estimating power, energy, and action integral in …estimating power, energy, and action integral in...
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ESTIMATING POWER, ENERGY, AND ACTION INTEGRAL IN
ROCKET-TRIGGERED LIGHTNING
By
VINOD JAYAKUMAR
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2004
Copyright 2004
by
Vinod Jayakumar
ACKNOWLEDGMENTS
I thank Dr. Vladimir Rakov for his infinite patience, guidance, and support
throughout my graduate studies. I would like to thank Dr. Martin Uman and Dr. Doug
Jordan for their valuable suggestions during the weekly lightning conference. I thank Dr.
Megumu Miki for responding to all my questions. I sincerely thank Jason Jerauld, Jens
Schoene, Rob Olsen, Venkateshwararao Kodali, and Brian De Carlo for helping me with
the data and software, and for other innumerable favors (without which I would not have
been able to complete my thesis). Research in my thesis was funded in part by the
National Science Foundation.
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TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................................................................................. iii
LIST OF TABLES............................................................................................................. vi
LIST OF FIGURES .......................................................................................................... vii
ABSTRACT..................................................................................................................... xiii
1 INTRODUCTION ........................................................................................................1
2 LITERATURE REVIEW .............................................................................................2
2.1 Cloud Formation and Electrification ......................................................................3 2.2 Natural Lightning Discharges.................................................................................5 2.3 Mechanism of NO Production by Lightning ........................................................10 2.4 Rocket-Triggered Lightning .................................................................................11
2.4.1 Classical Rocket-Triggered Lightning .......................................................11 2.4.2 Altitude Rocket-Triggered Lightning.........................................................13
2.5 Estimates of Peak Power and Input Energy in a Lightning Flash ........................14 2.5.1 Optical Measurements and Long Spark Experiments ................................14 2.5.2 Electrodynamic Model ...............................................................................17 2.5.3 Gas Dynamic Models .................................................................................24
3 ESTIMATING POWER AND ENERGY ..................................................................29
3.1 Methodology.........................................................................................................29 3.2 Experiment............................................................................................................31
3.2.1 Pockels Sensors ..........................................................................................31 3.2.2 Experimental Setup ....................................................................................35
3.3 Electric Field Waveforms .....................................................................................37 3.3.1 V-Shaped Signatures with ∆ERS = ∆EL.......................................................37 3.3.2 V-Shaped Signatures with ∆ERS < ∆EL and Field Flattening
within 20 µs .....................................................................................................38 3.3.3 Signatures with ∆ERS (t) < ∆EL and no Flattening within 20 µs.................39
3.4 Analysis of V-Shaped E-Field Signatures with ∆ERS = ∆EL ................................39 3.4.1 Data Processing ..........................................................................................39 3.4.2 Power and Input Energy .............................................................................42 3.4.3 Statistical Analysis .....................................................................................48
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3.4.4 Error Analysis.............................................................................................57 3.4.5 Channel Resistance and Radius..................................................................63
3.5 Analysis of V-shaped E-field Signatures with ∆ERS < ∆EL and Field Flattening within 20 µs............................................................................................................73
3.6 Analysis of V-Shaped E-field Signatures with ∆ERS (t) < ∆EL and no Field Flattening within 20 µs ..........................................................................................82
4 CHARACTERIZATION OF PULSES SUPERIMPOSED ON STEADY
CURRENTS ...............................................................................................................89
4.1 Initial Stage in Rocket-Triggered Lightning.........................................................89 4.1.1 Introduction ................................................................................................89 4.1.2 Statistical Characteristics of ICC Pulses ....................................................92
4.2 M-Components ...................................................................................................110 4.2.1 Introduction ..............................................................................................110 4.2.2 Statistical Characteristics of M-Components ...........................................110
5 RECOMMENDATIONS FOR FUTURE RESEARCH...........................................119
LIST OF REFERENCES.................................................................................................120
BIOGRAPHICAL SKETCH ...........................................................................................123
v
LIST OF TABLES
Table page 2-1: Lightning Energy Estimates .......................................................................................26
3-1: Summary of peak current and ∆EL statistics for 8 strokes exhibiting V-shaped electric field signatures with ∆ERS = ∆EL. ................................................................38
3-2: Summary of peak current and ∆EL statistics for 5 strokes with ∆ERS < ∆EL and flattening within 20 µs or so.....................................................................................38
3-3: Summary of peak current and ∆EL statistics for 18 strokes with ∆ERS (t) < ∆EL and no flattening within 20 µs.........................................................................................39
3-4: Power and energy estimates for strokes having V- shaped E-field signatures with ∆EL= ∆ERS. ...............................................................................................................47
3-5: Dependence of peak power and energy on errors in the value of E-field peak and its position on the time scale. ...................................................................................62
3-6: Resistance and channel radius for strokes having V- shaped E-field signatures with ∆ERS = ∆EL ...............................................................................................................72
3-7: Power and energy estimates for strokes having V- shaped E-Field Signatures with ∆ERS < ∆EL and field flattening within 20 µs or so...................................................75
3-8: Power estimates for strokes having V- shaped E-Field Signatures with ∆ERS (t) < ∆EL (t) and no flattening within 20 µs ......................................................................82
4-1: Summary of parameters (geometric means) of ICC pulses......................................108
4-2: Parameters of ICC pulses in Gaisberg tower flashes as a function of season. .........110
4-3: Geometric means of the various parameters of M-components and ICC pulses. ....117
vi
LIST OF FIGURES
Figure page 2-1: Electrical structure of a cumulonimbus........................................................................5
2-2: Natural lighting discharges for a cumulonimbus..........................................................6
2-3: Four types of discharges between cloud and ground....................................................7
2-4: Downward negative cloud-to-ground lightning ...........................................................8
2-5: Classical rocket-triggered lightning ...........................................................................12
2-6 Altitude rocket-triggered lightning..............................................................................13
2-7: Relative spectral response versus wavelength for the photodiode detector used by Krider (1966) and Krider et al. (1968). ....................................................................15
2-8: Measurement of photoelectric pulse of lightning.......................................................16
2-9: Conceptual flow of charge and energy.......................................................................19
2-10: Channel structure of lightning depicting the main channel, the branches (feeder channels) in the thundercloud, and branches below the thundercloud.....................20
2-11: Electrodynamic Model .............................................................................................23
2-12: The peak values of electric and magnetic fields produced by the return-stroke breakdown pulse for the case of rch = 0.15 cm, T=15,000ο K, ∆t = 500 ns, and Imax= 20 kA, plotted as functions of the radial distance from channel axis .............24
3-1: Illustration (not to scale) of the method used to estimate power, P(t), and energy, W(t), from measured lightning channel current, I(t), and vertical electric field, E(t), near the channel. ..............................................................................................30
3-2: Calibration of the Pockels sensor ...............................................................................32
3-3: Variation of the Pockels sensor output voltage as a function of the applied electric field...........................................................................................................................33
3-4: Comparison of the electric field waveforms simultaneously measured with a Pockels sensor and a flat-plate antenna, both at 5 m................................................34
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3-5: Comparison of magnitudes of the vertical electric field peaks measured with Pockels sensors and a flat-plate antenna, both at 5 m ..............................................35
3-6: Experimental setup .....................................................................................................36
3-7: V-shaped electric field signatures ..............................................................................37
3-8: Stroke S0013-1. ..........................................................................................................40
3-9: Scatter plot of screen current, IS vs. strike rod current, IR, for 2000 ..........................41
3-10: Time variation of electric field, current, power, and energy for stroke S006-4. .....42
3-11: Same as Figure. 3-10, but for Stroke S008-4 ...........................................................43
3-12: Same as Figure. 3-10, but for Stroke S0013-1. ........................................................43
3-13: Same as Figure. 3-10, but for Stroke S0013-4 .........................................................44
