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Atmospheric Research 116 (2012) 130–133
Contents lists available at SciVerse ScienceDirect
Atmospheric Research
journal homepage: www.elsevier.com/locate/atmos
Time-intervals between negative lightning strokes and the creation of new
ground terminations
Marco Antonio da Silva Ferro a,⁎, Marcelo Magalhães Fares Saba b, 1, Osmar Pinto Jr. b, 1
a
b
IAE, Institute of Aeronautics and Space, Atmospheric Science Division, Praça Marechal Eduardo Gomes, 50, Vila das Acácias, ZIP: 12228–904, São José dos Campos, SP, Brazil
INPE, National Institute for Space Research, Atmospheric Electricity Group; Av. dos Astronautas, 1758, Caixa Postal 515, 12201-970, São José dos Campos, SP, Brazil
a r t i c l e
i n f o
Article history:
Received 19 September 2011
Received in revised form 15 March 2012
Accepted 21 March 2012
Keywords:
Lightning
New channels
Channel conditioning
Interstroke time interval
a b s t r a c t
On average, negative cloud-to-ground (CG) lightning flashes produce 3 to 5 return strokes, and a
new ground termination is produced when any stroke after the first strikes the ground in a
different place. In order to understand better the physical factors that affect the formation of new
ground terminations, high-speed digital video cameras with time-resolutions and exposure times
ranging from 125 μs (8000 frames per second) to 2 ms (500 frames per second) were used to
record images of cloud-to-ground lightning in southern and southeastern Brazil and southern
Arizona (USA), between February 2003 and September 2007. Some relevant information
regarding the formation of new channels was obtained from the analysis of the previous
interstroke time intervals and the number of previous strokes following the same path to ground.
Although most of the subsequent strokes tend to follow the previously formed channel, this
tendency is not observed in the second stroke (that is, the first subsequent stroke). 52% of the new
channels occur in the second stroke. Contrary to what it was generally assumed in some past
studies (Kitagawa et al., 1962; Malan, 1956; Rakov and Uman, 1990; Rakov et al., 1994; Winn
et al., 1973), the formation of a new channel stroke is not clearly dependent on the interstroke
interval that precedes it. In general, most of the new channels occur after a single usage of the
channel and in these cases the previous interstroke time interval is not an important parameter.
However, when the channel is used more than once, a new channel occurs mostly after a long
interstroke interval.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Most negative cloud-to-ground (CG) lightning flashes
produce multiple strokes, and typically about half of these
flashes strike the ground in more than one place. The average
number of strike points per CG flash is in the range of 1.45 to
1.70, and the observed maximum number of strike points in
one flash is 6 (Rakov et al., 1994; Saba et al., 2006; Valine and
Krider, 2002). In recent decades several hypotheses have
been proposed for reasons that define the leader's choice
⁎ Corresponding author. Tel.: +55 12–3947 4553; fax: +55 12 3947 4551.
E-mail addresses: [email protected] (M.A.S. Ferro),
[email protected] (M.M.F. Saba), [email protected] (O. Pinto).
1
Tel.: + 55 12 3945 6768; fax: + 55 12 3945 6810.
0169-8095/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.atmosres.2012.03.010
between following the previously-formed channel or creating a new termination on the ground. Brook et al. (1962)
suggested that a small, steady current flowing in the lightning
channel during the interstroke period might be necessary to
maintain a level of ionization sufficient to allow a dart leader to
traverse the previous stroke's path. Later, Uman and Voshall
(1968) suggested that the temperature decay in the lightning
channel is sufficiently slow so that appreciable temperature and
electrical conductivity exist in the lightning channel tens of
milliseconds after the effective current cessation. Kitagawa et al.
(1962) found that when the preceding interstroke time interval
was shorter than 100 ms, a subsequent stroke could follow the
same channel. If the interstroke interval was longer than 100 ms,
a subsequent stroke, if any, would always be initiated by a
stepped-leader, thus establishing a different path to ground.
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M.A.S. Ferro et al. / Atmospheric Research 116 (2012) 130–133
131
Winn et al. (1973) reported new channel occurrence with a
preceding time interval shorter than 100 ms and suggested that
the redistribution of charge by a return stroke might increase the
local electric field inside the cloud and cause a completely new
leader to form.
Rakov and Uman (1990) and Rakov et al. (1994) reported
no changes in channel geometry after strokes of order 4 in
their data set, so an unalterable path to ground is apparently
established after at least four consecutive strokes down the
same channel.
They also reported that although 22 subsequent strokes
(out of 115) had a preceding time interval longer than 100 ms
(without evidence of a long continuing current) no new
terminations on the ground were observed in these cases,
contrary to what was predicted by Kitagawa et al. (1962).
