 1and 500 kHz. The digitising resolution of the data set usedin this study is not adequate to analyse the spectrum above500 kHz. Curve 1 corresponds to the mean spectrum of theentire pulse trains and curve 2 shows the mean spectrum of30 individual pulses. The spectral values are given in dB,which are 20 times the logarithm of base 10 of the magnitudeof Fourier transform of the electric (eld strength (in V/m).The peak of the spectrum of pulse trains is −76 dB and itoccurs approximately at 10 kHz while that of the individualpulses is −88 dB and it occurs around 16 kHz. Curve 3 ofFig. 6 depicts the frequency spectrum of 16 NBP trains. Theelectric (elds in this case are also normalised to 50 km as ithas been done in the previous calculation. In this case, thepeak occurs at 28 kHz with a value of −81 dB.
4. Discussion
In this study, we have observed that similar to negativereturn strokes, positive return strokes are also preceded bya train of bipolar pulses. As we have selectively recordedpositive CG *ashes with PBP trains, we are not able to givestatistics on the positive return strokes that are not precededby PBP trains. More than 80% of the PBP trains consist ofpulses with negative initial polarity (type a). In the studyof Ushio et al. (1998), 17 PBP trains out of 19, consist of
1052 C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047–1055
Table 1Statistics of the pulse characteristics of type b PBP trains
First region Second region
Flash ID PBP-RS T1 T2 T1 + T2 Pulse PBP PBP-RS T1 T2 T1 + T2 Pulse PBP Separationseparation (�s) (�s) (�s) separation train separation (�s) (�s) (�s) separation train between(ms) (�s) duration (ms) (�s) duration two pulse
(ms) (ms) regions (ms)
P930629.05 17 13 18 31 55 1.2 14 14 24 38 35 6.2 2.5P930629.18 23 13 14 27 68 1.2 17 16 16 32 49 2.6 6.4P930629.21 195 17 11 28 72 2.0 187 19 14 33 54 2.8 8.0P960724.04 27 15 14 29 66 1.2 25 4 10 14 33 2.0 1.4P960726.02 215 16 16 32 60 1.2 212 10 11 21 28 2.3 2.6P960709.21 10 6 7 13 49 0.8 9 10 10 20 26 0.6 1.2Mean 81 13 13 27 62 1.3 77 12 14 26 38 2.8 3.7SD 88 4 4 6 8 0.4 87 5 5 9 10 1.7 2.6
SD: Standard deviation. The other abbreviations are de(ned in the text.
Table 2Statistics of the pulse characteristics of type c PBP trains
Flash ID PBP-RS separation (ms) T1 (�s) T2 (�s) T1 + T2 (�s) Pulse separation (�s) PBP duration (ms)
P930629.01 23 10 12 22 49 2.9P930629.22 89 16 19 35 56 2.9P930722.21 30 11 13 24 35 1.8P960723.23 40 19 14 33 56 1.1P960706.76 37 22 19 41 60 1.9Mean 44 16 15 31 51 2.1SD 23 5 3 7 9 0.7
SD: Standard deviation. The other abbreviations are de(ned in the text.
pulses of negative initial polarity, thus, at both location typea is observed as the most common type of PBP trains. Inthe study of Gomes et al. (1997), 3 PBP trains out of 5,belong to type b. Only one pulse train consists of pulses withnegative initial polarity (type a) while the other is composedof pulses of positive initial polarity (type c).
The type a PBP trains consist of bipolar pulses with arelatively smooth zero-to-peak rising edge (Fig. 1b). Thisis in agreement with the observation of Ushio et al. (1998).In contrast, in the case of NBP trains, few sharp pulses areobserved to be superimposed on the leading edge of mostof the bipolar pulses. In NBP trains, these narrow spikes areobservable of the pulses of *ashes that were recorded evenat 200 km. The pulse characteristics of type a PBP trainsare con(ned in a broad range of values with signi(cantlylarge standard deviations (Fig. 2). The study of Ushio et al.(1998) recon(rms this diversity of the pulse characteristicsof PBP trains. According to their analysis, in PBP trains, thepulse width ranges from 5 to 52 �s. The pulse separationand PBP-RS separation have values from 10 to 180 �s andfrom 1 to 38 ms, respectively. The mean values of these pa-rameters are given in Table 3. All these mean values are at
least about twice less than the corresponding mean valuesof our data. The arrow pointers of Fig. 2 clearly demon-strate this di6erence. One reason may be that Ushio et al.(1998) have taken all the pulses in each train in their anal-ysis, while we have considered only the (ve largest pulsesfrom each pulse train. Secondly, the disagreement in pulsecharacteristics may be due to the di6erence in the types ofthunder storms to which the two data sets belong.