3-14: Same as Figure. 3-10, but for Stroke S0015-2 .........................................................44
3-15: Same as Figure. 3-10, but for Stroke S0015-4. ........................................................45
3-16: Same as Figure. 3-10, but for Stroke S0015-6 .........................................................46
3-17: Same as Figure. 3-10, but for Stroke S0023-3. ........................................................46
3-18: Histogram of peak current for strokes characterized by V- shaped electric field signatures with ∆ERS = ∆EL. .....................................................................................48
3-19: Histogram of ∆EL for strokes characterized by V- shaped electric field signatures with ∆ERS = ∆EL. ......................................................................................................49
3-20: Histogram of peak power for strokes characterized by V- shaped electric field signatures with ∆ERS = ∆EL. .....................................................................................50
3-21: Histogram for input energy for strokes characterized by V- shaped electric field signatures with ∆ERS = ∆EL. .....................................................................................51
3-22: Histogram for action integral for strokes characterized by V- shaped electric field signatures with ∆ERS = ∆EL. .....................................................................................52
3-23: Histogram of the risetime of current for strokes characterized by V-shaped electric field signatures with ∆ERS = ∆EL.................................................................53
3-24: Histogram of the 0-100 % risetime of power per unit length for strokes characterized by V-shaped electric field signatures with ∆ERS = ∆EL. ....................53
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3-25: Peak power vs. peak current for strokes characterized by V- shaped electric field signatures with ∆ERS = ∆EL......................................................................................54
3-26: Energy vs. peak current for strokes characterized by V- shaped electric field signatures with ∆ERS = ∆EL......................................................................................55
3-27: Peak power vs. ∆EL for strokes characterized by V- shaped electric field signature with ∆ERS = ∆EL. ......................................................................................................56
3-28: Energy vs. ∆EL for strokes characterized by V- shaped electric field signature with ∆ERS = ∆EL. ......................................................................................................56
3-29: Energy vs. Action Integral for strokes characterized by V- shaped electric field signature with ∆ERS = ∆EL........................................................................................57
3-30: Flash 0013, stroke 1; Correction factor of 1.6 is applied at the instant of negative E-field peak ..............................................................................................................59
3-31: Flash 0013, stroke 1; Correction factor of 1.6 is applied at the instant of negative E-field peak, which is shifted by 0.24 µs to the left in order to partially account for the ± 0.5 µs uncertainty in the position of the peak............................................60
3-32: Flash 0013, stroke 1; Correction factor of 1.6 is applied at the instant of negative E-field peak, which is shifted by 0.24 µs to the right in order to partially account for the ± 0.5 µs uncertainty in the position of the peak............................................61
3-33: Evolution of the various quantities for the first 0.58 µs for Flash S006, stroke 4....64
3-34: Evolution of the various quantities for the first 0.4 µs for Flash S008, stroke 4......65
3-35: Evolution of the various quantities for the first 1.4 µs for Flash S0013, stroke 1....66
3-36: Evolution of the various quantities for the first 1.2 µs for Flash S0013, stroke 4....67
3-37: Evolution of the various quantities for the first 2.1 µs for Flash S0015, stroke 2....68
3-38: Evolution of the various quantities for the first 1.5 µs for Flash S0014, stroke 4....69
3-39: Evolution of the various quantities for the first 1.3 µs for Flash S0015, stroke 6....70
3-40: Evolution of the various quantities for the first 5 µs for Flash S0023, stroke 3.......71
3-41: Evolution of channel radius for S0008-4..................................................................72
3-42: V-shaped signature with ∆EL> ∆ERS. ∆ERS represents the average electric field between 44 µs to 50 µs after the beginning of the return stroke (at 50 µs)..............76
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3-43: Histogram of peak current for strokes characterized by V-shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs. .........................................76
3-44: Histogram of ∆EL for strokes characterized by V-shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs. ..........................................................77
3-45: Histogram of peak power per unit length for strokes characterized by V-shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs. ....................77
3-46: Histogram of energy per unit length for strokes characterized by V-shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs. ....................78
3-47: Histogram of action integral for strokes characterized by V-shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs. .........................................78
3-48: Peak power vs. peak current for strokes characterized by V- shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs. .........................................79
3-49: Energy vs. peak current for strokes characterized by V- shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs. .........................................80
3-50: Energy vs. ∆EL for strokes characterized by V- shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs............................................................80
3-51: Energy vs. ∆EL for strokes characterized by V- shaped electric field signatures with ∆EL< ∆ERS and flattening within 20 µs............................................................81
3-52: Energy vs. Action integral for strokes characterized by V- shaped electric field signatures with ∆EL< ∆ERS and flattening within 20 µs...........................................81
3-53: Histogram of ∆EL for strokes characterized by V- shaped electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs. ........................................83
3-54: Histogram of ∆EL for strokes characterized by V- shaped electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs. ........................................84
3-55: Histogram of peak power for strokes characterized by V- shaped electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs........................85
3-56: Histogram of action integral for strokes characterized by electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs. ........................................86
3-57: Peak power vs. peak current for strokes characterized by electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs .........................................87
3-58: Peak power vs. ∆EL for strokes characterized by electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs .................................................88
4-1: Flash 03-31, bipolar flash ...........................................................................................90
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4-2: Flash 03-31 .................................................................................................................91
4-3: Definitions of parameters (peak, duration, rise time, half-peak width.......................92
4-4: Illustration of the removal of the background continuous current in computing charge and action integral ........................................................................................93
4-5: Histograms of the peak of the ICC pulses for 2002. ..................................................94
4-6: Histograms of the peak of ICC pulses for 2003. ........................................................95
4-7: Histogram of the peak of ICC pulses for 2002 and 2003. ..........................................95
4-8: Histogram of the duration of ICC pulses for 2002. ....................................................96
4-9: Histogram of the duration of ICC pulses for 2003. ....................................................96
4-10: Histogram of the duration of ICC pulses for 2002 and 2003. ..................................97
4-11: Histogram of the risetime of ICC pulses for 2002. ..................................................97
4-12: Histogram of the risetime of ICC pulses for 2003. ..................................................98
4-13: Histogram of the risetime of ICC pulses for 2002 and 2003....................................98
4-14: Histogram of the half-peak width of ICC pulses for 2002. ......................................99
4-15: Histogram of the half-peak width of ICC pulses for 2003. ......................................99
4-16: Histogram of the half-peak width of ICC pulses for years 2002 and 2003............100
4-17: Histogram of the charge of ICC pulses for 2002....................................................100
4-18: Histogram of the charge of ICC pulses for 2003....................................................101
4-19: Histogram of the charge of ICC pulses for years 2002 and 2003. .........................101
4-20: Histogram of the action integral of ICC pulses for years 2002..............................102
4-21: Histogram of the action integral of ICC pulses for years 2003..............................102
4-22: Histogram of the action integral of ICC pulses for years 2002 and 2003. .............103
4-23: Histograms of the peak of ICC pulses....................................................................104
4-24: Histograms of the duration of ICC pulses ..............................................................105
4-25: Histograms of the risetime of ICC pulses...............................................................106
4-26: Histograms of the half-peak width of ICC pulses ..................................................107
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4-27: Flash F0213 ............................................................................................................111
4-28: Flash F0213 ............................................................................................................112
4-29: Histogram of duration of M-component pulse. ......................................................112
4-30: Histogram of peak of M-component pulse.............................................................113
4-31: Histogram of risetime of M-component pulse........................................................113
4-32: Histogram of the half-peak width of M-component pulse. ....................................114
4-33: Histogram of the charge of M-component pulse....................................................114
4-34: Action integral of M-component pulse...................................................................115
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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
ESTIMATING POWER, ENERGY, AND ACTION INTEGRAL IN ROCKET-TRIGGERED LIGHTNING
By
Vinod Jayakumar
December 2004
Chair: Vladimir A. Rakov Cochair: Martin A. Uman Major Department: Electrical and Computer Engineering
The peak power and input energy for the triggered-lightning return strokes are
calculated as a function of time, using the vertical electric fields measured within 0.1 to
1.6 m of the lightning channel and the associated currents measured at the channel base.
