journal of atmospheric and solar-terrestrial...
TRANSCRIPT
Measurement of continuing charge transfer in rocket-triggered lightning
with low-frequency
magnetic sensor at
close range
Gaopeng Lu a, c, d, *, Yanfeng Fan b, Hongbo Zhang a, Rubin Jiang a, d, Mingyuan Liu a,
Xiushu Qie a , d, Steven
A. Cummer e ,
Congzheng Han a, f, Kun Liu a, f
aKey
Laboratory of Middle Atmosphere and Global Environment Observa tion, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China
bState Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, 100081, China
cState Key Laboratory of Numerical Mode ling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG) , Institute of Atmospheric Physics, Chinese Academy of
Sciences, Beijing, 100029, China
dCollaborative Innovation Center on Forecast and Evaluatio n of Meteorological
Disasters, Nanjing University of Information Science and Technology, Nanjing, Jiangsu,
210044, China
eElectrical and Computer Engineering Department, Duke University, Durham, NC, 27606, United States
fChengdu University of Information and Technology, Institute
of Electronic Engineering, Chengdu, Sichuan 610225, China
A R T I C L E
I N F O
Keywords:
Initial continuous current
Long continuing current
Rocket-and-wire triggered
lightning
Low-frequency magnetic sensor
A B S T R A C T
Based on the magnetic elds recorded with a compact low-frequency (LF) magnetic sensor deployed at 78 mfi
distance from the channel base, we reconstruct the time-resolved current waveform for the continuously dis-
charging processes
in classical rocket-and-wire triggered lightning ashes, including the initial continuous currentfl
(ICC) and long continuing current. Both the overall feature and the millisecond-scale slow variations (e.g.,
initial
current variation, ICC pulses and -components)
embedded in the channel-base current as measur ed with theM
conventional methods (such as the shunt
or Pearson coil) can be retrieved through
the numerical integral of close
LF magnetic signals. Despite the artifact caused by the magnetic elds radiated by the fast in-cloud processes, thefi
new approach has the advantage of
signi cantly reduced noise in
compariso n with the measurements of con-fi
ventional methods, and it is likely applicable to remotely measure the initial continuous current in upward
lightning from high objects and altitude-triggered lightning, as well as long continuing current in natural cloud-to-
ground (CG) lightning strokes that
occur at
suf ciently close range (e.g., within 100 m).fi
1. Introduction
During the classical rocket-and-wire triggered lightning where a
grounded metallic wire is unreeled from an ascending rocket (Newman
et al., 1967 Fieux et al., 1975 Fisher et al.,
1993 Lalande et al., 1998; ; ; ;
Zheng et al., 2013), when the upward leader (usually of positive polarity)
initiated from the wire undergoes a sustained development, the trig-
gering wire becomes a conduit where the electrical charge is
continu-
ously transported from the in-cloud reservoir to ground, forming a
characteristic process
called the initial continuous current (ICC) (Wang
et al., 1999). With typical amplitude of 20 300 A and a relatively long–
duration (typically longer than 100 ms), the ICC is usually responsible for
the major charge transfer in rocket-triggered lightning, and the total
charge transported by ICCs ranges between a few C and over 100
C
( ; ). It has been observed that the ICCHubert et al., 1984 Wang et al., 1999
is also present in altitude-triggered lightning (where the lower portion of
metallic wire is replaced with a nylon wire) (e.g., ) andZheng et al., 2013
upward lightning from tall objects (e.g., ;Miki et al., 2005 Diendorfer
et al., 2009 Zhou et
al., 2011; ).
In the rocket-triggered lightning experiment,
the ICC can be
directly
measured by applying a shunt (or a current transformer, such as Pearson
coil) installed
at the channel base (e.g., ;Wang et al., 1999 Biagi et al.,
2009 Yoshida et
al., 2010 Qie et al., 2011; ; ), which makes it possible to
precisely quantify the charge transfer from the thundercloud to ground.
However, one major problem is that the channel-base current spans a
wide dynamic range (from a few A
for the initial current pulses to tens of
kA for the return strokes) so that the conventional measurement is usu-
ally set with a bias toward the return strokes typically with peak
current
* Corresponding author. Key Laboratory of Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Sci-
ences, Beijing 100029, China.
E-mail address: [email protected] (G. Lu).
Contents
lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics
journal homepage:
www.elsevier.com/locate/jastp
https://doi.org/10.1016/j.jastp.2018.02.0 10
Received 10 November 2017; Received in revised form 8 February 2018; Accepted 25
February 2018
Available online 18 May 2018
1364-6826/ 2018 Published by Elsevier Ltd.©
Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
in excess of 10 kA (e.g., ; ), and thus weakYang et al., 2010 Lu et al., 2014
current pulses during the initial stage of
triggered lightning and small
variations superimposed on the initial
continuous current usually cannot
be precisely measured.
Also, for the current measurement with either shunt or Pearson coil, it
is required that the current ows along the desired path to groundfl
(usually the lightning rod that is grounded). Therefore, for the altitude-
triggered lightning where the triggering wire is not grounded and thus
the lightning charge transfer usually does not follow the lightning rod
( ; ; ), or upwardPinto et al., 2005 Saba et al., 2005 Zheng et al., 2013
lightning from high objects with a complicated grounding structure (e.g.,
Miki et al., 2005 Zhou et al., 2011 Lu et al., 2013 Wang et al., 2016; ; ; ), the
measurement of channel-base current requires substantially more efforts.
