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Radiation field pulses associated with the initiation of positive cloud to ground lightning flashes
Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055
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Radiation (eld pulses associated with the initiation of positive cloud to ground lightning *ashes Chandima Gomesa;∗ , Vernon Coorayb a Department
b Institute
of Physics, University of Colombo, Colombo 03, Sri Lanka 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 ground lightning *ashes were analysed. These pulses are bipolar in nature and somewhat similar in pulse characteristics to the breakdown pulses preceding negative cloud to ground lightning. However, in the case of these positive *ashes, the pulse characteristics of the pulse trains are con(ned in a much wider range of values than those of the pulse trains associated with negative return strokes. The leading edge of the pulses of the most commonly observed pulse trains that precede positive return strokes are relatively smooth, thus, di6erent from their counterparts associated with negative *ashes, in which case a few narrow pulses are superimposed on the rising edge of the bipolar pulses. Considering the initial polarity of pulses, four types of bipolar pulse trains preceding positive return strokes were identi(ed. For each type of pulse trains, statistics of pulse characteristics were given. In contrast, in the case of negative ground *ashes, the bipolar pulse trains were almost always composed of pulses of the same polarity as that of the succeeding return stroke. The possible causes of the observation of several 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 compared with the spectrum of the breakdown pulses of negative *ashes and those of negative return strokes. This spectrum of the preliminary breakdown pulse trains of positive ground *ashes is comparable with that of the preliminary breakdown pulse trains 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 return strokes of cloud to ground (CG) lightning have been observed 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 these publications, analysis were done only on pulse trains preceding 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 place inside 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 breakdown pulse (NBP) trains, if they precede negative return strokes and positive breakdown pulse (PBP) trains, if they precede positive return strokes. The main characteristics of the bipolar pulses in these pulse trains that can be found in the literature are as follows. A train of bipolar pulses occurs a few milliseconds to a few tens of milliseconds (some times more than 100 ms) before the (rst return stroke in a CG *ash. In most of the cases, bipolar pulses in NBP trains have an initial polarity similar to that of the return stroke succeeding them. Hereafter, negative polarity is assigned to
c 2004 Elsevier Ltd. All rights reserved. 1364-6826/$ - see front matter doi:10.1016/j.jastp.2004.03.015
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the (eld change due to the raising of negative charge away from earth or lowering of positive charge towards earth (i.e. the atmospheric electricity sign convention). Weidman and Krider (1979) have characterised the NBP trains, by analysing a large sample of *ashes recorded in Florida. The initial polarity of each pulse in a given sequence tends to be the same. A few fast narrow pulses have been superimposed on the initial half cycle. The bipolar overshoot is smaller in amplitude compared with the initial peak, in most cases. The frequency spectrum of few pulses of NBP trains has been given in Weidman et al. (1981), Weidman and Krider (1986) and Rakov et al. (1996). The time duration 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) of Gomes et al. (1997) was not statistically suIcient to make any conclusions quantitatively, on the characteristics of either individual pulses or pulse trains. Ushio et al. (1998) have analysed 19 PBP trains pertinent to winter thunder storms. They have conducted their measurements at the Hokuriku coast in Japan. The main results of the analysis of Gomes et al. (1997) and Ushio et al. (1998) are given in the discussion of this paper. The main objective of the present study is to investigate the nature and temporal characteristics of the PBP trains, of summer thunder storms in temperate regions. This information will be very useful in understanding the initiating mechanism of positive ground *ashes. We employ a statistically signi(cant sample, obtained during several frontal thunder storms in Sweden. The results are compared with the characteristics of PBP trains recorded in Japan, by Ushio et al. (1998), the only other study on PBP trains. We also compare our results with the characteristics of NBP trains recorded in Sweden and in elsewhere. Based on our observations, we discuss possible causes of the PBP trains. We also give the frequency spectrum of a set of PBP trains pertinent to distant located ground *ashes. As the study of Ushio et al. (1998) is based on winter lightning and the sample size of the study of Gomes et al. (1997) is small, this is the (rst detailed study on PBP trains, pertinent to summer thunder storms. 2. Experiment The measurements were conducted in Uppsala (latitude 59.8N and longitude 17.6E), Sweden during several frontal thunder storms in the periods of June, 1993 and June–August 1996. The measuring station is situated about 70 km inland of the Baltic sea. Only the positive *ashes with breakdown pulse activity were recorded. Hence, the data sets are selective. The approximate range of distances to the lightning strike location from the measuring station was 50–200 km as recorded by the LLP system in Sweden.
The vertical electric (elds were sensed by a *at plate antenna of which the capacitance to ground is 58 pF and the physical height is 1:88 m. After passing through a bu6er ampli(er (an operational ampli(er and a RC circuit which, together, acts as an active integrator), the signal was directly fed to a transient recorder, through a properly terminated and double screened 50 J coaxial cable of few meters. The rise time of the system is determined by the gain bandwidth product and the slew rate of the operational ampli(ers used in the bu6er circuit. These parameters were measured by simulating the antenna by a 58 pF capacitor and applying a square wave pulse to the system. The 10–90% rise time of the output of the antenna system for the step input voltage was less than 20 ns (according to the manufacturer’s information, the risetime response of the operational ampli(er is 10 ns). The decay time constant of the system is determined by the time constant of the RC circuit connected at the input stage of the operational ampli(er. The calculated decay time constant was about 20 ms. We checked the validity of this estimation by applying a step voltage input to the system. We found that the decay time does not deviate very much from 20 ms, during the operation. This time constant was much longer than the duration of the radiation pulses (few to few tens of microseconds) observed in this study. Note that, it is not our intention to investigate the static (eld characteristics of PBP trains, which may be several times longer than the decay time constant of our measuring system. Due to the same reason given above, we did not (lter the 50 Hz noise of the power system that has been interfered with our recorded waveforms. This measuring system was recently calibrated, by placing the *at plate antenna in a known (eld (in between a large wire mesh electrode, raised to a high potential, by connecting to a Max generator and a ground plane). However, we do not discuss the absolute amplitudes of pulses of the entire sample, as several *ashes were not located. The recording system consists of a LeCroy (1993 series) transient recorder with 1-Megabyte memory. The transient recorder was operated in the pre-trigger mode. The waveforms were continuously and selectively (only positive ground *ashes with PBP trains) recorded for 500 ms from a single negative trigger (sign convention as described in the Introduction). The sampling period was 50 ns and the pre-trigger delay time was set either 100 or 200 ms. Most of the (eld traces recorded were triggered by the PBP train. Seventy-one positive CG *ashes with PBP trains are analysed in this paper. In addition to these 71 cases, in one of the records, we have found that a pulse train very similar to a PBP train (which will be described in the next section), which does not precede a return stroke. We give a description of this *ash, too. The 16 breakdown pulse trains of negative *ashes, which have been used to calculate the frequency spectrum, for the comparison, were recorded at the same location with the same antenna and recording systems two of these negative *ashes were recorded in 1996 while the other 14 were recorded in 1998.