The data were acquired during the 2000 rocket-triggered lightning experiments at the
International Center for Lightning Research and Testing (ICLRT) at Camp Blanding,
Florida. Results were compared with estimates found in the literature, including those
based on gas-dynamic models, on electrostatic considerations, and optical measurements
and long spark experiments. We additionally examined the action integral, the variation
of resistance per unit length, and the radius of the lightning channel during the return-
stroke process. We also examined the correlation of various parameters. Our estimates for
energy and peak power are in reasonable agreement with those predicted by the gas
dynamic models found in the literature. Finally, pulses superimposed on the initial stage
xiii
current (ICC pulses) and similar pulses superimposed on the continuing current that
follows the return stroke process (M-component pulses) were analyzed for years 2002
and 2003, and compared with statistics found in the literature. The following comparisons
were made: (a) ICC pulses in triggered lightning recorded at the ICLRT in 2002 and 2003
(relatively high sampling rate) vs. their counterparts recorded earlier (relatively low
sampling rate), (b) ICC pulses in triggered lightning vs. those in object-initiated
lightning, and (c) ICC pulses in triggered lightning vs. M-component pulses in triggered
lightning. Duration, risetime, and half-peak width of ICC pulses were somewhat greater
than those of M-component pulses. Current peak, charge, and action integral of M-
components and ICC pulses were similar.
xiv
CHAPTER 1 INTRODUCTION
Lightning strikes are the cause of many deaths and injuries. Electromagnetic fields
and currents associated with lightning also can have deleterious effects on nearby
electronic devices. The energy of lightning is a fundamental quantity required, for
example, in estimating nitrogen oxide (NO) produced by lightning; which, in turn, is
needed in global climate-change studies. Trace gases produced by atmospheric electric
discharges are important to the ozone balance of the upper troposphere and lower
stratosphere. Atmospheric electric discharges might have played an important role in
generating the organic compounds that made life possible on Earth. Currently, there is no
consensus on lightning input energy. Estimates found in literature differ by one to two
orders of magnitude. Our study measured electric fields using Pockels sensors in the
immediate vicinity of the lightning channel, along with the channel base currents, to
estimate the energy and peak power in triggered lightning. We also analyzed the action
integral (energy per unit resistance at the strike point) and other parameters of return
strokes and pulses superimposed on steady currents in triggered lightning.
1
CHAPTER 2 LITERATURE REVIEW
Three approaches used to estimate lightning peak power and energy input were
found in the literature.
• The first approach is based on electrostatic considerations. The total electrostatic
energy of a lightning flash lowering charge Q to the ground can be estimated as
the product of Q and V, where V is the magnitude of the potential difference
between the lower boundary of the cloud charge source and ground (Rakov and
Uman, 2003). The typical value of Q for a cloud-to-ground flash is 20 C. The V is
estimated to be 50 to 500 MV (Rakov and Uman, 2003). Thus each flash
dissipates energy of roughly 1 to 10 GJ. Borovsky (1998), using electrostatic
considerations, estimated the energy associated with individual strokes to be
1×103–1.5×104 J/m, close to that predicted by gas dynamic models (Section
2.5.3).
• The second approach was described by Krider et al. (1968). Radiant power and
energy emitted within a given spectral region from a single-stroke lightning flash
are compared with those of a long spark whose electrical power and energy inputs
are known with fair accuracy. The value of lightning input energy per unit
channel length estimated by Krider et al. (1968) is 2.3 ×105 J/m.
The third approach involves the use of gas dynamic models proposed by a number of
researchers (Plooster. 1971; Paxton et al. 1986, 1990; Dubovoy et al. 1968, 1991; Hill
1977; Strawe 1979; Bizjaev et al. 1990). A short segment of a cylindrical plasma
2
3
column is driven by the resistive (joule) heating caused by a specified flow of electric
current as a function of time. Lightning input energy predicted by these models is one
to two orders of magnitude lower than that of Krider et al. (1968).
2.1 Cloud Formation and Electrification
The primary source of lightning is the cloud type, cumulonimbus (commonly
known as thundercloud). The process of charge generation and separation is called
electrification. Apart from the cumulonimbus, electrification can also take place in a
number of other cloud types (in stratiform clouds, for example; and in clouds produced
by forest fires, volcanic eruptions, and atmospheric charge separation in nuclear blasts.
According to Henry et al. (1994), eight types of thunderstorms are known. Among them,
some common in Florida are sea/land-breeze thunderstorms, oceanic thunderstorms, air-
mass thunderstorms, and frontal thunderstorms. Portier and Coin (1994) give other
classifications. The formation of air-mass clouds is explained next.
On a sunny day, Earth absorbs heat from the sun, causing both water vapor and air
to rise to higher atmospheric levels, forming clouds. The energy of the water vapor
decides the intensity of the thunderstorm; the hotter the air, the more water vapor it can
hold, and the more powerful the thunderstorm can be. When water vapor condenses, it
releases the same amount of energy required for heating water, to produce water vapor.
Convection causes warm, humid air to reach higher altitudes. The released energy heats
the surrounding atmosphere, which raises the cloud to higher altitudes, pulling the humid
air from below (setting a chain reaction); with an updraft velocity of round 30 m/s
forming cells. A cell is said to be in mature stage (actually this stage is related to the
cloud’s ability to generate lightning) when it reaches higher altitudes, and its top flattens,
forming an anvil. This kind of cloud formation can be divided into three stages
4
• Developing stage
-Starts with warm, rising air
-The updraft velocity increases with height
-Super-cooled water droplets are far above freezing level
-Small-scale process that electrifies individual hydrometeors takes place in this stage
• Mature stage
-The heaviest rains occur
-The downdraft occurs, due to frictional drag of the raindrops
-Evaporative cooling leads to negative buoyancy
-The top of the cloud forms an anvil
-The graupel-ice mechanism and the larger convective mechanism take place, leading
to electrical activity.
• Dissipating stage
-The downdraft takes over the entire cloud
-The storm deprives itself of supersaturated updraft air
-Precipitation decreases
-The cloud evaporates
Various measurements were made to estimate the distribution of charge within the
cloud. Initially, from ground-based measurements, it was assumed that the charge within
the cloud forms an electric dipole (positive charge region above negative charge region).
Simpson and Robinson (1941) made in-cloud measurements with balloons, and suggested
a tripole model with an additional positive charge at the base of the cloud. There is still
no consensus on the detailed distribution of charge within the cloud.
5
Figure 2-1: Electrical structure of a cumulonimbus. [Simpson, G. and Scrase, F.J.; “The
Distribution of Electricity in the Thunderclouds,” Proc. R. Soc. London Ser. A, 161: Figure. No.4. pp.315, 1937]
• Precipitation model: Heavy soft hail (graupel) with a fall speed > 0.3 m/s interacts
with lighter particles (ice crystals) in the presence of small water droplets. As a result,
heavy particles in cold regions (T<-15° C) acquire negative charge; heavy particles in
warm regions (T>-15° C) acquire positive charge. The second process (gravitational
force) separates the heavier and lighter charged particles, forming an electric tripole.
• Convection model: Charges are supplied by external sources such as corona and
cosmic rays. Separation of charges is accomplished by organized convection.
2.2 Natural Lightning Discharges
Lightning discharges can be classified as
• Cloud discharges
- Intracloud discharges
- Cloud-to-cloud discharges
- Cloud-to-air discharges
• Cloud-to-ground discharges
6
- Downward negative discharges
- Downward positive discharges
- Upward positive discharges
- Upward negative discharges
- Bipolar discharges
Figure 2-2: Natural lighting discharges for a cumulonimbus. Adapted from “Encyclopedia Britannica”
Most (47 to 75 %) discharges are cloud discharges and the rest are cloud-to-ground
discharges. Intracloud discharges are apparently most numerous in the cloud-discharge
category, compared to the intercloud and cloud-to-air discharges.