Therefore, the new technique is desired to improve the capability to
quantitatively measure the
initial continuous current in situations where
the lightning current ows through an unexpected strike point, such asfl
the indirect measurement of lightning currents with the -dot sensor asB
described by .Romero et al. (2012)
In this paper, we applied the method described by Fan et al. (2017b)
to reconstruct the current waveform of continuously discharging pro-
cesses in rocket-triggered lightning by measuring the
low-frequency (LF)
magnetic elds with a compact sensor deployed at 78 m distance fromfi
the channel base. These processes include the ICC present in nearly all
the rocket-triggered lightning, and the long continuing current that is
essentially the same as that in
the natural
cloud-to-ground (CG) lightning
strokes. The numerical integral of magnetic signals recorded at close
range is applied to retrieve the
time-resolved waveform of long
continuing current as measured by a
shunt at the channel base. One
prominent advantage of this method, in
addition to its portability to
implement and the wide range of deployment, is that the background
noise in the retrieved
current waveform is considerably reduced so that
the relatively slow
variations (such as -components with timescalesM
typically longer than 200 μs) as weak as a few hundred mA can be readily
resolved.
2. Measurement and data
The measurements examined in this
paper
were conducted during the
SHandong Arti cially Triggered Lightning Experiment (SHATLE) infi
summer of 2014 ( ). As shown in a, two magneticQie et al., 2011 Fig. 1
sensors
were deployed at distances of 78 m (on the roof of control room)
and 970 m (at
the main observation building), respectively, from the
lightning triggering site. The two induction coils of magnetic sensor
(made of one layer of solenoid varnished copper wire
winding around a
ferrite
rod with 1-cm diameter and 20-cm length) at 78 m range were
oriented in azimuthal and vertical direction (relative to
the lightning
channel), respectively; the
coils
of magnetic sensor at the main obser-
vation site were
oriented in north-south and east-west direction,
respectively. The channel-base current of classical rocket-triggered
lightning was measured
with both a
5-m shunt (with bandwidth ofΩ
0 3.2 MHz) and a Pearson
coil (with bandwidth of 0.9 Hz 1.5 MHz),– –
which have different measurement ranges of 8 A to 2 kA and 40 A to
30 kA, respectively.
The frequency response measured in the laboratory for the magnetic
sensors
used
in the observation, including the induction coil and ampli-
fying circuit, is plotted in b (e.g., ); the phaseFig. 1 Fan et al., 2017b
response of
sensor has not been quanti ed yet. The
3-dB bandwidth offi
the magnetic sensor is 6 330 kHz, and the sensor behaves roughly as a–
d /d sensor below 6 kHz. Therefore, the relatively fast processes withB t
time scales shorter than 100 μs in the triggered lightning (such as, leader
stepping, process, and return stroke) are mainly characterized by theK
B-sensor part; the relatively slow processes with time scales longer than
1
ms, such as continuing current (tens to hundreds of milliseconds)
( ;
; ), initialWilliams and Brook, 1963 Fisher
et al., 1993 Miki et
al., 2005
magnetic sensor is relatively high (e.g., ), the measurementLu et al.,
2016
data recorded in summer of 2014 is subject to a saturation for fast
magnetic pulses (e.g., with timescales less than 100 μs) with magnitude
greater than approximately 65 nT. In this paper, we mainly present the
results for two rocket-triggered lightning ashes
on August 18 andfl
August 23, respectively.
Fig. 2a shows the channel-base current measured for the triggered
lightning at 04:17:18 UTC on August 18 (the snapshot image of this event
is shown in the inset of
a), which was relatively complicated byFig. 1
comprising six
subsequent strokes with peak current ranging between
2 kA and 18 kA. As the channel-base current for
this event spans a broad
range from
~10 A to 20 kA, we plot
the time-resolved current waveform
in logarithmic scale by combining the measurements of channel-base
current with shunt and Pearson coil, respectively (namely replacing the
saturated portion
of shunt measurement with the Personal coil mea-
surement during the
same time period). It can be clearly seen that the
noise level of channel-base current measured during the SHATLE mea-
surement is about 8 A. The rst long excursion of ~230 ms duration wasfi
the initial continuous current (ICC) typical to classical rocket-triggered
lightning, which transferred a total of 18.4 C negative charge to
ground; the second long excursion
of ~260 ms after the sixth stroke was
caused by a long continuing current superposed with several millisecond-
Fig. 1.
(a) Sketch of magnetic eld measurements during the SHandong Arti-fi
ficial Triggered Lightning Experiment (SHATLE) during summer of 2014. (b)
Frequency response of the low-frequency magnetic eld sensor from the cali-fi
bration in the laboratory, showing that the magnetic sensor works as
a sensorB
between 6 kHz
and 340 kHz, and approximately as a dB/dt sensor below 6 kHz.
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
( ;
; ), initialWilliams and Brook, 1963 Fisher
et al., 1993 Miki et
al., 2005current and -component (typically longer than 2 ms) ( ),M Qie et al., 2014are primarily measured by the d /dt portion. As the sensitivity of ourB
scale variations called -components (e.g., ;M Rakov et al., 1995 Thottap-pillil et al., 1995 Campos et al., 2007; ),
similar to a triggered lightning
77
Fig. 2.
(a) Time-reso lved current waveform (absolute value in
the logarithmic scale) combined from the measurement with
shunt
and Pearson coil for the triggered lightning at 04:17:18
UTC on August
18, 2014, which contained the initial contin-
uous c urrent and six subsequent strokes, with the last stroke
followed by
a long continuing
current. (b) Time-resolved
current
waveform (absolute value in logarithmic scale)
measured with a 5-m shunt at the channel base for theΩ
rocket-triggered lightning at 16:29:52 UTC on August 23,
2014,
which only contained the initial continuous current. We
can see that the noise level of channel-base current is
approximately 8
A.