C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055
3. Results Most of the positive return strokes considered in this study were preceded by pulse trains with well structured bipolar pulses. Four types of PBP trains were identi(ed in the analysis: 57 *ashes contained PBP trains with pulses of negative initial polarity (henceforth termed type a). Six PBP trains had two distinct regions. The (rst region consists of pulses with positive initial polarity and the second with pulses of negative initial polarity (henceforth termed type b). Five PBP trains contain pulses of positive initial polarity (henceforth termed type c). In 3 cases, the PBP trains consist of pulses with irregular initial polarity and an erratic structure (henceforth termed type d). The labelling of pulse types as a, b, c, and d has been done by considering the descending order of the number of pulse trains that belongs to each category. Fig. 1 shows a type a PBP train, which is the most common type of pulse trains that precede positive CG *ashes. The *ash was struck approximately 80 km from the site. As it can be seen in Fig. 1b, the leading edge of the initial half cycle of the pulses is relatively smooth compared to that of the pulses in the other types of PBP trains (described below)
1.75
Electricfield(V/m)
1.25 0.75 0.25 -0.25 -0.75 -1.25 -1.75 0.1
0.3
0.5 0.7 Time (ms)
(a)
0.9
1.1
1.75
Electricfield (V/m)
1.25 0.75 0.25 -0.25 -0.75 -1.25 -1.75 0.25
(b)
0.3
0.35
0.4 0.45 Time (ms)
0.5
0.55
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 change de*ects upwards.
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and NBP trains. Fig. 2 depicts the distribution of the pulse characteristics of type a PBP trains. The duration of the pulse train was de(ned as the time between the regions of pulse activity at the beginning and at the end of the pulse train that have amplitudes of 10% of the maximum amplitude. In all 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 lower estimation for the distant *ashes than that for the nearby *ashes. T1 and T2 are approximate duration of the (rst half cycle and the second half cycle, respectively, of an individual bipolar pulse. Thus, T1 + T2 is approximately equal to the total pulse width. Pulse separation is the time interval between the crests of two adjacent bipolar pulses. Pulses with amplitude less than 10% of the maximum amplitude are neglected in estimating the pulse separation, as they are hard to be distinguished from the background noise. In each pulse train, 5 adjacent pulses were selected from the most active region (usually the (rst (ve pulses), to calculate the mean values of T1 , T2 and the pulse separation. Apart from being convenient in the analysis, there is no signi(cance in limiting the number of pulses to 5. The criteria used to obtain T1 , T2 and the pulse separation, may give higher values for each of these parameters, compared to the case where all the pulses of a PBP train are taken into account in the statistical analysis, as narrow and low amplitude pulses can be omitted. PBP-RS separation is the time duration between the highly active region of the pulse train and the return stroke. T1 and T2 have the same mean value of 19 s. The mean pulse width is 38 s. The pulse separation, the pulse train duration and the PBP-RS separation have the mean values of 96 s, 3 and 56 ms, respectively. All the above mean values di6er considerably from the mode value of the corresponding parameters. In cases of the pulse separation, the pulse duration and the PBP-RS separation, one may see that the distributions are double peaked. The arrows in the charts of Fig. 2 are pointed to the columns, to which the mean values of the corresponding parameters of several studies belong. Fig. 3 illustrates a type b PBP trains. The (rst region has pulses of positive initial polarity and the second region has pulses of negative initial polarity. The ground *ash of the PBP train in Fig. 3 has not been located. The two regions with pulses of opposite polarity are given in an expanded time scale. Pulses in the (rst region are very similar to those of NBP trains, in pro(le. A few narrow pulses are superimposed on the leading edge of the (rst half, which is a typical characteristic of pulses in NBP trains. The pulse pro(le of the second region is somewhat similar to that of type a PBP trains. However, in several type b pulse trains, one or two narrow unipolar spikes are superimposed on the (rst half of few 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 two regions of pulses with opposite polarity. The last row shows the mean of the characteristics, of the six cases. The separation between the two pulse regions is de(ned as the time
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C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055 16
10
10 8 6 4 2
N2 P2 4
2
Number of PB pulse trains
n= 57 X= 19 ∝s S= 9 ∝s
N1
P1
15
10
5
>200
151-200
121-150
101-120
81-90
91-100
71-80
n= 57 X= 3.0 ms S= 1.8 ms
8
P1 6
N1 4
2
(e)
T2: Width of the second half (µs)
>7.0
6.6-7.0
6.1-6.5
5.6-6.0
5.1-5.5
4.6-5.0
4.1-4.5
3.6-4.0
3.1-3.5
2.6-3.0
1.6-2.0
0.6-1.0
40-45
36-40
31-35
26-30
21-25
16-20
11-15
6-10
0-5
(b)
2.0-2.5
0
0
1.1-1.5
Pulse train duration (µs) 14
25
P2
N1
n= 57 X= 38 ∝s S= 16 ∝s
P1
Number of PB pulse trains
20
n= 57 X= 56 ms S= 48 ms
12
15
N2 10
P2 5
10
N1
8
P1 N2
6 4 2
(f)
>100
91-100
81-90
71-80
61-70
51-60
41-50
31-40
0-10
71-80
61-70
51-60
41-50
31-40
11-20
0-10
21-30
Width of PB pulses (µs)
21-30
0
0
11-20
Number of PB pulse trains
Pulse seperation (µs)
10
20
Number of PB pulse trains
61-70
21-30
(d)
T1: Width of the first half cycle (µs)
25
(c)
51-60
0
36-40
31-35
26-30
21-25
16-20
11-15
6-10
0-5
0
(a)
P1
6
41-50
12
n= 57 X= 96 ∝s S= 56 ∝s
8
Number of PB pulse trains
N1
Number of PB pulse trains
N1
n= 57 X= 19 ∝s S= 9 ∝s
31-40
P1 14
PBP-RS seperation (µs)
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 second half cycle of pulses (T2 ). (c) Width of pulses (T1 + T2 ). (d) Separation between individual pulses. (e) Pulse train duration. (f) Separation between 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 and N2: Weidman and Krider (1979) study of NBP trains. The other abbreviations are, n: Number of pulse trains; X : Mean value; S: Standard deviation.
duration between the most active places of the two regions. In several cases, there exists a low active period in between the two pulse regions. In the (rst region T1 , T2 , T1 + T2 and
Pulse separation have the mean values of 13, 13, 27 and 62 s, respectively. In the second region, the above values are 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 4.1
1
Electric field (V/m)
3.1 2.1
2
1.1 0.1 -0.9 -1.9 -2.9 0
0.5
1
(a)
1.5
2
2.5
3
3.5
4
Time (ms) 4.1
Electric field (V/m)
3.1 2.1 1.1 0.1 -0.9 -1.9 -2.9 0.3
0.4
(b)
0.5
0.6
0.7
0.8
2.1
2.2
Time (ms)
Electric field (V/m)
0.8
0.3
-0.2
-0.7
-1.2 1.7
(c)
1.8
1.9
2
Time (ms)
Fig. 3. Flash No. 960724.04 A type b PBP train. (a) The entire pulse 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 time scales. A positive (eld change de*ects upwards.