Most of cloud-to-ground discharges can be divided into four categories. They are
downward negative, downward positive, upward positive, upward negative. Upward
positive and upward negative discharges occur rarely, while 90% of the cloud-to-ground
discharges are downward negative discharges and 10% are downward positive
discharges. There are also discharges transporting both negative and positive charges to
7
ground. Such bipolar discharges are usually of upward type and constitute probably less
than 10% of all cloud-to-ground discharges.
Figure 2-3: Four types of discharges between cloud and ground.1. Downward negative 2. Upward negative 3.Downward positive 4. Upward positive. [M. A. Uman, The Lightning Discharge; Dover Publications, Minneola, New York; Figure.. 1-3, pp.9, 1987]
At t=0 ms, the thundercloud has a tripolar charge structure with positive charge in
the upper region, negative charge in the lower region, and a small pocket of positive
charge at the cloud base. Between t=0 and t=1 ms, preliminary breakdown occurs within
the cloud due to the local discharge between the pocket of positive charge at the base and
the primary negative charge. The local discharge neutralizes the positive charge at the
base and continues towards the ground as a stepped leader.
8
Figure 2-4: Downward negative cloud-to-ground lightning [V.A. Rakov, Uman, M.A, Lightning: Physics and Effects; Cambridge University Press, New York; 2003].
This leader consists of a narrow current-carrying core and a much wider radial
corona sheath. Between 1.10 ms to 19 ms, the stepped leader moves towards the ground
with an average speed of 105 to 106 m/s, exhibiting steps of some tens of meters in length
and separated by some tens of microseconds. At t= 20 ms, the stepped leader approaching
the ground causes the electric field near ground to exceed the breakdown value for air,
which in turn results in an upward positive leader extending from ground towards the
9
descending stepped leader. Between 20.0 and 20.1 ms, the downward leader attaches to
one of the upward leader branches and a return stroke is initiated with a typical peak
current of 30 kA. From t= 20.10 ms to 20.20 ms, the first return stroke propagates
upward towards the cloud in the ionized channel left behind by the stepped leader with a
speed of around 108 m/s. The return stroke neutralizes the negative charge (typically 5 C)
deposited by the leader (lowers negative charge to ground). At t= 40 ms, some in-cloud
processes called K and J processes occur inside the cloud. At t= 60 ms to 62 ms, a dart
leader propagates downward along the channel left by the first return stroke with an
average speed of 107 m/s [Uman, 1987]. The dart leader deposits a negative charge of the
order of 1C onto the channel.
When the dart leader reaches the ground, a second return stroke is initiated which
travels upward with an average speed of 108 m/s. A sequence of leader and return stroke
is called a stroke, with the average number of strokes per flash being 3 to 5 [Rakov and
Uman, 2003].
Positive cloud-to-ground discharges can originate from the upper positive charge
region or positive charge pocket at the cloud base (assuming the tripolar model of cloud
charge distribution). It starts with a downward propagating positive leader and connects
to a negative upward leader launched from the ground. Then an upward return stroke is
initiated which transfers positive leader charge to ground. Typically there are no
subsequent strokes in positive cloud-to-ground discharges. The typical values of first
stroke peak currents measured at ground for positive cloud-to-ground discharges is
35 kA, not much different from 30 kA for negative cloud to ground lightning. On the
other hand, larger currents are more probable in positive strokes that in negative ones.
10
2.3 Mechanism of NO Production by Lightning
During a return stroke, the lightning channel attains a peak temperature of 30,000° K
in a few microseconds. If the cooling of the channel takes place slowly, equilibrium
composition at a given temperature is established, i.e., the final constituents of the cold
air would be the same as the constituents prior to the return stroke. It has been shown by
Uman and Voshall (1968) and Picone et al (1981) that the residual hot channel cools
from around 10,000° K to 3,000° K in a few milliseconds. The time required by NO to
attain equilibrium concentration increases rapidly with decreasing temperature. Hence, as
the channel cools down to the ‘freeze out’ temperature, the temperature at which the
reactions that produce and destroy NO become too slow to keep NO in equilibrium
concentration, and hence NO remains at the density characteristic of the ‘freeze out’
temperature. Chemical reactions, which characterize the production of NO, are
O2 ↔ O + O
O + N2 → NO + N
O2 + N → NO + O
The reactions that compete with the NO producing reactions are shown below.
NO + N → O + N2
NO + N → O + N2
NO ↔ N + O
NO + NO → N2O + O
These equations assume importance as NO produced by natural processes decreases
the ozone (O3) concentration in the stratosphere via the dominant equation
NO + O3 → NO2 + O2
11
Ozone in the stratosphere is important to life because it shields the Earth from Sun’s
harmful ultraviolet radiation. Ozone in the troposphere acts as a greenhouse gas by
absorbing the infrared radiation.
2.4 Rocket-Triggered Lightning
The study of natural lightning is extremely difficult since it is impossible to
accurately predict its occurrence in space and time. For this reason, in order to study the
lightning properties a method to produce lightning artificially from natural thunderclouds
has been developed. The rocket and trailing wire technique is used to initiate lightning
that is referred to as rocket-triggered lightning.
Currently, rocket-triggered lightning (Rakov, 1999) can be produced in two
different ways:
• Classical rocket-triggered lightning. • Altitude rocket-triggered lightning.
2.4.1 Classical Rocket-Triggered Lightning
In classical triggering, the wire is continuous and is connected to the grounded
launcher. After the rocket is launched, it travels upward with a velocity of around 200
m/s. When the rocket reaches a height of around 200 m, an upward positive leader is
generated at the rocket tip, which travels with a velocity of around 105 m/s. The current
of the upward positive leader vaporizes the wire, and an initial continuous current (ICC)
follows for some hundred of milliseconds. During the formation of the upward positive
12
Figure 2-5: Classical rocket-triggered lightning [V. A. Rakov, “Lightning Discharges Triggered using Rocket-and-Wire Techniques," J. Geophys. Res., vol.100, pp.25711-25720, 1999]
leader, the so-called initial current variation (ICV) occurs, which is not shown in
Figure.2-5, but explained in the next paragraph. After the completion of ICC, there exists
a no current interval for a few tens of milliseconds that is followed by one or more leader/
return stroke sequences (Figure. 2-5). These leader/return stroke sequences are similar to
subsequent leader/return stroke sequences in natural lightning.
The ICV occurs when the triggering wire is replaced by the upward positive leader
plasma channel. The upward positive leader produces current in the tens to hundreds of
amperes range when measured at ground, and this current vaporizes the wire. At that
time, the current measured at ground goes to nearly zero since there is no well-
conducting path for the current to travel to the ground. Then a downward leader-like
process bridges the resultant gap and initiates a return stroke type process from the
13
ground. The latter leader/return stroke type sequence serves to re-establish the interrupted
current flow to ground.
Figure 2-6 Altitude rocket-triggered lightning [V. A. Rakov, “Lightning Discharges
Triggered using Rocket-and-Wire Techniques," J. Geophys. Res., vol.100, pp.25711-25720, 1999]
2.4.2 Altitude Rocket-Triggered Lightning
The altitude triggering technique uses an ungrounded wire in an attempt to
reproduce some of the features of the first stroke of the natural lightning which is not
possible using classical, grounded-wire triggering. Generally, the rocket extends three
sections, a 50 m long copper wire connected to the grounded launcher, a 400 m long
insulating Kevlar cable in the middle, and a 100 to 200-m long copper wire connected to
the rocket. The upper, floating wire is used for triggering and the lower, grounded wire
for intercepting the descending leader as discussed below. When the rocket reaches a
height of around 600 m an upward positive leader and a downward negative leader
14
(forming a bi-directional leader) are initiated, each propagating at a speed of 105 m/s. The
electric field produced by the downward negative leader initiates an upward connecting
positive leader from the grounded 50-m wire, which connects to the downward negative
leader. Finally, a return stroke is initiated which travels with a speed of 107-108 m/s and
catches up with the upward positive leader tip. After this stage, the processes are similar
to those of the classical rocket- triggered lightning.