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
Fig. 3. Channel-base current
(panel a) and magnetic elds (panels b and c for 78
m and 970 m distance, respectively) measured for the initial continuous currentfi
during the triggered lightning
ash at 04:17:18 UTC on August 18, 2014.fl
78
flash examined by .Zhou et al. (2013)
The initial continuous current shown
in a, as the only case ac-Fig. 2
quired during the summer campaign in 2014 without saturation in the
magnetic eld recorded at 78 m range, has been
examined byfi Fan et al.
(2017b) to describe the method to reconstruct the time-resolved current
from the close magnetic
eld. In some cases, the saturated magneticfi
signal can be retrieved with the signal of vertical coil ( ),Fan et al., 2017b
but
this method is not applicable to all cases. Hence, it remains desirable
to reduce the gain
of magnetic sensor in the future measurement. In this
paper, we analyze a different case shown in
b (with saturation in theFig. 2
recorded magnetic eld during
the initial current variation, see b)fi Fig. 6
for
a triggered lightning ash that only contained an ICC lasting aboutfl
350 ms, over which there
were several variations and the total charge
transfer accumulated to 46.3 C. Moreover, the long continuing current
in a will also be examined
to
demonstrate the possible applicationFig. 2
of our method to reconstruct the current waveform for long continuing
current in natural lightning strokes that happen to strike the ground at
close range.
3. Method and evaluation
The case of triggered lightning analyzed by isFan et al. (2017b)
revisited here to describe the method to reconstruct the current along the
triggered lightning channel from
close magnetic measurement. aFig. 3
shows the time-resolved current waveform recorded for the initial
continuous current (ICC) of triggered lightning at 04:17:18 UTC
on
August 18, including the initial current pulses associated with the
inception of a sustained upward positive leader when the rocket reached
an altitude of 245 m (above the ground level, AGL). The subsequent
variation (referred to as
initial current variation, ICV) lasting ~20 ms is
typical to classical rocket-triggered lightning, which is linked to the
disintegration of triggering wire and the following reestablishment
of
lightning current ( ; ). The abruptWang et al., 1999 Rakov et al., 2003
enhancement of current near the end of initial continuous current (at
about 18.99 s) was an ICC pulse with a relatively
long timescale (~8 ms).
The total charge transfer during the initial continuous current was about
18.5 C.
The low-frequency magnetic elds recorded at 78 m and 970 m dis-fi
tance are plotted in b and c, respectively. As shown in the gures,Fig. 3 fi
the magnetic elds of initial
current pulses are particularly clear in thefi
signals recorded at 78 m range. These magnetic
eld pulses were inves-fi
tigated by , and they can
be best simulated as the radia-Lu et al. (2016)
tion from the downward propagation of current pulse generated at the
upper end of the ascending metallic wire ( ).Fan et al., 2017a
The initial current variation is rather complicated in this case, so as
the corresponding magnetic elds acquired at 78 m range. showsfi Fig. 4
the current waveform
of ICV ( a) and associated magnetic eldsFig. 4 fi
recorded at 78 m ( b) and 970 m ( c), respectively. The mag-Fig. 4 Fig. 4
netic signals recorded at two different distances are signi cantlyfi
different. The magnetic eld measured at 78 m range mainly exhibitsfi
millisecond-scale variations throughout the duration of ICV, while the
magnetic eld recorded at 970 m range exhibits a burst of fast pulsesfi
mainly during the early stage of ICV. This burst of magnetic pulses could
be
attributed to the step-like progression
of positive leader (Lu et al.,
2014), and it has been shown that these magnetic pulses are radiated
in
the vicinity of ascending positive
leader ( ).Zheng et al., 2018
During the rocket-triggered lightning
experiment at SHATLE, the
triggering wire usually has been unreeled over 100 m when the upward
positive leader becomes successful to launch the sequence of
initial
current pulses ( ; ). Before the ICC becomesLu et al., 2016 Fan
et al., 2017a
discernible (typically a
few ms after the inception of positive leader) in
the current measured at the channel base, the positive leader still pro-
gresses upward at a speed varying widely in the range of 2 104 m/s
to
2.1 105 m/s ( ; ). Consequently, for aBiagi et al., 2009 Jiang et al., 2013
vertical current ow with timescales longer than 1 ms, the magnetic eldfl fi
at 78 m range from the channel base is related to the current through the
Biot-Savart law. Based on the computation of magnetic eld radiated byfi
the current ow along the lightning channel, showedfl Fan et al. (2017b)
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
Fig. 4. Channel-base current (panel a) and magnetic elds (panels b and c for 78 m and 970 m distance, respectively) measured for the initial current variation (ICV)fi
of the triggered lightning at 04:17:18 UTC on August 18, 2014.
79
that for current pulses
with
characteristics timescales 1 ms, the current
waveform along the lightning path can be reconstructed by numerically
integrating the magnetic signal recorded at 78 m range, i.e.,
' 0
d
d 'd ' (1)
where is the conversion coef cient
at time (which does
not vary(t) fi t
considerably with time when the channel length is more than 200 m),
and is the magnetic eld measured at 78 m range. The exact value ofB fi
should depend on the distance from the channel base, while it is
also
affected by many factors present in the eld experiment that are dif cultfi fi
to be considered in a
theoretical derivation. Therefore, in our work, we
determine the value of by comparing the measured channel-base cur-
rent and the numerical integral result of
close magnetic eld; for the vefi fi
cases of rocket-triggered lightning in SHATLE during 2014, there is
~0.00195 0.00005 A/nT ( ). Generally speaking, theFan
et al., 2017b
magnetic signal acquired by the sensor contains measurement noise
(from both data acquisition device and background), but the numerical
integral of this term typically goes to zero. Also, the DC offset in the
magnetic measurement is appropriately removed before the numerical
integral.