duration of the (rst region and the second region are 1.3 and 2:8 ms, respectively. The mean PBP-RS separations for the two regions are 81 and 77 ms, respectively. The pulse characteristics 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 shows the pulse characteristics of this type of PBP trains. The pulse
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characteristics, T1 , T2 , T1 + T2 and Pulse separation have the mean values of 16, 15, 31 and 51 s, respectively. The pulse train duration and the PBP-RS separation have mean values of 2.1 and 44 ms, respectively. Pulse characteristics and pulse pro(le of this type of PBP trains is comparable with those of NBP trains. All 3 PBP trains with pulses of irregular polarity continue upto the return stroke. Fig. 5 epitomises one such case. In all 3 cases, this irregular pulse activity is visible for a few milliseconds, even after the return stroke. However, Cooray (1984) has observed that in a number of positive return strokes, a pulse burst appeared immediately after the return stroke, in which case, there was no pulse activity immediately prior to the return stroke. Hence, one cannot con(dently conclude, that the pulses, which appear after the return stroke, in the 3 type d PBP trains, are continuations of the respective PBP trains. The time from the beginning of the PBP train to the return stroke, in these 3 cases are 33, 27, and 22 ms. The width of these pulses is in the range of 5–15 s, while the pulse separation is in the range of 20–80 s. Thus, in pulse train duration, pulse width and pulse separation, this type of PBP trains are considerably different from chaotic pulse trains, that occur mainly in association with negative subsequent strokes (Gomes et al., 1998). Fig. 6 (curve 1 and 2) shows the frequency spectrum of the electric (elds of 23 type a PBP trains normalised to a distance of 50 km. The *ashes of these pulse trains have occurred at a distance of 50–150 km. Thus, the higher frequency part of the spectrum may be reduced due to the propagation e6ects. The spectrum is calculated between 1 and 500 kHz. The digitising resolution of the data set used in this study is not adequate to analyse the spectrum above 500 kHz. Curve 1 corresponds to the mean spectrum of the entire pulse trains and curve 2 shows the mean spectrum of 30 individual pulses. The spectral values are given in dB, which are 20 times the logarithm of base 10 of the magnitude of Fourier transform of the electric (eld strength (in V/m). The peak of the spectrum of pulse trains is −76 dB and it occurs approximately at 10 kHz while that of the individual pulses is −88 dB and it occurs around 16 kHz. Curve 3 of Fig. 6 depicts the frequency spectrum of 16 NBP trains. The electric (elds in this case are also normalised to 50 km as it has been done in the previous calculation. In this case, the peak occurs at 28 kHz with a value of −81 dB. 4. Discussion In this study, we have observed that similar to negative return strokes, positive return strokes are also preceded by a train of bipolar pulses. As we have selectively recorded positive CG *ashes with PBP trains, we are not able to give statistics on the positive return strokes that are not preceded by PBP trains. More than 80% of the PBP trains consist of pulses with negative initial polarity (type a). In the study of Ushio et al. (1998), 17 PBP trains out of 19, consist of
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C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055
Table 1 Statistics 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 separation (s) (s) (s) separation train separation (s) (s) (s) separation (ms) (s) duration (ms) (s) (ms)
PBP train duration (ms)
Separation between two pulse regions (ms)
P930629.05 P930629.18 P930629.21 P960724.04 P960726.02 P960709.21 Mean SD
17 23 195 27 215 10 81 88
6.2 2.6 2.8 2.0 2.3 0.6 2.8 1.7
2.5 6.4 8.0 1.4 2.6 1.2 3.7 2.6
13 13 17 15 16 6 13 4
18 14 11 14 16 7 13 4
31 27 28 29 32 13 27 6
55 68 72 66 60 49 62 8
1.2 1.2 2.0 1.2 1.2 0.8 1.3 0.4
14 17 187 25 212 9 77 87
14 16 19 4 10 10 12 5
24 16 14 10 11 10 14 5
38 32 33 14 21 20 26 9
35 49 54 33 28 26 38 10
SD: Standard deviation. The other abbreviations are de(ned in the text. Table 2 Statistics 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 P930629.22 P930722.21 P960723.23 P960706.76 Mean SD
23 89 30 40 37 44 23
10 16 11 19 22 16 5
12 19 13 14 19 15 3
22 35 24 33 41 31 7
49 56 35 56 60 51 9
2.9 2.9 1.8 1.1 1.9 2.1 0.7
SD: Standard deviation. The other abbreviations are de(ned in the text.
pulses of negative initial polarity, thus, at both location type a is observed as the most common type of PBP trains. In the study of Gomes et al. (1997), 3 PBP trains out of 5, belong to type b. Only one pulse train consists of pulses with negative initial polarity (type a) while the other is composed of pulses of positive initial polarity (type c). The type a PBP trains consist of bipolar pulses with a relatively smooth zero-to-peak rising edge (Fig. 1b). This is in agreement with the observation of Ushio et al. (1998). In contrast, in the case of NBP trains, few sharp pulses are observed to be superimposed on the leading edge of most of the bipolar pulses. In NBP trains, these narrow spikes are observable of the pulses of *ashes that were recorded even at 200 km. The pulse characteristics of type a PBP trains are con(ned in a broad range of values with signi(cantly large standard deviations (Fig. 2). The study of Ushio et al. (1998) recon(rms this diversity of the pulse characteristics of PBP trains. According to their analysis, in PBP trains, the pulse width ranges from 5 to 52 s. The pulse separation and PBP-RS separation have values from 10 to 180 s and from 1 to 38 ms, respectively. The mean values of these parameters are given in Table 3. All these mean values are at
least about twice less than the corresponding mean values of our data. The arrow pointers of Fig. 2 clearly demonstrate this di6erence. One reason may be that Ushio et al. (1998) have taken all the pulses in each train in their analysis, while we have considered only the (ve largest pulses from each pulse train. Secondly, the disagreement in pulse characteristics may be due to the di6erence in the types of thunder storms to which the two data sets belong. The mean pulse characteristics of type a PBP trains are considerably larger than those values of NBP trains that have been observed in Sweden (Gomes et al., 1997) and somewhat similar to that have been observed in Florida (Weidman and Krider, 1979). A summary of the results of these two studies on NBP trains is given in Table 4 along with the results of the present study on type a PBP trains. The arrow pointers of the distribution charts (Fig. 2) indicate, that the mean pulse characteristics of NBP trains of Swedish thunder storms lie in the vicinity of the mode value of the corresponding charts of type a PBP trains. The mean pulse width and the mean PBP-RS separation of type a PBP trains are comparable with the same mean values obtained by Weidman and Krider (1979) for NBP trains. The mean pulse
2
1.5
1.5
1
Electric field (V/m)
Electric field (V/m)
C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055
1 0.5 0
0.5
0
-0.5
-1
-0.5
-1.5
-1 0
0.1
0.2
0.3
(a)
0.4
0.5
0.6
0.7
0.8
0.9
4
1
4.5
5
(a)
Time (ms)
5.5
6
Time (ms) 1.5
2
1
Electric field (V/m)
1.5
Electric field (V/m)
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1
0.5
0
0.5
0
-0.5
-0.5 -1
(b)
4.5
0.45
0.5
0.55
0.6
0.65
4.55
4.6
4.65
(b)
4.7
4.75
4.8
4.85
4.9
Time (ms)
Time (ms)
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), is somewhat higher than that of this study, but Fig. 2d shows that there is a considerable number of pulse trains that belongs to a range of pulse separation close to 130 s. Thus, we (nd no clear distinction between the pulse characteristics of NBP trains and those of the most commonly observed type of PBP trains. Note that, the pulse characteristics of Ushio et al. (1998) are similar to those of NBP trains that were reported in Gomes et al. (1997). One of the interesting observations of this study is the 6 type b PBP trains. The electric (eld perturbation of these pulse trains starts with pulses of positive polarity, of which the characteristics are very similar to that of NBP trains observed in Sweden. The amplitude of pulses gradually decays and after a quiescent period of a few milliseconds, another pulse train with pulses of opposite initial polarity occurs. There is little di6erence in the pulse characteristics of the two regions other than the polarity reversal (Table 2). Totally, type b constitutes about 12% of PBP trains, observed in Sweden (observations of both Gomes et al., 1997 and this study). This type of pulse trains have not been reported before, in connection with any lightning event.
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.
-75 -80 Electric field spectrum (dB)
-1 0.4
-85 -90
1
3 2
-95 -100 -105 -110 -115 -120 -125 103
104 105 Frequency (Hz)
106
Fig. 6. The mean frequency spectrum of (1) The electric (elds of the 23 distant located type a PBP trains (2) Thirty individual pulses of type 1 PBP trains (3) Sixteen distant located NBP trains. The electric (elds are normalised to 50 km. The spectral values are given in dB, which are 20 times the logarithm of base 10 of the magnitude of Fourier transform of the electric (eld strength (in V/m).