2.5 Estimates of Peak Power and Input Energy in a Lightning Flash
There are various methods to estimate the peak power and energy dissipated in
lightning discharges. Some of them are described in the following sections.
2.5.1 Optical Measurements and Long Spark Experiments
Krider et al. (1968) estimated the average energy per unit length and peak power
per unit length to be 2.3×105 J/m and 1.2×109 W/m. Their optical measurements were
similar to those performed by Krider (1966) and are described below. A calibrated silicon
photodiode and an oscilloscope were used as a fast-response lightning photometer
covering the visible and near-infrared regions of the spectrum from 0.4 to 1.1 µm.
Simultaneous still photographs of the discharge channels were taken to determine the
dependence of the photoelectric pulse profile on the type of lightning and the geometry of
its channel. The photodiode detector consisted of an Edgerton, Germeshausen and Grier
model 560-561 ‘lite-mike’ and ‘detector head’ (Krider, 1966). The photodiode and
associated circuitry were linear over a wide range of incident light levels (within 5% over
7 decades) and had a response time of less than 1 µs. The calibrated relative response of
the detector is shown in Figure. 2-7. Photographs of the lightning channels were taken
with an Ansco Memar 35-mm camera, which had a focal length of 45 mm. Using the
15
same experimental setup optical measurements have been performed on a 4-meter air
spark produced by the Westinghouse 6.4 MV impulse generator.
Figure 2-7: Relative spectral response versus wavelength for the photodiode detector
used by Krider (1966) and Krider et al. (1968).
The basic principle of power and energy estimation is as follows. The spark current
and voltage were recorded as functions of time, enabling the calculation of electrical
power and total energy input. The radiant power reaching the detector is proportional to
the voltage measured at the output of the optical detector. Considering the spark channel
to be straight (to avoid taking the dependence of the radiant power on the azimuth of the
channel), the radiant power output from the light source is determined from equation (1).
It has been demonstrated experimentally using the long spark that the distance
dependence of the radiant flux is 1/R2. Hence, the total radiant power emitted in all
directions within the detector bandwidth is given by
P= (V/K) × (4πR2/ A) (1)
where, V is the optical detector output voltage, K is the pulse calibration factor, R is the
distance from the light source to the detector, and A is the sensitive area of the detector.
The radiant power vs. time curve can be integrated to obtain the total radiant energy
emitted in a given bandwidth. The radiative efficiency is calculated by comparing the
value of total radiant energy to that of measured electrical energy input. Making a
16
simplifying assumption that the radiative efficiencies for laboratory spark and lightning
are the same, power and input energy can be found for a lightning stroke whose total
radiant energy is known from measurements. In this experiment, knowing the
approximate location of the lightning, cloud base height was obtained from the U.S.
Weather Bureau. Using the cloud base height, which determined the length of the
lightning channel that was visible, the size of the photographic image, and knowledge of
the camera focal length one can estimate the distance to the channel. This method
assumes that the channel is vertical. Figure 2-8 b shows the photoelectric voltage pulse
corresponding to the lightning whose still picture is shown in Figure 2-8a.
Figure 2-8: Measurement of photoelectric pulse of lightning. a) Still photograph of a typical cloud-to-ground lightning at a distance of 6 km. b) the corresponding photoelectric voltage pulse [E.P. Krider, “Some photoelectric observations of Lightning”, J.Geophys. Res., Figure. 2-3, pp.3096-3097, 1966].
For the lightning stroke which was under study in Krider, (1968), the cloud base
height was 1.8 km, and the distance calculated from the photographic image size was 8.2
km. At this distance and maximum signal at the optical detector, the peak power radiated
17
from the lightning stroke is calculated to be 1.1×1010 W, or dividing by the channel
length, to be as 6.2×106 W/m. The corresponding measurements for the spark were made
at a distance of 23 m, and the peak radiant power within the detector bandwidth was
obtained to be 1.0×1010 W/m. The peak electrical power dissipated in the spark was
obtained accurately from the direct traces of current recorded as a function of voltage. At
the time of peak power, the current was 4.2×103 A and the voltage
1.8×106 V, yielding a peak electrical power input of 7.6×109 W, or 1.9×109 W/m
Comparison of the radiant and electrical peak powers for the long spark yields a radiative
efficiency of 0.52%. Assuming the same radiative efficiency for the lightning at the
instant of peak radiant power, the peak electrical power dissipated in the lightning stroke
is 2.1×1012 W. Dividing this value by the channel length, peak electrical power dissipated
per unit length is obtained as 1.2×109 W/m.
The electrical energy per unit length dissipated in the long spark is obtained by
integrating over time the product of the current and voltage values obtained from the
traces taken during the experiment. Comparing the radiant and electrical energy values,
the average radiative efficiency of 0.38% is obtained for the spark. Applying the same
radiative efficiency to lightning, the total average energy dissipation per unit length is
estimated to be 2.3×105 J/m. This value is in agreement with the thunder theory data of
Few et al. (1969), but is one to two orders of magnitude larger than the values predicted
by gas-dynamic models (Section 2.5.3).
2.5.2 Electrodynamic Model
We present here the electrodynamic model proposed by Borovsky (1995), which
describes dart leaders and return strokes as electromagnetic waves that are guided along
18
conducting lightning channels. The downward propagating dart leader deposits negative
charge onto the channel and deposits electrostatic energy around the channel. The
subsequent upward-propagating return stroke drains the negative charge off the channel
and heats the channel by expending the stored electrostatic energy. The net result is that
the negative charge is lowered from the cloud to the ground and the energy is transferred
from the cloud to the channel. This electrodynamic model also accounts for the flow of
energy associated with the flow of charge. In this model energy dissipated per unit length
in lightning channels is calculated as a relation to the linear charge density on the channel
and not to the cloud-to-ground electrostatic potential difference.
This model serves as a tool to visualize the dynamics of lightning during the dart-
leader and return stroke phases. Figure 2-9 illustrates the concept of the model.
The amount of energy deposited on the lightning channel can be estimated based on
electrostatic considerations, from the following expression (Uman, 1984, 1987)
W/L = (Qlow × ∆Φtot)/ Lmain (2)
Where, Qlow is the amount of charge lowered from the cloud to ground, ∆Φtot is the
electrostatic potential difference between the thundercloud charge region and the ground,
and Lmain is the length of the main channel.
19
Figure 2-9: Conceptual flow of charge and energy. a) Flow of charge during a dart leader
and return stroke. b) Flow of energy during the dart leader and return stroke. [J. E. Borovsky, “Lightning Energetics: Estimates of energy dissipation in channels, channel radii, and channel-heating risetimes”, J.Geophys. Res., vol.103, Figure.1, pp.11538, May. 1998]
The above relation for W/L is unreliable due to the following reasons,
• The amount of energy expended in the branch channels is unknown.
• Difficulty in estimating ∆Φtot (requires integration of the height varying electric field from the ground level to the cloud charge source).
• As seen in Figure 2-10, which gives a sketch of the structure of lightning channel, the energy expended in creating the “feeder” channels in the cloud and branches
20
(for first strokes only) has to be included. The total length of the channel network is very difficult to estimate.
Cloud base
Figure 2-10: Channel structure of lightning depicting the main channel, the branches (feeder channels) in the thundercloud, and branches below the thundercloud.
Borovsky (1995) gives a more accurate estimate of the energy dissipation by considering
the stored electrostatic energy density around the channel. Electrostatic energy density
(ED) is given by (3).
ED = ε0 E2/2 (3)
Where, ε0 = 8.85× 10-12 F/m and E is the electric field at a distance r from the channel. E
is given by (4).
E = ρL / (2π ε0 r) (4)
Where, ρL is the charge per unit channel length.