The mean pulse characteristics of type a PBP trains areconsiderably larger than those values of NBP trains that havebeen observed in Sweden (Gomes et al., 1997) and some-what similar to that have been observed in Florida (Weidmanand Krider, 1979). A summary of the results of these twostudies on NBP trains is given in Table 4 along with theresults of the present study on type a PBP trains. The arrowpointers of the distribution charts (Fig. 2) indicate, that themean pulse characteristics of NBP trains of Swedish thunderstorms lie in the vicinity of the mode value of the correspond-ing charts of type a PBP trains. The mean pulse width andthe mean PBP-RS separation of type a PBP trains are com-parable with the same mean values obtained by Weidmanand Krider (1979) for NBP trains. The mean pulse
C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047–1055 1053
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time (ms)
-1
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0
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1
1.5
2
Ele
ctric
fiel
d (V
/m)
0.4 0.45 0.5 0.55 0.6 0.65
Time (ms)
-1
-0.5
0
0.5
1
1.5
2
Ele
ctric
fiel
d (V
/m)
(a)
(b)
Fig. 4. Flash No. P290793.22 (a) A part of a type c BP train. (b)Several pulses of the same pulse train in an expanded time scale.A positive (eld change de*ects upwards.
separation that was given in Weidman and Krider (1979), issomewhat higher than that of this study, but Fig. 2d showsthat there is a considerable number of pulse trains that be-longs to a range of pulse separation close to 130 �s. Thus, we(nd no clear distinction between the pulse characteristics ofNBP trains and those of the most commonly observed typeof PBP trains. Note that, the pulse characteristics of Ushioet al. (1998) are similar to those of NBP trains that werereported in Gomes et al. (1997).
One of the interesting observations of this study is the 6type b PBP trains. The electric (eld perturbation of thesepulse trains starts with pulses of positive polarity, of whichthe characteristics are very similar to that of NBP trains ob-served in Sweden. The amplitude of pulses gradually decaysand after a quiescent period of a few milliseconds, anotherpulse train with pulses of opposite initial polarity occurs.There is little di6erence in the pulse characteristics of thetwo regions other than the polarity reversal (Table 2). To-tally, type b constitutes about 12% of PBP trains, observedin Sweden (observations of both Gomes et al., 1997 andthis study). This type of pulse trains have not been reportedbefore, in connection with any lightning event.
4 4.5 5 5.5 6
Time (ms)
-1.5
-1
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0
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Ele
ctric
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4.5 4.55 4.6 4.65 4.7 4.75 4.8 4.85 4.9
Time (ms)
-1
-0.5
0
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1
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Ele
ctric
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d (V
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(a)
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Fig. 5. Flash No. P230793.07 (a) A part of a type d PBP train. (b)Several pulses of the same pulse train in an expanded time scale.A positive (eld change de*ects upwards.
Frequency (Hz)
-125
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-115
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-105
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-95
-90
-85
-80
-75
Ele
ctric
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ectr
um (d
B) 1
2
3
103 105104 106
Fig. 6. The mean frequency spectrum of (1) The electric (eldsof the 23 distant located type a PBP trains (2) Thirty individualpulses of type 1 PBP trains (3) Sixteen distant located NBP trains.The electric (elds are normalised to 50 km. The spectral valuesare given in dB, which are 20 times the logarithm of base 10of the magnitude of Fourier transform of the electric (eld strength(in V/m).
1054 C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047–1055
Table 3The pulse characteristics of PBP trains as given in Ushio et al. (1998)
No of pulses Mean Standard deviation Range of values
Pulse width (�s) 132 18.8 7.9 5–52Pulse separation (�s) 219 54.2 35.7 10–180PBP-RS separation (ms) 19∗ 12 1–38
The mean duration of the PBP trains has been estimated to be around 1 ms.∗No. of pulse trains.
Table 4
Parameter Weidman and Gomes et al. (1997) ThisKrider (1979) study
T1 (�s) 9 19T2 (�s) 14 19Pulse 41 23 38width (�s)Pulse 130 63 96separation (�s)NBP/PBP 1.7 3.0duration (ms)NBP/PBP-RS 82 26 56separation (ms)
In the (ve cases of type c PBP trains, the pulse character-istics are not much di6erent from that of NBP trains (Table3). The pulses of this type are also very similar in pro(leto those of the NBP trains observed at the same location.However, in the cases of type b, and type c PBP trains, thenumbers of events are insuIcient to provide a good com-parison with the previously observed data.
Another interesting outcome of this study is the obser-vation of the several types of pulse trains (4 types) withpulses of di6erent initial polarity. In contrast, negativeground *ashes that we have observed very seldom consistof preliminary breakdown pulse trains with pulses of initialpolarity opposite to that of the succeeding return stroke. Infew cases, that we have observed, negative return strokeswere preceded, by a pulse train with pulses of polarity op-posite to that of the return stroke. However, these pulsesmostly resemble the characteristics of the pulses of isolatedcloud *ashes, rather than that of the preliminary breakdownpulses.