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C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055
Table 3 The pulse characteristics of PBP trains as given in Ushio et al. (1998)
Pulse width (s) Pulse separation (s) PBP-RS separation (ms)
No of pulses
Mean
Standard deviation
Range of values
132 219 19∗
18.8 54.2 12
7.9 35.7
5–52 10–180 1–38
The mean duration of the PBP trains has been estimated to be around 1 ms. ∗ No. of pulse trains. Table 4 Parameter T1 (s) T2 (s) Pulse width (s) Pulse separation (s) NBP/PBP duration (ms) NBP/PBP-RS separation (ms)
Weidman and Krider (1979)
Gomes et al. (1997)
This study
41
9 14 23
19 19 38
130
63
96
1.7
3.0
26
56
82
In the (ve cases of type c PBP trains, the pulse characteristics are not much di6erent from that of NBP trains (Table 3). The pulses of this type are also very similar in pro(le to those of the NBP trains observed at the same location. However, in the cases of type b, and type c PBP trains, the numbers of events are insuIcient to provide a good comparison with the previously observed data. Another interesting outcome of this study is the observation of the several types of pulse trains (4 types) with pulses of di6erent initial polarity. In contrast, negative ground *ashes that we have observed very seldom consist of preliminary breakdown pulse trains with pulses of initial polarity opposite to that of the succeeding return stroke. In few cases, that we have observed, negative return strokes were preceded, by a pulse train with pulses of polarity opposite to that of the return stroke. However, these pulses mostly resemble the characteristics of the pulses of isolated cloud *ashes, rather than that of the preliminary breakdown pulses. In cases where the discharge channel is vertical, the positive initial polarity of pulses indicates an uplifting of positive charge and the negative initial polarity indicates a lowering of positive charge (opposite directions if negative charge is considered). On the other hand, if breakdown events take place in a non-vertical geometry, then, a positive, charge moved away from the observer, gives the same result as in the case that the same charge is uplifted in a vertical channel. Such horizontally extended pre-leader breakdown events, associated with negative ground *ashes, have been observed by Rhodes (1989), Rhodes and Krehbiel (1989)
and Rhodes et al. (1994), using radio-interferometric methods. Some of these observed discharge paths are not straight but curved in 3-D space. Thus, one possibility of the observation of pulse trains with di6erent initial pulse polarity and polarity reversal may be the di6erence and change in horizontal direction of charge transfer. However, it may be noted that, in connection with negative lightning, NBP trains are observed only of single initial pulse polarity. Contrary to the above discussion, one may speculate different initiating mechanisms for positive return strokes, on the following observations of PBP trains. Unlike in the case of the NBP trains, PBP trains are of several types. Furthermore, the distributions of most of the pulse characteristics of PBP trains are signi(cantly diversi(ed and double peaked. These two observations may be indications which show that these pulse trains are due to discharge processes in between di6erent charge regions of the cloud (e.g. main negative and positive charge centres, positive charge pocket, screening layers, irregularly located charge regions as reported in recent studies, etc.). The existence of several mechanisms of initiation of positive CG *ashes is supported by the study of Rust et al. (1981). They have reported that, in spring and summer thunder storms, there are several regions of the cloud from which positive lightning emanate (i.e. from high on the back of the main storm tower, through the wall cloud and from the downshear anvil etc.). However, further studies in locating radiation sources in the cloud with simultaneous (eld measurements at ground are required to backup the above speculation of the di6erent initiating processes of positive CG lightning. As it was shown in Fig. 6 the spectrum of NBP has a peak that occurs at a higher frequency than that of the spectrum of PBP trains does. One reason for this shift of peak may be the sharp pulses superimposed on the leading edge of the bipolar pulses of negative *ashes. However, in general, the two spectrums are not very di6erent from each other. As these pulse trains have propagated several tens of kilometres over the land, the high frequency part of the pulses may be attenuated 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
detailed study of these pulse trains observed in summer thunder storms. (2) Considering the initial polarity of pulses, 4 types of bipolar pulse trains preceding positive return strokes were identi(ed. Those are, pulse trains with (I) bipolar pulses of negative initial polarity, (II) two regions; one with bipolar pulses of positive initial polarity, followed by a train of bipolar pulses with negative initial polarity (III) bipolar pulses of positive initial polarity, and (IV) pulses of irregular initial polarity. (3) The majority of pulse trains (57 pulse trains) belongs to the (rst category, while 6 come under the second category, 5 under the third, and 3 under the fourth. (4) The main type of pulse trains consist of pulses with relatively smooth leading edge of the initial half cycle, which is in contrary to the preliminary breakdown pulses of negative ground *ashes, where few sharp, narrow, unipolar pulses are superimposed on the positive leading edge. The pulse characteristics have largely diversi(ed distributions. Thus, a clear discrimination cannot be prescribed for these pulse characteristics and those of NBP trains observed in Sweden and that in Florida (in fact, the two data sets of NBP trains show considerable di6erence from each other). According to the distributions of pulse characteristics, we conclude that the results of Ushio et al. (1998), on PBP trains, are not different from those of ours. Furthermore, the pulse structure of PBP trains as described in Ushio et al. (1998) is similar to that of this study. (5) The second type of pulse trains with two distinct regions is a new observation in lightning literature. They constitute about 12% of the total PBP trains observed in Sweden. (6) We described the observation of several types of preliminary breakdown pulse trains in connection with positive return strokes as due to two possible reasons. One is the geometrical alignment of the channel, which may give rise to both types of polarities and even the reversal of the polarity. The second possibility is the breakdown between several combinations of regions in the cloud that may give rise to the ground *ash. For both explanations further investigations should be conducted to make proper conclusions. (7) The spectrum of the main type of PBP trains, which has the peak at about 10 kHz, is somewhat similar to that of NBP trains and that of negative return strokes. However, the spectrum of individual pulses in the PBP train, has a peak at about 16 kHz, a frequency, which is higher than that for the entire pulse train. Acknowledgements Authors thank Prof. Viktor Scuka for placing excellent research facilities at their disposal. Financial assistance given by the IPPS of the International Science Programs, Uppsala University, and the Swedish Natural Science Foundation for the research grant G-AA/GU 01448-315 are greatly acknowledged.
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References Beasley, W.H., Uman, M.A., Rustan, P., 1982. Electric (elds preceding cloud-to-ground lightning *ashes. Journal of Geophysical Research 87, 4883–4902. Brook, M., 1992. Breakdown electric (elds inside summer and winter storm-clouds: inferences based on initial lightning leader wave form. Proceedings of the International Conference on Atmospheric Electricity. Clarence, N.D., Malan, D.J., 1957. Preliminary discharge processes in lightning *ashes to ground. Quarterly Journal of the Royal Meteorology Society 83, 161–172. Cooray, V., 1984. Further characteristics of positive radiation (elds from lightning in Sweden. Journal of Geophysical Research 89, 11807–11815. Gomes, C., Thottappillil, R., Scuka, V., 1997. Bipolar electric (eld pulses in lightning *ashes over Sweden. Proceedings of the 12th International Zurich Symposium on EMC, 31F4. Gomes, C., Cooray, V., Fernando, M., Jayaratne, C., 1998. Chaotic pulse trains that mainly associated with negative subsequent lightning return strokes. Proceedings of the 24th International Conference on Lighting Protection, Birmingham, UK. Norinder, H., Knudsen, E., 1957. Pre-discharges in relation to subsequent lightning strokes. Arkiv foer Geofysik 2, 551–571. Ogawa, T., 1993. Initiation of lightning in Clouds. Journal of Atmospheric Electricity, 121–132. Rakov, V.A., Uman, M.A., Ho6man, G.R., Masters, M.W., Brook, M., 1996. Bursts of pulses in lightning electromagnetic radiation: observations and implications for lightning test standards. IEEE Transactions on EMC 38 (2), 156–164. Rhodes, C.T., 1989. Interferometric observations of VHF radiation from lightning. Ph.D. Thesis, New Mexico Institute of Mining and Technology, Socorro, USA. Rhodes, C.T., Krehbiel, P.R., 1989. Interferometric observations of a single cloud-to-ground *ash. Geophysical Research Letters 16, 1169–1172. Rhodes, C.T., Shao, X.M., Krehbiel, P.R., Thomas, R.J., Hayenga, C.O., 1994. Observations of lightning phenomena using radio interferometry. Journal of Geophysical Research 99, 13059–13082. Rust, W.D., MacGorman, D.R., Arnold, R.T., 1981. Positive cloud-to-ground lightning *ashes in severe storms. Geophysical Research Letters 8 (7), 791–794. Ushio, T., Kawasaki, Z.-I., Matsuura, K., Wang, D., 1998. Electric (elds of initial breakdown in positive ground *ash. Journal of Geophysical Research 103, 14135–14140. Weidman, C.D., Krider, P., 1979. The radiation (eld wave forms produced by intracloud lightning discharge processes. Journal of Geophysical Research 84, 3159–3164. Weidman, C.D., Krider, P., Uman, M.A., 1981. Lightning amplitude spectra in the interval from 100 kHz to 20 MHz. Geophysical Research Letters 8, 931–934. Weidman, C.D., Krider, E.P., 1986. The amplitude spectra of lightning radiation (elds in the interval from 1 to 20 MHz. Radio Science 21, 964–970.