21
Note that equations (3) and (4), employ SI units, while the corresponding equations given
by Borovsky (1995) are in CGS metric unit system. The original equations and
conversion can be found in the Appendix. The stored energy per unit length equation is
derived as follows. If all the charge resides on the channel, the electric field very close to
the channel exceeds the air-breakdown limit Ebreak. Ebreak is approximately equal to 2×106
V/m (Cobine, 1941). At locations along the lightning channel where E exceeds Ebreak,
conductivity increases rapidly, facilitating the movement of free charge, thus reducing the
electric field E. So, around the channel the electric field will be approximately equal to
Ebreak, up to a radius of rbreak.
rbreak= ρL / (2π ε0 Ebreak ) (5)
Beyond rbreak, the electric field falls off as 1/r. Thus, the radial dependence of the
electrostatic energy density residing around the channel is given by,
ED = ε0 Ebreak 2/2 if r ≤ rbreak (6)
ED = ρL 2/8π2 r2 ε0 if r ≥ rbreak (7)
Total amount of electrostatic energy per unit length stored around the channel is given by
Wstored /L = ∫ (ED) 2πr dr (8) ∞
Because of the radial dependence of elect
broken into
Wstored /L = ∫ (ED
Substituting the appropriate expressions f
Wstored /L = ρL2/(4π ε0 rbreak)+
Where rcut is the cutoff radius that is intro
logarithmically diverging as r → ∞. The
0
rostatic energy, the above integral can be) 2πr dr +r
or ED in e
ρL2 /(4π ε
duced to p
physically
∞
∫ (ED) 2πr dr (9) break0
rbreak
quation (9),
0) ln( rcut/rbreak) (10)
revent the integral from
reasonable choice for rcut is the radius
22
at which the electric field of the channel equals the background electric field. Therefore,
rcut is given by the expression
rcut = ρL/ (2π ε0 Ecloud ) (11)
Thus the total electrostatic energy per unit length is given by
Wstored /L = ρL2/4π ε0 [1/ rbreak + ln( Ebreak / Ecloud)] (12)
Where,ρL = charge per unit length of the channel.
Ebreak = breakdown electric field.
Ecloud = background electric field.
In Borovsky (1998), the electric field under the thundercloud is taken to be the
background electric field. Ebreak value is taken to be 2.0 × 10 6
V/m. For Ecloud, two
limiting values taken are 1×104 V/m and 4×105 V/m.
The value of ρL is typically chosen in the range 1×10-4 C/m and 5×10-4 C/m, with
the dart-leader loaded channel being at the lower end of this range and stepped-leader
loaded channel being at the upper end of this range. This model estimates the energy per
unit channel length to be about 1×103 and 1.5×104 J/m for the dart-leader and stepped
leader respectively. These values are in good agreement with estimates of gas-dynamic
models of lightning [Rakov and Uman, 2003] considered in Section 2.5.3.
Borovsky (1998), whose electrodynamic model is illustrated in Figure 2-11, also
estimates the initial and final channel radii. Taking the charge per unit channel length
ρL = 4×10-4 C/m and number density of atoms in unexpanded channel, ηatomic = 5.0×1019
cm-3, the initial radius of the return stroke channel can be estimated to be 0.32 cm. The
values chosen for ηatomic and ρL are appropriate for a stepped–leader channel. In the
calculation of final radius (channel radius after expansion) the values of channel
23
parameters chosen are εdisso =9.8 eV, εioniz= 14.5 eV, Tinit= 30000 °K, Tatmos= 300 °K,
where Tinit is the temperature of the channel before expansion, Tatmos is the temperature of
the ambient air outside the channel, εdisso and εioniz are the dissociation and ionization
constants. The final channel radius is estimated to be about 4.7 cm. Similarly, for the
dart-leader channel, the initial and final radii are found to be 0.26 cm and 3.8 cm. In this
latter case, the values chosen for ρL and ηatomic are 1×10-4 C/m and 5.0×1018 cm- 3.
(a) (b)
Figure 2-11: Electrodynamic Model. (a) Downward propagating dart leader that loads charge and electrostatic energy onto a lightning channel, (b) upward-propagating return stroke that drains charge off the channel and uses up the stored electrostatic energy [J. E. Borovsky, “An electrodynamic description of lightning return strokes and dart leaders: Guided wave propagation along conducting cylindrical channels”, J.Geophys. Res., Figure. 10, pp. 2717, Feb. 1995].
According to this model, the vertical (longitudinal) electric field outside the channel
decreases with increasing the distance r as –loge (0.9 γout r), which is a slowly varying
24
function of r, where γout is the external wave number (Borovsky, 1995). γout is chosen to
be (8.8 + i 9.4)×10-5 cm-1 for the return stroke break-down pulse used in the illustration of
the variation of the horizontal and vertical components of the electric field shown in
Figure 2-12.
Figure 2-12: The peak values of electric and magnetic fields produced by the return-
stroke breakdown pulse for the case of rch = 0.15 cm, T=15,000ο K, ∆t = 500 ns, and Imax= 20 kA, plotted as functions of the radial distance from channel axis. rch is the channel radius, T is the channel temperature, ∆t is the rise time of the wave e-iwt, where w is the angular frequency(ω = 1/∆t, i e. ∆t is around one sixth of the time period of the sine wave), and Imax is the peak current . [J. E. Borovsky, “An electrodynamic description of lightning return strokes and dart leaders: Guided wave propagation along conducting cylindrical channels”, J.Geophys. Res., Figure. 8, pp.2712, Feb. 1995]
2.5.3 Gas Dynamic Models
Gas dynamic models consider a short segment of a cylindrical plasma column
driven by the resistive (Joule) heating caused by a specified flow of electric current as a
function of time. Rakov and Uman (1998) review essentially all the gas dynamic models
25
found in the literature. The basic assumptions in the most recent models are: 1) the
plasma column is straight and cylindrical; 2) the algebraic sum of all the charges is zero;
3) local thermodynamic equilibrium exists at all times. The initial conditions of the
lightning channel are temperature of the order of 10000°K, channel radius of the order of
1 mm, and pressure equal to ambient (1 atm) or mass density equal to ambient (of the
order of 10-3 g/cm3), the latter two conditions representing, respectively, the older and
newly created channel sections. The initial condition assuming the ambient pressure best
represents the upper part of the of the leader channel, since that part had sufficient time to
expand and attain equilibrium with the surrounding air. The initial condition of ambient
density is most suitable for the recently created, bottom part of the leader channel. At
each time step: 1) electrical energy sources; 2) the radiation energy sources; 3) Lorentz
force are computed and the gas dynamic equations are solved for the thermodynamic and
flow parameters of the plasma.
The energy input is determined as follows. The plasma channel is visualized as a
set of concentric annular zones, in which the gas properties are assumed constant. For a
known temperature and mass density, plasma composition can be obtained from the Saha
equation (Paxton et al. (1986, 1990), Plooster (1971)) or from tables of precompiled
properties of air in thermodynamic equilibrium (Hill (1971), Dubovoy et al. (1991,
1995)). The plasma conductivity can be computed from the plasma composition,
temperature and mass density. The current is distributed among the annular zones as if
they were a set of resistors connected in parallel. Using the cross-sectional distribution of
current and plasma conductivity, the amount of electrical energy input can be computed
for each of the annular zones.
26
The energy is deposited at the center of the channel in the form of heat, which is
transported to cooler outer regions in the form of radiation. The radiative properties of air
are complex functions of frequency and temperature. Radiation at a given frequency can
be absorbed and re-radiated at different frequencies while traversing the channel in the
outward direction. Paxton et al. (1986, 1990) and Dubovoy et al. (1991, 1995) used
tables of radiative properties of hot air to determine absorption coefficients as a function
of temperature for a number of selected frequency intervals to solve the equation of
radiative energy transfer in the diffusion approximation.
The pinch effect due to the interaction of electric current with its own magnetic
field was included in the gas dynamic model of Dubovoy et al. (1991, 1995). This
phenomenon counteracts the channel’s gas dynamic expansion, resulting in 10-20%
increase in input energy for the same input current because of reduced channel size.