Krehbiel (1981) showed evidences that abrupt changes in the
cloud's electric field could cause a “cut off” of the channel
current, leaving a residual negative charge at about 1 km above
the ground. This residual charge would then be responsible for
the deviation of the next leader to a completely new path to
ground. Shao et al. (1995) described one flash that produced
“negative partial channels,” or attempted leaders propagating
downward in the original channel, and being deviated near the
region where the current was cut off. Mazur et al. (1995) also
suggested that the negative charge associated with the current
cut off and the development of attempted leaders may play a
significant role in altering the geometry of the negative leaders
that initiate new paths to ground. Although Mazur et al. (1995),
Valine and Krider (2002) and Saba et al. (2006) confirmed the
observation made by Rakov et al. (1994) that the probability of
creation of a new channel decreases drastically when the order
of the stroke increases, some of them reported the occurrence of
new channels in strokes of an order greater than five (Ferro et
al., 2009). They also observed a high percentage of new channels
created after there had been just one stroke in the previous
channel.
This study analyzes when the interstroke time interval is
an important factor in the formation of a new channel in
multiple-stroke flashes.
study. Flashes with channels that were obscured by precipitation, the terrain, or that were too diffuse were discarded.
Only flashes with different grounding contacts for different strokes were analyzed. Branching that occurs during the
leader propagation and which may create “forked strokes”
when simultaneously touching the ground (Ballarotti et al.,
2005; Kong et al., 2009) or “root branching”, i.e. branches that
occur only at distance of from 20 to 50 meters from the ground,
(Schonland et al., 1935) is not a part of this study.
In order to determine the stroke polarity, we used data from
two Vaisala lightning location systems, BrasilDAT, in Brazil, and
the NLDN, in the United States. More information on the
characteristics of these networks are provided by Pinto et al.
(2006) and Orville (2008), Cummins and Murphy (2009) and
Stall et al. (2009), respectively. GPS time-synchronization was
used to match strokes between cameras and networks (to an
accuracy better than 1 ms). A flash was considered to have
negative polarity if all the detected strokes had negative polarity.
2. Instrumentation and data collection techniques
Table 1 shows geometric mean (GM) values for interstroke
time intervals preceding subsequent strokes that remained in
a PEC and for intervals preceding subsequent strokes that
produced an NGC. Results from two past studies are also
shown for comparison.
It is important to observe in Table 1 that the interval
sample size for strokes producing an NGC has increased
significantly in this study on previous ones. Saba et al. (2006)
used a sample of NGC strokes that was nearly 2.5 times larger
than the sample used by Rakov et al. (1994), whereas in the
A RedLake MotionScope 8000S high-speed digital video
camera (at 1000 frames per second) was used to record
images of CG flashes in southern Brazil between December
2006 and March 2007. In August 2007 this camera, along
with a Photron PCI-512 high-speed digital camera (at 4000 to
8000 frames per second), was used to record several flashes
in Tucson, Arizona. The high-speed video images were GPS
time-stamped and stored in the cameras until a signal was
received from an external source and the trigger point could
be set to determine how many video frames were read out
before the event of interest. Each trigger pulse was initiated
manually by an operator when a flash was observed within
the camera's field-of-view. The spatial separation that was
resolved by the cameras was 2 to 60 m for flashes occurring
at distances of 1 to 30 km from the cameras. For more details
on the operation and accuracy of high-speed cameras for
lightning observations, see Saba et al. (2006).
A sub-set of 186 negative CG flashes containing at least one
stroke following a new channel to ground was used for this
3. Results and discussion
Out of 736 subsequent strokes contained in 186 selected
flashes, 291 (40%) produced a new ground contact (NGC), and
445 remained in a pre-existing channel (PEC). Subsequent
strokes occurring after a new channel that returned to a
previously used channel (21 out of 736) were considered PEC.
In the group of 291 strokes that produced an NGC, 83% (242)
occurred immediately after an initial stroke – that is, the first
stroke down a given channel, as defined by Krehbiel et al.
(1979) and Mazur et al. (1995) – and 17% (49) occurred after
two or more had traversed the previous channel.
The following sections analyze the influence of the preceding
interval, the stroke order, and the number of strokes in the
previous channel, in order to investigate the role of the time
interval in the new channel formation process.
3.1. Relation between new channel formation and preceding
interstroke interval
Table 1
Statistics for interstroke time intervals that precede subsequent PEC and
NGC.
Subsequent
PEC
NGC
All (PEC and NGC)
This work
Saba et al.
(2006)
Rakov et al.