www.elsevier.com/locate/jastp
Radiation (eld pulses associated with the initiation of positive cloud to ground lightning *ashes Chandima Gomesa;∗ , Vernon Coorayb a Department
b Institute
of Physics, University of Colombo, Colombo 03, Sri Lanka 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 ground lightning *ashes were analysed. These pulses are bipolar in nature and somewhat similar in pulse characteristics to the breakdown pulses preceding negative cloud to ground lightning. However, in the case of these positive *ashes, the pulse characteristics of the pulse trains are con(ned in a much wider range of values than those of the pulse trains associated with negative return strokes. The leading edge of the pulses of the most commonly observed pulse trains that precede positive return strokes are relatively smooth, thus, di6erent from their counterparts associated with negative *ashes, in which case a few narrow pulses are superimposed on the rising edge of the bipolar pulses. Considering the initial polarity of pulses, four types of bipolar pulse trains preceding positive return strokes were identi(ed. For each type of pulse trains, statistics of pulse characteristics were given. In contrast, in the case of negative ground *ashes, the bipolar pulse trains were almost always composed of pulses of the same polarity as that of the succeeding return stroke. The possible causes of the observation of several 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 compared with the spectrum of the breakdown pulses of negative *ashes and those of negative return strokes. This spectrum of the preliminary breakdown pulse trains of positive ground *ashes is comparable with that of the preliminary breakdown pulse trains 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 return strokes of cloud to ground (CG) lightning have been observed 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 these publications, analysis were done only on pulse trains preceding 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 place inside 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 breakdown pulse (NBP) trains, if they precede negative return strokes and positive breakdown pulse (PBP) trains, if they precede positive return strokes. The main characteristics of the bipolar pulses in these pulse trains that can be found in the literature are as follows. A train of bipolar pulses occurs a few milliseconds to a few tens of milliseconds (some times more than 100 ms) before the (rst return stroke in a CG *ash. In most of the cases, bipolar pulses in NBP trains have an initial polarity similar to that of the return stroke succeeding them. Hereafter, negative polarity is assigned to
c 2004 Elsevier Ltd. All rights reserved. 1364-6826/$ - see front matter doi:10.1016/j.jastp.2004.03.015
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the (eld change due to the raising of negative charge away from earth or lowering of positive charge towards earth (i.e. the atmospheric electricity sign convention). Weidman and Krider (1979) have characterised the NBP trains, by analysing a large sample of *ashes recorded in Florida. The initial polarity of each pulse in a given sequence tends to be the same. A few fast narrow pulses have been superimposed on the initial half cycle. The bipolar overshoot is smaller in amplitude compared with the initial peak, in most cases. The frequency spectrum of few pulses of NBP trains has been given in Weidman et al. (1981), Weidman and Krider (1986) and Rakov et al. (1996). The time duration 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) of Gomes et al. (1997) was not statistically suIcient to make any conclusions quantitatively, on the characteristics of either individual pulses or pulse trains. Ushio et al. (1998) have analysed 19 PBP trains pertinent to winter thunder storms. They have conducted their measurements at the Hokuriku coast in Japan. The main results of the analysis of Gomes et al. (1997) and Ushio et al. (1998) are given in the discussion of this paper. The main objective of the present study is to investigate the nature and temporal characteristics of the PBP trains, of summer thunder storms in temperate regions. This information will be very useful in understanding the initiating mechanism of positive ground *ashes. We employ a statistically signi(cant sample, obtained during several frontal thunder storms in Sweden. The results are compared with the characteristics of PBP trains recorded in Japan, by Ushio et al. (1998), the only other study on PBP trains. We also compare our results with the characteristics of NBP trains recorded in Sweden and in elsewhere. Based on our observations, we discuss possible causes of the PBP trains. We also give the frequency spectrum of a set of PBP trains pertinent to distant located ground *ashes. As the study of Ushio et al. (1998) is based on winter lightning and the sample size of the study of Gomes et al. (1997) is small, this is the (rst detailed study on PBP trains, pertinent to summer thunder storms. 2. Experiment The measurements were conducted in Uppsala (latitude 59.8N and longitude 17.6E), Sweden during several frontal thunder storms in the periods of June, 1993 and June–August 1996. The measuring station is situated about 70 km inland of the Baltic sea. Only the positive *ashes with breakdown pulse activity were recorded. Hence, the data sets are selective. The approximate range of distances to the lightning strike location from the measuring station was 50–200 km as recorded by the LLP system in Sweden.
The vertical electric (elds were sensed by a *at plate antenna of which the capacitance to ground is 58 pF and the physical height is 1:88 m. After passing through a bu6er ampli(er (an operational ampli(er and a RC circuit which, together, acts as an active integrator), the signal was directly fed to a transient recorder, through a properly terminated and double screened 50 J coaxial cable of few meters. The rise time of the system is determined by the gain bandwidth product and the slew rate of the operational ampli(ers used in the bu6er circuit. These parameters were measured by simulating the antenna by a 58 pF capacitor and applying a square wave pulse to the system. The 10–90% rise time of the output of the antenna system for the step input voltage was less than 20 ns (according to the manufacturer’s information, the risetime response of the operational ampli(er is 10 ns). The decay time constant of the system is determined by the time constant of the RC circuit connected at the input stage of the operational ampli(er. The calculated decay time constant was about 20 ms. We checked the validity of this estimation by applying a step voltage input to the system. We found that the decay time does not deviate very much from 20 ms, during the operation. This time constant was much longer than the duration of the radiation pulses (few to few tens of microseconds) observed in this study. Note that, it is not our intention to investigate the static (eld characteristics of PBP trains, which may be several times longer than the decay time constant of our measuring system. Due to the same reason given above, we did not (lter the 50 Hz noise of the power system that has been interfered with our recorded waveforms. This measuring system was recently calibrated, by placing the *at plate antenna in a known (eld (in between a large wire mesh electrode, raised to a high potential, by connecting to a Max generator and a ground plane). However, we do not discuss the absolute amplitudes of pulses of the entire sample, as several *ashes were not located. The recording system consists of a LeCroy (1993 series) transient recorder with 1-Megabyte memory. The transient recorder was operated in the pre-trigger mode. The waveforms were continuously and selectively (only positive ground *ashes with PBP trains) recorded for 500 ms from a single negative trigger (sign convention as described in the Introduction). The sampling period was 50 ns and the pre-trigger delay time was set either 100 or 200 ms. Most of the (eld traces recorded were triggered by the PBP train. Seventy-one positive CG *ashes with PBP trains are analysed in this paper. In addition to these 71 cases, in one of the records, we have found that a pulse train very similar to a PBP train (which will be described in the next section), which does not precede a return stroke. We give a description of this *ash, too. The 16 breakdown pulse trains of negative *ashes, which have been used to calculate the frequency spectrum, for the comparison, were recorded at the same location with the same antenna and recording systems two of these negative *ashes were recorded in 1996 while the other 14 were recorded in 1998.