The waveform
of channel current retrieved from
the magnetic eldfi
shown in b using the
numerical integral described above is plottedFig.
3
in a. The major features present in the shunt measurement,Fig. 5
including all the millisecond-scale variations around the time with
abundant
de ections in the current
waveform,
are present in thefl
retrieved current waveform. The good consistency between the retrieved
current waveform and the measured channel-base current is largely
caused by that the ICC is mainly composed of
charge transfer processes
with time scales longer than 1 ms. The initial current pulses are not
retrieved as these pulses are of microsecond scale; for the impulsive
current pulses with relatively short time scales (e.g., 10 μs for the initial
current pulses), there is an approximately linear correlation between
channel-base current and magnetic eld measured with our sensor atfi
78 m distance ( ). As the initial current pulses do
notLu et al., 2016
contribute considerably to charge transfer, the total charge transfer
during the initial continuous current calculated from the retrieved cur-
rent waveform (about 18.45 C) is in good
agreement with the
measurement.
To quantitatively demonstrate that the noise level in the channel
current retrieved from the numerical integral of close magnetic eldfi
measurement is substantially reduced in comparison with the direct
channel-base current measurement, we performed the Fourier transform
of retrieved current waveform (e.g., ). Note that theRomero et al., 2012
numerical integral is also
applied to the component above 6 kHz,
and
therefore we need to compensate by dividing the result of Fourier
transform with the following factor
1
1 2j 6000(2)
As shown in b,
the result of Fourier transform
for the retrievedFig. 5
current waveform indicates that the reconstructed waveform is generally
consistent with the measurement below about 6 kHz. The at partfl
beginning approximately at 30 kHz is
from the background noise.
Therefore, from a technical perspective, the noise oor in the channel-fl
based
current retrieved from close magnetic eld is only about 1/10 offi
that for the
measurement with a shunt (namely the close magnetic eldfi
measurement can be used to detect the slow current variation as weak as
a few hundred mA).
We also applied the method described above to the magnetic eldsfi
measured at 970 m range, and the channel current cannot be recon-
structed with good consistency. The reason is that at a distance compa-
rable to or larger than the
vertical scale of lightning channel (about
several
hundred meters), in addition to the magnetic
eld from thefi
current along the vertical lightning channel, the magnetic signal received
by the sensor also contains a
signi cant portion of radiation eld fromfi fi
the in-cloud processes, such as -processes (as
shown in ).K Fig. 3
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
Fig. 5. (a) Waveform of
initial continuous current reconstructed from the low-frequency magnetic eld recorded at 78 m distance from the channel base. (b)
Fourierfi
transform of channel-base current from the measurement with
a shunt (black line) and the current waveform reconstructed from the integral of close magnetic eldfi
(blue line). (For interpretation of the references to color in this gure legend, the
reader is referred to the Web version of this article.)fi
80
Nevertheless, further theoretical and experimental studies are desired to
determine the range within which the method proposed in this paper is
applicable to reconstruct the channel-base current
waveform. As
mentioned earlier in this section, the magnetic eld measured at 970 mfi
distance from the channel distance is dominated by the radiation from
the in-cloud processes. For the magnetic eld measured at 970 m range,fi
the signal contains more de ections caused by
fast discharges.fl
4. Measurement of initial continuous current
In this section, we apply the method described above to reconstruct
the 350-ms long initial
continuous current shown in b. As shown inFig. 2
Fig. 6b, the magnetic signal recorded at 78 m distance is subject to
saturation for some part associated with the portion of initial continuous
current with magnitude up to about 2100 A. Therefore, we only analyze
the current measurement without saturated magnetic signals in associ-
ation. During the ICC, there are several distinct enhancements, including
the major current surge starting at about 52.65 s.
4.1. Reconstructed current waveform
With the method described in section , we numerically
integrate the3
current waveform (for the same time interval) measured at the channel
Fig. 6.
Measurement for the rocket-triggered lightning at 16:29:52 UTC on August 23, 2014: (a) the channel-base current measured with a 5-m shunt; and (b) theΩ
low-frequency (LF) magnetic eld measured at 78 m distance from the channel base. The magnetic
measurement is saturated at the interval (during the
initial currentfi
variation) marked by the thick gray line.
Fig. 7. Comparison between the shunt measurement of initial continuous cur-
rent and the current waveform reconstructed from the LF magnetic eld at 78 mfi
distance for the triggered lightning
at 16:29:52 UTC on
August 23, 2014. The
red color marks the segment of channel-base current
that is not reconstructed
due to the saturation of magnetic
signal measured at 78 m range. (For
inter-
pretation of the references to color in this gure legend, the reader is referred tofi
the Web version of this article.)
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
close magnetic elds recorded at 78 m range to reconstruct the currentfiwaveform along the lightning channel. For the convenience of
compar-
ison, we add an appropriate offset to the retrieved current waveform
shown in (and also for other retrieved current waveforms shown inFig. 7
this paper).