In cases where the discharge channel is vertical, the posi-tive initial polarity of pulses indicates an uplifting of positivecharge and the negative initial polarity indicates a loweringof positive charge (opposite directions if negative charge isconsidered). On the other hand, if breakdown events takeplace in a non-vertical geometry, then, a positive, chargemoved away from the observer, gives the same result asin the case that the same charge is uplifted in a verticalchannel. Such horizontally extended pre-leader breakdownevents, associated with negative ground *ashes, have beenobserved by Rhodes (1989), Rhodes and Krehbiel (1989)
and Rhodes et al. (1994), using radio-interferometric meth-ods. Some of these observed discharge paths are not straightbut curved in 3-D space. Thus, one possibility of the ob-servation of pulse trains with di6erent initial pulse polarityand polarity reversal may be the di6erence and change inhorizontal direction of charge transfer. However, it may benoted that, in connection with negative lightning, NBP trainsare observed only of single initial pulse polarity.
Contrary to the above discussion, one may speculate dif-ferent initiating mechanisms for positive return strokes, onthe following observations of PBP trains. Unlike in the caseof the NBP trains, PBP trains are of several types. Further-more, the distributions of most of the pulse characteristics ofPBP trains are signi(cantly diversi(ed and double peaked.These two observations may be indications which show thatthese pulse trains are due to discharge processes in betweendi6erent charge regions of the cloud (e.g. main negative andpositive charge centres, positive charge pocket, screeninglayers, irregularly located charge regions as reported in re-cent studies, etc.). The existence of several mechanisms ofinitiation of positive CG *ashes is supported by the studyof Rust et al. (1981). They have reported that, in springand summer thunder storms, there are several regions of thecloud from which positive lightning emanate (i.e. from highon the back of the main storm tower, through the wall cloudand from the downshear anvil etc.). However, further stud-ies in locating radiation sources in the cloud with simulta-neous (eld measurements at ground are required to backupthe above speculation of the di6erent initiating processes ofpositive CG lightning.
As it was shown in Fig. 6 the spectrum of NBP has a peakthat occurs at a higher frequency than that of the spectrumof PBP trains does. One reason for this shift of peak maybe the sharp pulses superimposed on the leading edge of thebipolar pulses of negative *ashes. However, in general, thetwo spectrums are not very di6erent from each other. Asthese pulse trains have propagated several tens of kilometresover the land, the high frequency part of the pulses may beattenuated due to the propagation e6ects.
5. Conclusions
(1) Similar to negative ground *ashes, positive ground*ashes also precede breakdown pulse trains. This is the (rst
C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047–1055 1055
detailed study of these pulse trains observed in summer thun-der storms.
(2) Considering the initial polarity of pulses, 4 types ofbipolar pulse trains preceding positive return strokes wereidenti(ed. Those are, pulse trains with (I) bipolar pulses ofnegative initial polarity, (II) two regions; one with bipolarpulses of positive initial polarity, followed by a train of bipo-lar pulses with negative initial polarity (III) bipolar pulsesof positive initial polarity, and (IV) pulses of irregular ini-tial polarity.
(3) The majority of pulse trains (57 pulse trains) belongsto the (rst category, while 6 come under the second category,5 under the third, and 3 under the fourth.
(4) Themain type of pulse trains consist of pulses with rel-atively smooth leading edge of the initial half cycle, which isin contrary to the preliminary breakdown pulses of negativeground *ashes, where few sharp, narrow, unipolar pulsesare superimposed on the positive leading edge. The pulsecharacteristics have largely diversi(ed distributions. Thus,a clear discrimination cannot be prescribed for these pulsecharacteristics and those of NBP trains observed in Swedenand that in Florida (in fact, the two data sets of NBP trainsshow considerable di6erence from each other). Accordingto the distributions of pulse characteristics, we conclude thatthe results of Ushio et al. (1998), on PBP trains, are not dif-ferent from those of ours. Furthermore, the pulse structureof PBP trains as described in Ushio et al. (1998) is similarto that of this study.
(5) The second type of pulse trains with two distinctregions is a new observation in lightning literature. Theyconstitute about 12% of the total PBP trains observed inSweden.
(6) We described the observation of several types of pre-liminary breakdown pulse trains in connection with positivereturn strokes as due to two possible reasons. One is the ge-ometrical alignment of the channel, which may give rise toboth types of polarities and even the reversal of the polar-ity. The second possibility is the breakdown between sev-eral combinations of regions in the cloud that may give riseto the ground *ash. For both explanations further investiga-tions should be conducted to make proper conclusions.
(7) The spectrum of the main type of PBP trains, whichhas the peak at about 10 kHz, is somewhat similar to that ofNBP trains and that of negative return strokes. However, thespectrum of individual pulses in the PBP train, has a peakat about 16 kHz, a frequency, which is higher than that forthe entire pulse train.
Acknowledgements
Authors thank Prof. Viktor Scuka for placing excellent re-search facilities at their disposal. Financial assistance givenby the IPPS of the International Science Programs, Upp-sala University, and the Swedish Natural Science Founda-tion for the research grant G-AA/GU 01448-315 are greatlyacknowledged.
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