C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055
3. Results Most of the positive return strokes considered in this study were preceded by pulse trains with well structured bipolar pulses. Four types of PBP trains were identi(ed in the analysis: 57 *ashes contained PBP trains with pulses of negative initial polarity (henceforth termed type a). Six PBP trains had two distinct regions. The (rst region consists of pulses with positive initial polarity and the second with pulses of negative initial polarity (henceforth termed type b). Five PBP trains contain pulses of positive initial polarity (henceforth termed type c). In 3 cases, the PBP trains consist of pulses with irregular initial polarity and an erratic structure (henceforth termed type d). The labelling of pulse types as a, b, c, and d has been done by considering the descending order of the number of pulse trains that belongs to each category. Fig. 1 shows a type a PBP train, which is the most common type of pulse trains that precede positive CG *ashes. The *ash was struck approximately 80 km from the site. As it can be seen in Fig. 1b, the leading edge of the initial half cycle of the pulses is relatively smooth compared to that of the pulses in the other types of PBP trains (described below)
1.75
Electricfield(V/m)
1.25 0.75 0.25 -0.25 -0.75 -1.25 -1.75 0.1
0.3
0.5 0.7 Time (ms)
(a)
0.9
1.1
1.75
Electricfield (V/m)
1.25 0.75 0.25 -0.25 -0.75 -1.25 -1.75 0.25
(b)
0.3
0.35
0.4 0.45 Time (ms)
0.5
0.55
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 change de*ects upwards.
1049
and NBP trains. Fig. 2 depicts the distribution of the pulse characteristics of type a PBP trains. The duration of the pulse train was de(ned as the time between the regions of pulse activity at the beginning and at the end of the pulse train that have amplitudes of 10% of the maximum amplitude. In all 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 lower estimation for the distant *ashes than that for the nearby *ashes. T1 and T2 are approximate duration of the (rst half cycle and the second half cycle, respectively, of an individual bipolar pulse. Thus, T1 + T2 is approximately equal to the total pulse width. Pulse separation is the time interval between the crests of two adjacent bipolar pulses. Pulses with amplitude less than 10% of the maximum amplitude are neglected in estimating the pulse separation, as they are hard to be distinguished from the background noise. In each pulse train, 5 adjacent pulses were selected from the most active region (usually the (rst (ve pulses), to calculate the mean values of T1 , T2 and the pulse separation. Apart from being convenient in the analysis, there is no signi(cance in limiting the number of pulses to 5. The criteria used to obtain T1 , T2 and the pulse separation, may give higher values for each of these parameters, compared to the case where all the pulses of a PBP train are taken into account in the statistical analysis, as narrow and low amplitude pulses can be omitted. PBP-RS separation is the time duration between the highly active region of the pulse train and the return stroke. T1 and T2 have the same mean value of 19 s. The mean pulse width is 38 s. The pulse separation, the pulse train duration and the PBP-RS separation have the mean values of 96 s, 3 and 56 ms, respectively. All the above mean values di6er considerably from the mode value of the corresponding parameters. In cases of the pulse separation, the pulse duration and the PBP-RS separation, one may see that the distributions are double peaked. The arrows in the charts of Fig. 2 are pointed to the columns, to which the mean values of the corresponding parameters of several studies belong. Fig. 3 illustrates a type b PBP trains. The (rst region has pulses of positive initial polarity and the second region has pulses of negative initial polarity. The ground *ash of the PBP train in Fig. 3 has not been located. The two regions with pulses of opposite polarity are given in an expanded time scale. Pulses in the (rst region are very similar to those of NBP trains, in pro(le. A few narrow pulses are superimposed on the leading edge of the (rst half, which is a typical characteristic of pulses in NBP trains. The pulse pro(le of the second region is somewhat similar to that of type a PBP trains. However, in several type b pulse trains, one or two narrow unipolar spikes are superimposed on the (rst half of few 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 two regions of pulses with opposite polarity. The last row shows the mean of the characteristics, of the six cases. The separation between the two pulse regions is de(ned as the time
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C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055 16
10
10 8 6 4 2
N2 P2 4
2
Number of PB pulse trains
n= 57 X= 19 ∝s S= 9 ∝s
N1
P1
15
10
5
>200
151-200
121-150
101-120
81-90
91-100
71-80
n= 57 X= 3.0 ms S= 1.8 ms
8
P1 6
N1 4
2
(e)
T2: Width of the second half (µs)
>7.0
6.6-7.0
6.1-6.5
5.6-6.0
5.1-5.5
4.6-5.0
4.1-4.5
3.6-4.0
3.1-3.5
2.6-3.0
1.6-2.0
0.6-1.0
40-45
36-40
31-35
26-30
21-25
16-20
11-15
6-10
0-5
(b)
2.0-2.5
0
0
1.1-1.5
Pulse train duration (µs) 14
25
P2
N1
n= 57 X= 38 ∝s S= 16 ∝s
P1
Number of PB pulse trains
20
n= 57 X= 56 ms S= 48 ms
12
15
N2 10
P2 5
10
N1
8
P1 N2
6 4 2
(f)
>100
91-100
81-90
71-80
61-70
51-60
41-50
31-40
0-10
71-80
61-70
51-60
41-50
31-40
11-20
0-10
21-30
Width of PB pulses (µs)
21-30
0
0
11-20
Number of PB pulse trains
Pulse seperation (µs)
10
20
Number of PB pulse trains
61-70
21-30
(d)
T1: Width of the first half cycle (µs)
25
(c)
51-60
0
36-40
31-35
26-30
21-25
16-20
11-15
6-10
0-5
0
(a)
P1
6
41-50
12
n= 57 X= 96 ∝s S= 56 ∝s
8
Number of PB pulse trains
N1
Number of PB pulse trains
N1
n= 57 X= 19 ∝s S= 9 ∝s
31-40
P1 14
PBP-RS seperation (µs)
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 second half cycle of pulses (T2 ). (c) Width of pulses (T1 + T2 ). (d) Separation between individual pulses. (e) Pulse train duration. (f) Separation between 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 and N2: Weidman and Krider (1979) study of NBP trains. The other abbreviations are, n: Number of pulse trains; X : Mean value; S: Standard deviation.
duration between the most active places of the two regions. In several cases, there exists a low active period in between the two pulse regions. In the (rst region T1 , T2 , T1 + T2 and
Pulse separation have the mean values of 13, 13, 27 and 62 s, respectively. In the second region, the above values are 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 4.1
1
Electric field (V/m)
3.1 2.1
2
1.1 0.1 -0.9 -1.9 -2.9 0
0.5
1
(a)
1.5
2
2.5
3
3.5
4
Time (ms) 4.1
Electric field (V/m)
3.1 2.1 1.1 0.1 -0.9 -1.9 -2.9 0.3
0.4
(b)
0.5
0.6
0.7
0.8
2.1
2.2
Time (ms)
Electric field (V/m)
0.8
0.3
-0.2
-0.7
-1.2 1.7
(c)
1.8
1.9
2
Time (ms)
Fig. 3. Flash No. 960724.04 A type b PBP train. (a) The entire pulse 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 time scales. A positive (eld change de*ects upwards.