In general, the current waveform can be retrieved with fairly
good consistency with the channel-base current recorded with shunt,
including all the small millisecond-scale variations. During the initial
continuous current, the lightning channel remains illuminated and the
conductivity of lightning channel is very high (on the order of 10 4 s/m)
( ), and there
is no considerable attenuation
in the currentRakov, 1998
along lightning channel. Therefore, the current along the triggered
lightning channel should be almost equal to
the channel-base current as
measured by the shunt.
We also calculate the total charge transfer during the time interval of
numerical integral, which is 44.5 C according to the shunt measure-
ment, in good agreement with that (also 44.5 C) calculated with the
current waveform (for the same time interval) measured at the channelbase.
In the following section
,
we will examine the channel current4.2
waveform reconstructed for two major ICC variations indicated in Fig. 6
(as
ICC pulse and ICC pulse b, respectively).
4.2. Initial continuous current variations
We rst examine a large current pulse superposed on the ICC wave-fi
form that
appeared near 52.65 s in a (denoted as ICC pulse a). TheFig. 6
duration of this ICC variation pulse was longer than 10 ms, and it con-
tains multiple variations in the current waveform. a shows theFig. 8
magnetic signals recorded at 78 m distance from the channel base, and
we
can see that the initial part of this current pulse was superposed with a
long sequence of
microsecond-scale magnetic pulses, which could also be
observed in the magnetic eld recorded at 970 m (not shown).fi
81
As shown in b, the
current waveform reconstructed from theFig. 8
close magnetic eld is in excellent agreement with the measurement.fi
When we zoom in to
examine the reconstructed current waveform, we
can still identify some small
variations with amplitude of ~0.1 A that
might be associated with the aforementioned microsecond-scale mag-
netic pulses. However, since the current variation on this
magnitude
could not be identi ed from the measured channel-base current, wefi
cannot conclude whether the reconstructed small variations
are artifacts.
Figure 9 shows the current waveform reconstructed for another ICC
pulse at about 52.773 s in a (indicated as ICC pulse b). As shown inFig. 6
Fig. 8a, the magnetic eld recorded at 78 m distance exhibits a fast pulsefi
around the onset of the current enhancement, which is
similar to a case
examined
by , ). However, in our case examinedFan et al. (2017b Fig. 8
here, the
polarity of magnetic pulse appears to
be opposite to that asso-
ciated with ensuing current pulse. The presence of fast magnetic pulse in
the numerical integral of close
magnetic eld causes an artifact in thefi
reconstructed current waveform. Other than that, the general features,
including the rise time and magnitude of enhancement, are correctly
obtained.
5. Measurement of long continuing current
In some rocket-triggered lightning ashes and also lightning to highfl
objects, one of the subsequent return strokes will be followed by a long
continuing current (e.g., , ). In this section, weZhou et al., 2011 2013
applied our method to the long continuing current (i.e., the second long
excursion of
~270 ms
in a) that followed the sixth return stroke ofFig. 2
triggered lightning at 04:17:19 UTC on August 18, 2014, demonstrating
the likelihood of using numerical integral of close magnetic
elds tofi
remotely detect long continuing current in natural CG strokes.
Fig. 8. Measurement for a large current variation during the initial continuing current: (a) the magnetic eld measured at 78 m distance; (b) current waveform (withfi
offset) reconstructed from
the numerical integral of magnetic eld recorded at 78 m range in comparison with the channel-base measurement .fi
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
Fig. 9. Measurement
for another current variation during the initial continuous current: (a)
magnetic eld recorded at 78 m
range; (b) time-resolved current measuredfi
at
the channel base in comparison with the current waveform (with offset) reconstructed
from the numerical integral of magnetic eld recor ded at 78 m range.fi
82
The channel-base current and broadband magnetic elds recorded atfi
78 m for the long continuing current are plotted in a and b,Fig. 9
respectively. As shown in the gure, there are several -componentsfi M
superimposed on the long continuing current (e.g., Thottappillil
et al.,
1995 Flache et al., 2008 Zhou et al., 2013; ; ), leading to
current surges
with magnitude up to 1 kA. The magnetic signal recorded at 78 m was
saturated at
the moments corresponding to the return stroke (with peak
current of 19 kA) and the largest -component that peaks at aboutM
1400 A.
5.1. Reconstructed current waveform
Due to the saturation of magnetic pulse driven by the relatively strong
M-component
(with current peaking at 1.4 kA) at about 19.237 s, we
processed the magnetic signals measured at 78 m distance in two sepa-
rated time intervals divided by the -component that caused the satu-M
ration. The current waveform retrieved from the numerical integral of
several
fast pulses superposed on a millisecond-scale pulse, including one
relatively large pulse around the onset of the -component, which isM
interpreted as a -change. Due to the presence of this fast pulse, there isK
an artifact in the retrieved current waveform for the -componentM
( b).Fig. 12
Fig. 13 shows the measurements for an -component with a rela-M
tively complicated waveform by consisting of
three consecutive current
pulses (e.g., ). As shown
in a, there is noZhou et al., 2013 Fig. 13
discernible magnetic signal associated with the onset of rst currentfi
pulse, while the second current pulse appears to be initiated by a fast
K -process, similar to the case shown in a. Therefore, the LFFig. 12
magnetic signal recorded at 78 m distance is interesting in that it in-
dicates the
physical process leading
to -component as well as the directM
measurement of current enhancement at the channel base due to the
charge transfer in the -component.M
6. Discussions and summary
Fig. 10. Channel-base current (panel a) and magnetic eld measured at 78 m range (panel b) during
the long continuing current of triggered lightning at 04:17:18fi
UTC on August 18, 2014.