Table 2-1,which is found in Rakov and Uman (1998), summarizes predictions of
the various gas dynamic models for the input energy and percentages of this energy
converted to kinetic energy and radiated from the channel. Additionally included in Table
2-1 are energy estimates based on experimental data (Krider et al; see Section 2.5.1 and
on electrostatic considerations (Uman, 1987; Borovsky, 1998; see Section 2.5.2). Brief
comments on each of these estimates follow the table.
Table 2-1: Lightning Energy Estimates [Rakov and Uman, 1998]. Source Current
Peak, kA
Input Energy, ×103 J/m
% Converted to Kinetic Energy
% of Energy Radiated
Hill (1971,1977) 21 15 (~3)
9+ (at 25 µs) 2*+ (at 25 µs)
Plooster (1971) 20 2.4 4 (at 35 µs) 50 (at 35 µs) Paxton et al. (1986,1990)
20 4 2 (at 64 µs) 69 (at 64 µs)
27
Continued Table 2-1. Source Current
Peak, kA
Input Energy, ×103 J/m
% Converted to Kinetic
Energy
% of Energy Radiated
Dubovoy et al. (1991,1995)
20 3 - 25 (at ≥55 µs)
Borovsky (1998) - 0.2-10 - Krider et al. (1968) Single-
stroke flash
230 - 0.38#
Uman (1987) (200-2000) -
+ Incorrect due to a factor of 20-30 error in electrical conductivity. * Estimated by subtraction of the internal and kinetic energies from the input energy shown in figure 1 of Hill (1977). # Only radiation in the wavelength range from 0.4 to 1.1 µm.
Hill (1971, 1977) overestimates the input energy by a factor of 5 or so due to the
underestimation of electrical conductivity. The corrected value is given in the
parentheses. Plooster’s (1971) model gives a crude radiative transport mechanism
adjusted to the expected temperature profile. Paxton et a/. (1986, 1990) gives individual
temperature dependent opacities for several wavelength intervals. Dubovoy et al.’s
(1991, 1995) model is in principle the same as the previous one, except for the fact that
the pinch effect was taken into account. Uman (1987) estimates the input energy by
assuming that tens of coulombs are lowered from a height of 5 km to ground. An
assumption made is that the potential difference between the ground and charge center
inside the cloud is 108-109 V.
Krider et al. (1968) estimated the average energy per unit length and peak power to
be 2.3×105 J/m and 1.2×109 W/m (see Section 2.5.1). In this experiment the radiative
efficiencies of the long spark and the lightning channel are assumed to be constant in the
wavelength range of 0.4 to 1.1 µm. Since the input energy for long spark energy is
known, the radiative efficiency can be determined (0.38%) and applied to the lightning
28
return stroke. This estimate appears to be consistent with the thunder theory of Few
(1965, 1995).
Borovsky (1995, 1998) describes the dart leaders and return strokes as
electromagnetic waves that are guided along the conducting channels (see Section 2.5.2).
In this electrodynamic representation of lightning, the stored electrostatic energy Wstored
around a charged channel is the source of power for a return stroke. Borovsky, based on
electrostatic considerations, estimates the energy per unit channel length to be around
1×103–1.5×104 J/m, which is consistent with that predicted by the gas dynamic models.
Hence, there are one to two orders of magnitude differences in the estimates of
energy per unit length. The higher end of the energy range is likely to have included a
significant fraction of the energy dissipated by processes other than the return stroke.
These include the in-cloud discharge processes like the one in which charges are
collected from isolated hydrometeors in volumes measured in cubic kilometers and
transported into the developing leader channel. Additional experiments are required to
resolve the up to two orders of magnitude uncertainty in the estimate of lightning energy
input. In chapter 3, we will attempt to estimate lightning energy using recently acquired
experimental data for rocket-triggered lightning.
: CHAPTER 3 ESTIMATING POWER AND ENERGY
3.1 Methodology
Power per unit length and energy per unit length, each as a function of time are
estimated from the vertical (longitudinal) component of the electric field in the immediate
vicinity of the triggered-lightning channel and associated lightning return stroke current.
Additionally, channel resistance per unit length and channel radius are estimated. The
vertical electric field was measured by Miki et al. (2002) using a Pockels sensor placed at
a radial distance of 0.1 m from, and at a height of 0.1 m above the tip of the 2-m vertical
rod. The measured field was assumed to be equal to the longitudinal electric field inside
the channel. Indeed, according to Borovsky (1995), the longitudinal electric field at radial
distances of 10 cm and 1.6 m from the channel axis differs from the field at the channel
axis only by 2.1 ×10-4 % and 18 ×10-4 %, respectively (see Ez in Figure 2-12). The
average values of leader electric field changes (approximately equal to return stroke field
changes) at 0.1 to 1.6 m, 15 m, and 30 m from the lightning channel are 577 kV/m,
105kV/m, and 60 kV/m, respectively (Miki et al., 2002; Schoene et al., 2003, JGR).
Lightning current was measured at the base of the 2-m strike rod. We will assume that
this current is representative of the current flowing in the lightning channel at a height of
the Pockels sensor. Under these assumptions, the power and energy per unit length can be
estimated as P(t) = I(t) E(t) and W(t) = ∫ P(τ)dτ , respectively (Figure. 3-1). This energy is
associated with joule heating of the lightning channel and can be viewed as the input
energy for the return-stroke process that is spent for ionization, channel expansion, and
t
0
29
30
production of electromagnetic (including optical) and acoustical radiation from the
channel.
Figure. 3-1: Illustration (not to scale) of the method used to estimate power, P(t), and
energy, W(t), from measured lightning channel current, I(t), and vertical electric field, E(t), near the channel.
Very close (0.1 to 1.6 m) vertical electric fields and associated channel-base
currents were obtained for 36 strokes in nine triggered lightning flashes (see section 3.2
for the experimental setup). Out of 36 strokes, only 31 strokes in 12 flashes were suitable
for the analysis presented here. For the remaining strokes, though the current records
were available, the corresponding electric field records were saturated. All the acquired
electric field signatures can be divided in three types: 1) “classical” V-shaped signature
with return-stroke electric field change ∆ERS being approximately equal to the leader
electric field change ∆EL (∆ERS = ∆EL); 2) V-shaped signature with ∆ERS being
appreciably smaller ∆EL; 3) same as 2, but with the return stroke portion exhibiting no
flattening that is expected to occur within 20 µs or so of the beginning of the return
stroke. These three types of waveforms are illustrated in Figure 3-7. The reason for the
31
residual electric field some tens of microseconds after the return stroke for Types 2 and 3
is apparently due to the fact that the return stroke fails to neutralize all the leader charge
in the corona sheath surrounding the channel core (Kodali et al., 2003). The statistics for
peak power and energy are produced separately for the three types of electric field
signatures (no energy estimates for Type 3). Since Type 1 represents the “classical”
leader/return stroke sequence, while Types 2 and 3 indicate the presence of an additional,
slower process involved in the removal of charge from the channel (not all the
electrostatic energy deposited along the channel by the leader is tapped by the return
stroke), all the analysis concerning the channel resistance per unit length and channel
radius is presented only for Type 1. Further, the power and energy estimates for Type 2
were performed after adjusting the E- field waveforms to account for the residual field.
Events of Type 3 were used for estimating peak power only.
3.2 Experiment
Experimental data used in Section 3 have been acquired at the International Center for
Lightning Research and Testing (ICLRT) at Camp Blanding, Florida, in 2000. The
experiment was a joint University of Florida / CRIEPI, Japan, project, which is described
by Miki et al. (2002).
3.2.1 Pockels Sensors
The Pockels sensors used in this experiment had a stated dynamic range of 20 kV/m
to 1 MV/m (Miki et al., 2002). The lower measurement limit was determined by noise. In
this study we applied filtering (moving time averaging; see section 3.4.1) that allowed us
to significantly reduce this noise and, hence, to lower the measurement limit. The residual
noise translated to noise in power waveforms but did not materially influence energy
estimates.