duration of the (rst region and the second region are 1.3 and 2:8 ms, respectively. The mean PBP-RS separations for the two regions are 81 and 77 ms, respectively. The pulse characteristics 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 shows the pulse characteristics of this type of PBP trains. The pulse
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characteristics, T1 , T2 , T1 + T2 and Pulse separation have the mean values of 16, 15, 31 and 51 s, respectively. The pulse train duration and the PBP-RS separation have mean values of 2.1 and 44 ms, respectively. Pulse characteristics and pulse pro(le of this type of PBP trains is comparable with those of NBP trains. All 3 PBP trains with pulses of irregular polarity continue upto the return stroke. Fig. 5 epitomises one such case. In all 3 cases, this irregular pulse activity is visible for a few milliseconds, even after the return stroke. However, Cooray (1984) has observed that in a number of positive return strokes, a pulse burst appeared immediately after the return stroke, in which case, there was no pulse activity immediately prior to the return stroke. Hence, one cannot con(dently conclude, that the pulses, which appear after the return stroke, in the 3 type d PBP trains, are continuations of the respective PBP trains. The time from the beginning of the PBP train to the return stroke, in these 3 cases are 33, 27, and 22 ms. The width of these pulses is in the range of 5–15 s, while the pulse separation is in the range of 20–80 s. Thus, in pulse train duration, pulse width and pulse separation, this type of PBP trains are considerably different from chaotic pulse trains, that occur mainly in association with negative subsequent strokes (Gomes et al., 1998). Fig. 6 (curve 1 and 2) shows the frequency spectrum of the electric (elds of 23 type a PBP trains normalised to a distance of 50 km. The *ashes of these pulse trains have occurred at a distance of 50–150 km. Thus, the higher frequency part of the spectrum may be reduced due to the propagation e6ects. The spectrum is calculated between 1 and 500 kHz. The digitising resolution of the data set used in this study is not adequate to analyse the spectrum above 500 kHz. Curve 1 corresponds to the mean spectrum of the entire pulse trains and curve 2 shows the mean spectrum of 30 individual pulses. The spectral values are given in dB, which are 20 times the logarithm of base 10 of the magnitude of Fourier transform of the electric (eld strength (in V/m). The peak of the spectrum of pulse trains is −76 dB and it occurs approximately at 10 kHz while that of the individual pulses is −88 dB and it occurs around 16 kHz. Curve 3 of Fig. 6 depicts the frequency spectrum of 16 NBP trains. The electric (elds in this case are also normalised to 50 km as it has been done in the previous calculation. In this case, the peak occurs at 28 kHz with a value of −81 dB. 4. Discussion In this study, we have observed that similar to negative return strokes, positive return strokes are also preceded by a train of bipolar pulses. As we have selectively recorded positive CG *ashes with PBP trains, we are not able to give statistics on the positive return strokes that are not preceded by PBP trains. More than 80% of the PBP trains consist of pulses with negative initial polarity (type a). In the study of Ushio et al. (1998), 17 PBP trains out of 19, consist of
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C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055
Table 1 Statistics 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 separation (s) (s) (s) separation train separation (s) (s) (s) separation (ms) (s) duration (ms) (s) (ms)
PBP train duration (ms)
Separation between two pulse regions (ms)
P930629.05 P930629.18 P930629.21 P960724.04 P960726.02 P960709.21 Mean SD
17 23 195 27 215 10 81 88
6.2 2.6 2.8 2.0 2.3 0.6 2.8 1.7
2.5 6.4 8.0 1.4 2.6 1.2 3.7 2.6
13 13 17 15 16 6 13 4
18 14 11 14 16 7 13 4
31 27 28 29 32 13 27 6
55 68 72 66 60 49 62 8
1.2 1.2 2.0 1.2 1.2 0.8 1.3 0.4
14 17 187 25 212 9 77 87
14 16 19 4 10 10 12 5
24 16 14 10 11 10 14 5
38 32 33 14 21 20 26 9
35 49 54 33 28 26 38 10
SD: Standard deviation. The other abbreviations are de(ned in the text. Table 2 Statistics 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 P930629.22 P930722.21 P960723.23 P960706.76 Mean SD
23 89 30 40 37 44 23
10 16 11 19 22 16 5
12 19 13 14 19 15 3
22 35 24 33 41 31 7
49 56 35 56 60 51 9
2.9 2.9 1.8 1.1 1.9 2.1 0.7
SD: Standard deviation. The other abbreviations are de(ned in the text.
pulses of negative initial polarity, thus, at both location type a is observed as the most common type of PBP trains. In the study of Gomes et al. (1997), 3 PBP trains out of 5, belong to type b. Only one pulse train consists of pulses with negative initial polarity (type a) while the other is composed of pulses of positive initial polarity (type c). The type a PBP trains consist of bipolar pulses with a relatively smooth zero-to-peak rising edge (Fig. 1b). This is in agreement with the observation of Ushio et al. (1998). In contrast, in the case of NBP trains, few sharp pulses are observed to be superimposed on the leading edge of most of the bipolar pulses. In NBP trains, these narrow spikes are observable of the pulses of *ashes that were recorded even at 200 km. The pulse characteristics of type a PBP trains are con(ned in a broad range of values with signi(cantly large standard deviations (Fig. 2). The study of Ushio et al. (1998) recon(rms this diversity of the pulse characteristics of PBP trains. According to their analysis, in PBP trains, the pulse width ranges from 5 to 52 s. The pulse separation and PBP-RS separation have values from 10 to 180 s and from 1 to 38 ms, respectively. The mean values of these parameters are given in Table 3. All these mean values are at
least about twice less than the corresponding mean values of our data. The arrow pointers of Fig. 2 clearly demonstrate this di6erence. One reason may be that Ushio et al. (1998) have taken all the pulses in each train in their analysis, while we have considered only the (ve largest pulses from each pulse train. Secondly, the disagreement in pulse characteristics may be due to the di6erence in the types of thunder storms to which the two data sets belong. The mean pulse characteristics of type a PBP trains are considerably larger than those values of NBP trains that have been observed in Sweden (Gomes et al., 1997) and somewhat similar to that have been observed in Florida (Weidman and Krider, 1979). A summary of the results of these two studies on NBP trains is given in Table 4 along with the results of the present study on type a PBP trains. The arrow pointers of the distribution charts (Fig. 2) indicate, that the mean pulse characteristics of NBP trains of Swedish thunder storms lie in the vicinity of the mode value of the corresponding charts of type a PBP trains. The mean pulse width and the mean PBP-RS separation of type a PBP trains are comparable with the same mean values obtained by Weidman and Krider (1979) for NBP trains. The mean pulse
2
1.5
1.5
1
Electric field (V/m)
Electric field (V/m)
C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055
1 0.5 0
0.5
0
-0.5
-1
-0.5
-1.5
-1 0
0.1
0.2
0.3
(a)
0.4
0.5
0.6
0.7
0.8
0.9
4
1
4.5
5
(a)
Time (ms)
5.5
6
Time (ms) 1.5
2
1
Electric field (V/m)
1.5
Electric field (V/m)
1053
1
0.5
0
0.5
0
-0.5
-0.5 -1
(b)
4.5
0.45
0.5
0.55
0.6
0.65
4.55
4.6
4.65
(b)
4.7
4.75
4.8
4.85
4.9
Time (ms)
Time (ms)
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), is somewhat higher than that of this study, but Fig. 2d shows that there is a considerable number of pulse trains that belongs to a range of pulse separation close to 130 s. Thus, we (nd no clear distinction between the pulse characteristics of NBP trains and those of the most commonly observed type of PBP trains. Note that, the pulse characteristics of Ushio et al. (1998) are similar to those of NBP trains that were reported in Gomes et al. (1997). One of the interesting observations of this study is the 6 type b PBP trains. The electric (eld perturbation of these pulse trains starts with pulses of positive polarity, of which the characteristics are very similar to that of NBP trains observed in Sweden. The amplitude of pulses gradually decays and after a quiescent period of a few milliseconds, another pulse train with pulses of opposite initial polarity occurs. There is little di6erence in the pulse characteristics of the two regions other than the polarity reversal (Table 2). Totally, type b constitutes about 12% of PBP trains, observed in Sweden (observations of both Gomes et al., 1997 and this study). This type of pulse trains have not been reported before, in connection with any lightning event.