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
ration. The current waveform retrieved from the numerical integral ofclose magnetic signals is shown in . Again, the reconstructedFig.
11current waveform (with a signi cantly reduced noise level) is in goodfi
agreement with the measurement, including all the -components.M
Our method makes it possible to remotely measure the time-resolved
waveform of continuing current in natural cloud-to-ground (CG) light-
ning for which the
stroke location is usually unpredictable and thus the
direct measurement of channel-base current
is almost impossible. How-
ever, it should be noted that the treatment of continuing current in
nat-
ural CG strokes could be more
complicated than that in the rocket-
triggered lightning as investigated here. First of all, the stroke channel
of natural lightning usually contains some tortuosity even for the lowest
portion above the ground. Moreover, a precise estimation of distance
from the stroke channel is challenging. Further analyses are desired to
explore
the feasibility to use the dB/dt sensor as an effective tool to
remotely measure the continuing
current in natural CG strokes.
5.2. M-components
The -component superimposed on the long continuing current canM
be well retrieved with our method. shows the reconstructionFig. 12
result for one -component that exhibits a relatively simple currentM
waveform. The magnetic eld recorded at 78 m distance ( a) showsfi Fig. 12
6. Discussions and summary
In this paper, we apply the method presented by toFan et al. (2017b)
reconstruct the channel current of rocket-triggered lightning on
the basis
of low-frequency magnetic elds recorded with a compact magneticfi
sensor at
78 m range. The main advantage
of this method to retrieve the
time-resolved current waveform with relatively long time
scale is that it
is a remote sensing approach that does
not require the current ow tofl
propagate through a desired location.
Provided that the magnetic sensor
is installed suf ciently close to the lightning channel, the currentfi
waveform could be
reconstructed through the numerical integral of
recorded magnetic signals.
This method is developed on the basis of two factors. First of all, the
noise present in
the magnetic eld measurement is of random distribu-fi
tion, and
the numerical integral of background
measurement almost goes
to zero. Therefore, the reconstructed current waveform is essentially
noiseless. Secondly, the main charge transfer contributing to the
continuing current usually has the time scale longer than 1 ms, and
therefore the associated lightning magnitude eld at 78 m range isfi
dominated by
the induction component ( ), which isFan et al., 2017b
approximately proportional to the current along the lightning channel.
In this paper, we examine both cases of initial continuous current
and
long continuing current with the close magnetic measurement. The same
83
Fig. 11. Comparison between the direct measurement of channel-base current and the current waveform (with offset) reconstructed from the magnetic eld recordedfi
at
78 m range for two segments of long
continuing current
separated by the largest -component (which caused the saturation of magnetic eld recorded at 78 mM fi
range) in a.Fig. 10
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
method has been applied by to the initial continuousFan
et al. (2017b)
current measured for other rocket-triggered lightning during summer of
2014, and
the
conversion coef cient is almost identical (with
variationfi
3%) for the lightning cases
at the
same range. Therefore, the recon-
struction result is reliable when the channel length is suf ciently longfi
(e.g., 200 m), and the moderate inclination
of lightning channel does
not considerably affect the conversion coef cient. This is very critical forfi
the practical application of this technique in the lightning measurement.
One drawback of our method is that the signals from in-cloud
processes
might cause some artifacts in the retrieved current waveform.
Our
method could also be applied to the altitude-triggered lightning
( ), where the lowest portion of triggering wire is nylonSaba et al., 2005
and thus the direct measurement of initial continuous current is almost
impossible. The measurement of altitude-triggered lightning has rarely
been reported except for a few cases where the downward leader
happened to strike the lightning rod (e.g., ). AlthoughZheng et al., 2013
Fig. 12. (a) Low-frequency magnetic signal recorded at 78 m distance from the channel base for an -component during the long continuing current of triggeredM
lightning at 04:17:18 UTC on August 18, 2014. (b) Time-resolved -component current measured at the channel base in comparison with the current waveform (withM
offset) retrieved from the numerical integral of magnetic signal shown in
panel a.
84
the actual stroke location is always displaced from the rod, the stroke
distance usually still satis es that assumption that
the inductionfi
component dominates the magnetic eld.fi
Also, the classical rocket-triggered lightning is similar to the upward
lightning initiated from tall buildings, where the ICC is also commonly
observed
( ; ). Although the situation isFlache et al., 2008 Zhou et
al., 2011
often
complicated
due to the actual geometry of structures, our method
is
probably suitable to remotely measure the initial continuous current of
upward lightning from tall buildings. As one step toward developing a
remote sensing method that is capable of
measuring continuing current
References
Biagi, C.J., Jordan, D.M., Uman, M.A., Hill, J.D., Beasley, W.H., Howard, J., 2009. High-
speed video observations of rocket-and-wire initiated lightning. Geophys. Res.
Lett.
36, L15801. .https://doi.org/10.1029/2009GL038525
Campos, L.Z.S., Saba, M.M.F., Pinto Jr., O.,
Ballarotti, M.G., 2007. Waveshapes of
continuing currents and properties of M-components in natural negative
cloud-to-
ground lightning from high-speed video observations . Atmos. Res. 84, 302 310 – .
Diendorfer, G., Pichler, H., Mair, M., 2009.
Some parameters of negative upward-initiated
lightning to the Gaisberg Tower (2000-2007). IEEE Trans. Electromagn C. 51
(3),
443 452– .