(1994)
N
GM, ms
N
GM, ms
N
GM, ms
445
291
736
62
66
64
253
101
354
60
68
61
232
38
270
56
92
60
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study herein the sample of strokes producing an NGC is nearly
2.5 times larger than that used by Saba et al. (2006).
Rakov et al. (1994) found a GM value for interstroke
interval preceding strokes producing an NGC 53% greater than
the GM value for all the intervals together. This percentage
drops to only 12% in the subsequent work by Saba et al. (2006)
and to 6% in the work herein. Note that as the sample size
increases and becomes more statistically significant, the GM
values for time intervals preceding an NGC tend to approach
the typical GM values for the general interstroke interval. This
fact means that although time interval may play a role in the
creation of a new channel, it is probably not the predominant
factor.
3.2. Relation between new channel formation and stroke order
Since a conductive channel between the charge center in
the cloud and the ground is established by a flash's first stroke,
all subsequent strokes are expected to use the same path.
However, this is true for only 50% of the negative CG flashes
(Rakov and Uman, 1990; Saba et al., 2006). According to Rakov
and Uman (1990), the reason second strokes have the greatest
probability of creating new channels is related to the fact that
the first stroke of the flash often does not create the conditions
capable of supporting the propagation of the subsequent
leader all the way to ground.
They suggest that the multiplicity of strokes in one channel
affects the chances of forming a new or altered channel. That
is, the channel status depends on the number of strokes that
conditioned the channel previously.
In our study, out of 291 strokes producing an NGC, 140
(48%) were second strokes. Fig. 1 shows the distribution of
strokes producing an NGC according to their stroke order. In
line with past studies, our study shows that the percentage of
subsequent leaders that create an NGC rapidly decreases
when stroke order increases.
3.3. Relation between new channel formation and number of
strokes in a previous channel
Fig. 2. Occurrence of new channel formation in terms of the number of
strokes in the previous channel.
in the previous channel, 8 (6.8%) after two consecutive
strokes have used the same channel, 2 (1.7%) after three, and
1 (0.9%) after four consecutive strokes have used the same
channel. Valine and Krider (2002) also reported that 72% of
the changes in channel geometry in their study occurred after
there had been just one stroke in the previous channel. In our
study, a total of 242 out of 291 (83%) changes in channel
geometry occurred after there had been just one stroke in the
previous channel, thus confirming that an unalterable path to
ground does not usually occur after only one stroke (Fig. 2).
However, one of the key results of the work herein comes
from the analysis of the remaining 49 (17% of 291) cases.
These 49 new channel events produced an NGC after 2 to 7
strokes repeatedly traversed the previous channel. Interestingly, the average interstroke time interval preceding these
new channel events (119.8 ms) was about 3.5 times greater
than the average interval between previous strokes that
follow the same channel (34.4 ms) in the same data subset.
These results are summarized in Table 2.
Note that the interstroke interval GM value for PEC strokes
in Table 2 is considerably lower than the general GM value for
PEC strokes in Table 1. Therefore, the longer time that is
required in these cases is probably due to the intense ionization
of the previous channel caused both by the consecutive usage
of the channel and the lower interstroke time between them
(Table 2).
4. Summary
Saba et al. (2006) observed that out of 117 new channels,
106 (90.6%) occurred after the occurrence of only one stroke
This study found that when a statistically significant
amount of data is analyzed, the time interval preceding the
occurrence of a new channel tends to approach the typical
interstroke interval mean value.
This study observed that 83% of new channels occurred
following the first stroke down a given channel. Thus, it
corroborates that after only one stroke an unalterable path to
ground is not usually established. Only 17% of the new channels
occur when more than one return stroke goes down the same
channel. For the remaining 17% of new channel events, a longer
Table 2
Interstroke time interval between strokes using a PEC and interstroke time
interval preceding a NGC after two or more strokes down the same channel
for a same subset of flashes.
Fig. 1. Distribution of new channel strokes according to their stroke order.
The numbers on the bars represent the number of strokes.
Interstroke time interval
N
GM, ms
Between strokes remaining in a PEC
Preceding a NGC after two or more
strokes in the same channel
78
49
34.4
119.8
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M.A.S. Ferro et al. / Atmospheric Research 116 (2012) 130–133
preceding interval (about 3.5 times greater than the average
interval between previous strokes that follow the same
channel in the same data subset) is an important factor in
new channel formation.
Therefore, this study suggests that a larger interstroke time
interval is an important factor in the creation of a new channel
only when two or more strokes have used the previous
channel. Factors other than time interval are prevalent for the
occurrence of new channels in second strokes or initial strokes,
which accounts for most NGC cases.
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