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.
-75 -80 Electric field spectrum (dB)
-1 0.4
-85 -90
1
3 2
-95 -100 -105 -110 -115 -120 -125 103
104 105 Frequency (Hz)
106
Fig. 6. The mean frequency spectrum of (1) The electric (elds of the 23 distant located type a PBP trains (2) Thirty individual pulses of type 1 PBP trains (3) Sixteen distant located NBP trains. The electric (elds are normalised to 50 km. The spectral values are given in dB, which are 20 times the logarithm of base 10 of the magnitude of Fourier transform of the electric (eld strength (in V/m).
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C. Gomes, V. Cooray / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1047 – 1055
Table 3 The pulse characteristics of PBP trains as given in Ushio et al. (1998)
Pulse width (s) Pulse separation (s) PBP-RS separation (ms)
No of pulses
Mean
Standard deviation
Range of values
132 219 19∗
18.8 54.2 12
7.9 35.7
5–52 10–180 1–38
The mean duration of the PBP trains has been estimated to be around 1 ms. ∗ No. of pulse trains. Table 4 Parameter T1 (s) T2 (s) Pulse width (s) Pulse separation (s) NBP/PBP duration (ms) NBP/PBP-RS separation (ms)
Weidman and Krider (1979)
Gomes et al. (1997)
This study
41
9 14 23
19 19 38
130
63
96
1.7
3.0
26
56
82
In the (ve cases of type c PBP trains, the pulse characteristics are not much di6erent from that of NBP trains (Table 3). The pulses of this type are also very similar in pro(le to those of the NBP trains observed at the same location. However, in the cases of type b, and type c PBP trains, the numbers of events are insuIcient to provide a good comparison with the previously observed data. Another interesting outcome of this study is the observation of the several types of pulse trains (4 types) with pulses of di6erent initial polarity. In contrast, negative ground *ashes that we have observed very seldom consist of preliminary breakdown pulse trains with pulses of initial polarity opposite to that of the succeeding return stroke. In few cases, that we have observed, negative return strokes were preceded, by a pulse train with pulses of polarity opposite to that of the return stroke. However, these pulses mostly resemble the characteristics of the pulses of isolated cloud *ashes, rather than that of the preliminary breakdown pulses. In cases where the discharge channel is vertical, the positive initial polarity of pulses indicates an uplifting of positive charge and the negative initial polarity indicates a lowering of positive charge (opposite directions if negative charge is considered). On the other hand, if breakdown events take place in a non-vertical geometry, then, a positive, charge moved away from the observer, gives the same result as in the case that the same charge is uplifted in a vertical channel. Such horizontally extended pre-leader breakdown events, associated with negative ground *ashes, have been observed by Rhodes (1989), Rhodes and Krehbiel (1989)
and Rhodes et al. (1994), using radio-interferometric methods. Some of these observed discharge paths are not straight but curved in 3-D space. Thus, one possibility of the observation of pulse trains with di6erent initial pulse polarity and polarity reversal may be the di6erence and change in horizontal direction of charge transfer. However, it may be noted that, in connection with negative lightning, NBP trains are observed only of single initial pulse polarity. Contrary to the above discussion, one may speculate different initiating mechanisms for positive return strokes, on the following observations of PBP trains. Unlike in the case of the NBP trains, PBP trains are of several types. Furthermore, the distributions of most of the pulse characteristics of PBP trains are signi(cantly diversi(ed and double peaked. These two observations may be indications which show that these pulse trains are due to discharge processes in between di6erent charge regions of the cloud (e.g. main negative and positive charge centres, positive charge pocket, screening layers, irregularly located charge regions as reported in recent studies, etc.). The existence of several mechanisms of initiation of positive CG *ashes is supported by the study of Rust et al. (1981). They have reported that, in spring and summer thunder storms, there are several regions of the cloud from which positive lightning emanate (i.e. from high on the back of the main storm tower, through the wall cloud and from the downshear anvil etc.). However, further studies in locating radiation sources in the cloud with simultaneous (eld measurements at ground are required to backup the above speculation of the di6erent initiating processes of positive CG lightning. As it was shown in Fig. 6 the spectrum of NBP has a peak that occurs at a higher frequency than that of the spectrum of PBP trains does. One reason for this shift of peak may be the sharp pulses superimposed on the leading edge of the bipolar pulses of negative *ashes. However, in general, the two spectrums are not very di6erent from each other. As these pulse trains have propagated several tens of kilometres over the land, the high frequency part of the pulses may be attenuated 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
detailed study of these pulse trains observed in summer thunder storms. (2) Considering the initial polarity of pulses, 4 types of bipolar pulse trains preceding positive return strokes were identi(ed. Those are, pulse trains with (I) bipolar pulses of negative initial polarity, (II) two regions; one with bipolar pulses of positive initial polarity, followed by a train of bipolar pulses with negative initial polarity (III) bipolar pulses of positive initial polarity, and (IV) pulses of irregular initial polarity. (3) The majority of pulse trains (57 pulse trains) belongs to the (rst category, while 6 come under the second category, 5 under the third, and 3 under the fourth. (4) The main type of pulse trains consist of pulses with relatively smooth leading edge of the initial half cycle, which is in contrary to the preliminary breakdown pulses of negative ground *ashes, where few sharp, narrow, unipolar pulses are superimposed on the positive leading edge. The pulse characteristics have largely diversi(ed distributions. Thus, a clear discrimination cannot be prescribed for these pulse characteristics and those of NBP trains observed in Sweden and that in Florida (in fact, the two data sets of NBP trains show considerable di6erence from each other). According to the distributions of pulse characteristics, we conclude that the results of Ushio et al. (1998), on PBP trains, are not different from those of ours. Furthermore, the pulse structure of PBP trains as described in Ushio et al. (1998) is similar to that of this study. (5) The second type of pulse trains with two distinct regions is a new observation in lightning literature. They constitute about 12% of the total PBP trains observed in Sweden. (6) We described the observation of several types of preliminary breakdown pulse trains in connection with positive return strokes as due to two possible reasons. One is the geometrical alignment of the channel, which may give rise to both types of polarities and even the reversal of the polarity. The second possibility is the breakdown between several combinations of regions in the cloud that may give rise to the ground *ash. For both explanations further investigations should be conducted to make proper conclusions. (7) The spectrum of the main type of PBP trains, which has the peak at about 10 kHz, is somewhat similar to that of NBP trains and that of negative return strokes. However, the spectrum of individual pulses in the PBP train, has a peak at about 16 kHz, a frequency, which is higher than that for the entire pulse train. Acknowledgements Authors thank Prof. Viktor Scuka for placing excellent research facilities at their disposal. Financial assistance given by the IPPS of the International Science Programs, Uppsala University, and the Swedish Natural Science Foundation for the research grant G-AA/GU 01448-315 are greatly acknowledged.
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