Fan, Y., Lu, G., Zhang, H.,
et al.,
2017a. Observations and simulations of magnetic eldsfi
radiated by initial current pulses in rocket-triggered lightning. High. Volt. Eng. 43
Fig. 13. (a) Low-frequency magnetic signal recorded at 78 m range for a complicated -component of three current pulses during the long continuing current ofM
triggered lightning at
04:17:18 UTC on August 18, 2014. (b) Time-resolved -component current measured at the channel base in comparison with the
currentM
waveform (with offset) retrieved from the numerical integral of magnetic
signal shown in panel a.
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
in natural CG strokes (at close range
100 m) with relatively high res-olution and dynamic range, we deployed a method to retrieve the
time-resolved waveform of channel
current with signi cantly reducedfi
noise, although it remains necessary to investigate how the signi cantlyfi
tilted or tortuous channel sections might affect the results. In addition to
providing an alternative means to remotely measure the current in
rocket-and-wire triggered lightning, the method presented in
this work
makes it possible to record
the current waveform.
Magnetic eld measurements have been applied
to measure thefi
current of return strokes in rocket-triggered lightning (e.g., Yang et al.,
2008 2010 Romero, ) or lightning striking the instrumented towers (e.g.,
et al., 2012). Therefore, it is desirable to develop
a comprehensive
pro-
cedure that remotely measures the current waveform
in both
rocket-triggered lightning and upward lightning from tall buildings.
Acknowledgment
This work was nancially supported by National Key Basic Researchfi
and Development Program (973) (2014CB441405), "The Hundred Tal-
ents Program" of Chinese Academy of Sciences (2013068), Natural
Sci-
ence Foundation of Excellent Youth Program of China (41622501), and
National Natural Science Foundation of China (41305005, 41574179,
41605122). The data for the analyses in this paper are available upon
request from
Gaopeng Lu ( )[email protected]
Appendix A. Supplementary
data
Supplementary data related to this article can be found at
https://doi.
org/10.1016/j.jastp.2018.02.010.
radiated by initial current pulses in rocket-triggered lightning. High. Volt. Eng. 43(3), 987 993 (in Chinese)– .
Fan, Y., Lu, G., Jiang, R., Zhang, H., Liu, M., Qie, X., 2017b. Application of low-frequency
magnetic sensor for remote measurement of
the initial continuous current in rocket-
triggering lightning. Chin. J. Atmos. Sci. 41 (5), 1027 1036.– https://doi.org/
10.3878/j.issn.1006-9895.1702.16248.
Fieux, R.P.R., Gary, C.H., Hubert, P., 1975. Arti cially triggered lightning above land.fi
Nature 257, 212 214– .
Fisher,
R.J., Schnetzer,
G.H., Thottappillil, R., Rakov, V.A., Uman, M.A., Goldberg, J.D.,
1993. Parameters of triggered-lightning ashes in Florida and Alabama. J. Geophys.fl
Res. 98, 22887 22902– .
Flache, D., Rakov, V.A., Heidler, F., Zischank, W., Thottappillil, R., 2008. Initial-stage
pulses in upward lightning: leader/return stroke versus M-component mode of charge
transfer to ground. Geophys. Res. Lett. 35, L13812. https://doi.org/10.1029/
2008GL034148.
Hubert, P., Laroche, P., Eybert-Berard, A., Barret, L., 1984. Triggered lightning in New
Mexico. J. Geophys. Res. 89, 2511 2521– .
Jiang, R., Qie, X., Wang, C., Yang, J., 2013. Propagating features of upward positive
leaders in the initial stage of rocket-triggered lightning. Atmos. Res. 129, 90 96– .
Lalande,
P., Bondiou-Clergerie, A., Laroche, P., Eybert-Berard, A., Berlandis, J.P.,
Bador, B., Bonamy, A., Uman, M.A., Rakov, V.A., 1998. Leader properties determined
with
triggered
lightning techniques. J. Geophys. Res. 103 (D12), 14,109 14,115.–
https://doi.org/10.1029/97JD02492.
Lu, W., Chen, L., Ma, Y., Rakov, V.A., Gao, Y., Zhang, Y., Yin, Q., Zhang, Y., 2013.
Lightning attachment process involving connection of the downward negative leader
to the lateral surface of the upward connecting leader. Geophys. Res. Lett. 40,
5531 5535. .– https://doi.org/10.1002/2013GL058060
Lu, G., Jiang, R., Qie, X., Zhang, H., Sun, Z., Liu, M., Wang, Z., Liu, K., 2014. Burst of
intracloud current pulses during the initial continuous current of a rocket-triggered
lightning ash. Geophys. Res. Lett. 41 .fl https://doi.org/10.1002/201 4GL062127
Lu, G., Zhang, H., Jiang, R., Fan, Y., Qie, X., Liu, M., Sun, Z., Wang, Z., Tian, Y., Liu, K.,
2016. Characterization
of initial current pulses in negative rocket-triggere d lightning
with
sensitive magnetic sensor. Radio Sci. 51 https://doi.org/10.100 2/
2016RS005945.
Miki, M., Rakov, V.A., Shindo, T., Diendorfer, G.,
Mair,
M., Heidler, F., Zischank, W.,
Uman, M.A., Thottappillil, R., Wang, D., 2005. Initial stage in lightning initiated from
tall objects and in rocket-triggered lightning. J. Geophys. Res. 110, D02109. https://
doi.org/10.1029/2003JD004474.
85
Newman, M.M.,
Stahmann, J.R., Robb, J.D., Lewis,
E.A., Martin, S.G., Zinn, S.V., 1967.
Triggered lightning strokes at very close range. J. Geophys. Res. 72, 4761 4764– .
Pinto
Jr., O., Pinto, I.R.C.A., Saba, M.M.F., Solorzano, N.N., Guedes, D., 2005. Return
stroke peak current observations of negative
natural and triggered lightning in
Brazil.
Atmos. Res. 76, 493 502– .
Qie, X., Jiang, R., Wang, C., Yang, J., Wang, J., Liu, D., 2011. Simult aneously measured
current, luminosity, and electric eld pulses in a rocket-triggered lightning ash.fi fl
J. Geophys. Res. 116, D10102. .https://doi.org/10.10 29/2010JD015331
Qie, X., Jiang, R., Yang, J., 2014. Characteristics of
current pulses in rocket-triggered
lightning. Atmos. Res. 135, 322 329– .
Rakov, V.A., 1998. Some
inferences on the propagation mechanisms of dart leaders and
return strokes. J. Geophys. Res. 103 (D2), 1879 1887– .
Rakov, V.A., Thottappillil, R., Uman, M.A., Barker, P.P., 1995. Mechanism of the lightning
M component. J. Geophys. Res. 100, 25,701 25,710– .
Rakov, V.A., Crawford, D.E., Kodali, V., Idone, V.P., Uman, M.A., Schnetzer, G.H.,
Rambo, K.J., 2003. Cutoff and reestablishmen t of current in rocket-triggered
lightning. J. Geophys. Res. 108 (D23), 4747. https://doi.org/10.102 9/
2003JD003694.
Romero, C., Paolone, M., Rubinstein, M.,
Rachidi, F., Rubinstein, A., Diendorfer, G.,
Schulz, W., Daout, B., K alin, A., Zweiacker, P., 2012. A system for the measurements
of lightning currents at the S antis Tower. Elec. Power Syst. Res. 82 (1), 34–43.
Saba, M.M.F., Pinto Jr., O., Solorzano, N.N.,
Eybert-Berard, A., 2005. Lightning current
observation of an altitude-triggered ash. Atmos. Res. 76, 402 411fl – .
Thottappillil, R., Goldberg, J.D., Rakov, V.A., Uman, M.A., Fisher, R.J., Schnetzer, G.H.,
1995. Properties of M components from currents measured at triggered lightning
channel base. J. Geophys. Res. 100 (D12), 25711 25720.– https://doi.org/10.1029/
95JD02734.
Wang, D., Rakov, V.A., Uman, M.A., Fernandez, M.I., Rambo, K.J., Schnetzer, G.H.,
Fisher, R.J., 1999. Characterization of the initial stage of negative rocket-triggered
lightning. J. Geophys. Res. 104 (D4), 4213 4222.– https://doi.org/10.102 9/
1998JD200087.
Wang, Z., Qie, X., Jiang, R., Wang, C., Lu, G., Sun, Z., Liu, M., Pu, Y., 2016. High-speed
video observation of stepwise propagation of a natural upward positive leader.
J. Geophys. Res. Atmos.
121 .https://doi.org/10.1002/2016JD02 5605
Williams, D.P., Brook, M., 1963. Magnetic measurements of thunderstorm currents: 1.
continuing currents in
lightning. J. Geophys. Res. 68 (10), 3243 3247– .
Yang, J., Qie, X., Zhang, G., Wang, H., 2008. Magnetic eld
measuring system and currentfi
retrieval in arti cially
triggering lightning experiment. Radio Sci. 43, RS2011.fi
https://doi.org/10.1029/2007RS003753.
Yang, J., Qie, X., Zhang, G., Zhang, Q., Feng, G., Zhao, Y., Jiang , R., 2010. Characteristics
of channel base currents and close magnetic elds in triggered ashes in SHATLE.fi fl
J. Geophys. Res. 115, D23102. .https://doi.org/10.102 9/2010JD014420
Yoshida, S., Biagi, C.J., Rakov, V.A., Hill, J.D., Stapleton, M.V., Jordan, D.M., Uman, M.A.,
Morimoto,
T., Ushio, T., Kawasaki, Z.-I., 2010. Three-dimensional imaging of upward
positive leaders
in triggered lightning using
VHF broadband digital interferometers.
Geophys. Res. Lett. 37, L05805. .https://doi.org/10.1029/200 9GL042065
Zheng, D., Zhang, Y., Lu, W., Zhang, Y., Dong, W.,
Chen, S., Dan, J., 2013. Characteristics
of return stroke currents of classical and altitude triggered lightning in GCOELD in
China. Atmos. Res. 129 130, 67 68– – .
Zheng, T., Lu, G., Tan, Y., Fan, Y., Jiang, R., Zhang, H., Liu, M., Qie, X., 2018. Observation
and analysis of polarity reversal for the burst-type magnetic pulses during the
propagation of upward positive leader in rocket-triggered lightning. Chin. J. Atmos.
Sci. 41 (4) (in Chinese).https://doi.org/10.3878/j.issn.1006-9895.1702.1 6248
Zhou, H., Diendorfer, G., Thottappillil, R., Pichler, H., Mair, M., 2011. Characteristics of
upward bipolar lightning ashes observed at the Gaisberg Tower. J. Geophys. Res.fl
116, D13106.
.https://doi.org/10.1029/2011JD015634
Zhou, E., Lu, W., Zhang, Y., Zhu, B., Zheng, D., Zhang, Y., 2013. Correlation analysis
between the channel current and
luminosity of initial continuou s
and continuing
current processes in
an arti cially triggered lightning ash. Atmos. Res. 129 130,fi fl –
79 89– .
G. Lu et al. Journal of Atmospheric and Solar-Terrestrial Physics 175 (2018) 76 86–
86