32
During the calibration in laboratory in Japan, the 2/50 µs voltage waveform from a
1-MV impulse generator was applied across a plane-plane gap formed by two electrodes
separated by 2-3 m for creating fields less than 1 MV/m and 0.1-0.2 m for creating fields
between 1 and 2 MV/m (Miki et al., 2002). 1.2-kV and 18 kV generators were also used.
The calibration setup is shown in Figure 3-2. The electric field was obtained by dividing
the voltage by the gap length, h (Figure. 3-2). The Pockels sensor was placed in this gap
and its output voltage was measured. The variation of the sensor output voltage as a
function of the external electric field is shown in Figure 3-3. The sensor output voltage
varies linearly with the E-field, and this linear relationship was applied to all
measurements analyzed here, even when the field values were less than the lowest field
used in the calibration process.
Field calibration of the Pockels sensors was performed at the ICLRT and
accomplished by comparing the outputs of Pockels sensors with that of a flat-plate
antenna, both installed 5 m from the triggered-lightning channel. Figure 3-4 shows
1.2-kV 18-kV, or 1-MV Impulse Voltage Generator
Figure 3-2: Calibration of the Pockels sensor. Courtesy Megumu Miki of CRIEPI, Tokyo, Japan. h=2 or 3 m for creating fields less than 1 MV/m and h=0.1 or 0.2 m for creating fields between 1 and 2 MV/m.
examples of the two types of observed electric field waveforms, termed slow and fast,
measured simultaneously with a Pockels sensor and a flat-plate antenna.
33
The flat-plate antenna was calibrated theoretically [e.g., Uman, 1987], and the
Pockels sensors were calibrated (up to about 2 MV/m) in plane-plane gaps by CRIEPI
Figure 3-3: Variation of the Pockels sensor output voltage as a function of the applied electric field: (a) sensor No.6 (used to measure vertical electric field component) (b) sensor No.7 (used to measure horizontal electric field component). Courtesy Megumu Miki of CRIEPI, Tokyo, Japan.
34
personnel (see above). Figure 3-5 shows a scatter plot of the magnitude of the vertical
electric field due to lightning measured with the Pockels sensor versus that measured
with the flat-plate antenna. Figures 3-4 and 3-5 show that the magnitudes of slow
waveforms are essentially the same for the flat-plate antenna and the Pockels sensor
records. However, the magnitudes of the relatively fast waveforms measured with the
Pockels sensor are on average about 60% of those measured using the flat-plate antenna.
This implies that electric field peaks measured using Pockels sensors may be
underestimates by 40% or so, provided that the frequency content of the electric field in
the immediate vicinity of the channel is not much different from that of relatively fast
waveforms at 5 m. The difference in the response of the Pockels sensors to slow and fast
waveforms is presumably caused by the insufficient upper frequency response of 1 MHz
of the Pockels sensor measuring system. If the frequency content is higher very close to
Figure 3-4: Comparison of the electric field waveforms simultaneously measured with a Pockels sensor and a flat-plate antenna, both at 5 m.
35
the channel than at 5 m, the field peaks measured by the Pockels sensors may be
underestimated by more than 40%.
Figure 3-5: Comparison of magnitudes of the vertical electric field peaks measured with
Pockels sensors and a flat-plate antenna, both at 5 m. Pockels sensors No.6 and No.7 were subsequently used for measuring the vertical and horizontal electric field components, respectively, in the immediate vicinity of the lightning channel. [M. Miki, V.A. Rakov, K.J. Rambo, G.H. Schnetzer, and M.A. Uman; "Electric Fields Near Triggered Lightning Channels Measured with Pockels Sensors," J. Geophys. Res., vol.107 (D16), Figure. 4, pp. 4, 2002]
3.2.2 Experimental Setup
Pockels sensors were installed on the underground rocket launching facility at the
ICLRT [Rakov et al., 2000, 2001; Crawford et al., 2001], as shown in Figure 3-6.
36
Figure 3-6: Experimental setup. [M. Miki, V.A. Rakov, K.J. Rambo, G.H. Schnetzer, and M.A. Uman; "Electric Fields Near Triggered Lightning Channels Measured with Pockels Sensors," J. Geophys. Res., vol.107 (D16), Figure. 3, pp. 3, 2002]
The vertical field sensor was placed at a radial distance of 0.1 m from, and at a
height of 0.1 m above the tip of the 2-m vertical strike rod, and the horizontal field sensor
was placed directly below it. A metal ring having a radius of 1.5 m was installed around
the strike rod. The ring was connected to the base of the strike rod, which was grounded.
Since the lightning channel could attach itself either to the strike rod or the ring, the
horizontal distance between the channel and the Pockels sensor varied between 0.1 m to
1.6 m. The corresponding lightning currents are measured using a current viewing
resistor (shunt), placed at the base of the strike rod. Currents were also measured using a
different method as discussed in Section 3.4.1. There are two types of current records: 1)
low-current records, whose duration is about 250 ms and the measurement range is from
–2 kA to 2 kA. The amplitude resolution of low-current records is about 1.8 A. The
sampling interval was 100 ns; 2) high-current records, whose duration is 50 µs and the
measurement range is from –31 kA to 18 kA. The resolution of high-current records was
37
about 450 A. The current sampling interval was 20 ns. The sampling interval for electric
field records was 0.5 µs.
3.3 Electric Field Waveforms
3.3.1 V-Shaped Signatures with ∆ERS = ∆EL
In this type, the return stroke apparently neutralizes all the charge deposited by the
leader and thus the entire waveform exhibits a V-shaped signature in which the leader
and return stroke field changes are nearly equal to each other. The rise time of the return
stroke electric field is of the order of 1 µs.
∆EL ∆ERS = ∆EL
∆EL ∆ERS < ∆EL
∆EL ∆ERS (t) < ∆EL
Time, µs
Figure 3-7: V-shaped electric field signatures with a) the return stroke field change, ∆ERS, being equal to the leader field change, ∆EL. b) ∆ERS < ∆EL, field flattening within 20 µs or so of the beginning of the return stroke (of the bottom of the V), c) ∆ERS (t) < ∆EL, no flattening within 20 µs.
38
Table 3-1: Summary of peak current and ∆EL statistics for 8 strokes exhibiting V-shaped electric field signatures with ∆ERS = ∆EL.
Peak Current, kA
∆EL, kV/m
Min Max Mean GM Min Max Mean GM
9.85
21.5
16.6
15.5
52.5
305
122
109
3.3.2 V-Shaped Signatures with ∆ERS < ∆EL and Field Flattening within 20 µs
These electric field waveforms are characterized by residual electric fields (Kodali
et al., 2003) and, hence, residual charge (and associated electrostatic field energy) that is
apparently dissipated via a slower process lasting in excess of some hundreds of
microseconds, other than the return stroke. Therefore, such waveforms cannot be used
with confidence for estimating the input energy of a lightning return-stroke using the
method illustrated in Figure. 3-1, which is based on the assumption that all the
electrostatic field energy of the leader is converted to the Joule heating of the channel by
the return stroke. However, these waveforms can be used to estimate the peak power,
which is expected to occur within the first few hundred nanoseconds, long before the
flattening takes place. We also used these waveforms for computing the input energy
after adjusting them to eliminate the residual field. The statistics for the peak current and
∆EL for this category of strokes are given in Table 3-2.
Table 3-2: Summary of peak current and ∆EL statistics for 5 strokes with ∆ERS < ∆EL and flattening within 20 µs or so.
Peak Current, kA ∆EL, kV/m Min Max Mean GM Min Max Mean GM 15.4 26.3 20.6 19.6 105 227 160 155
39
3.3.3 Signatures with ∆ERS (t) < ∆EL and no Flattening within 20 µs
In these E-field signatures, the electric field after the beginning of the return stroke
continues to increase during a time interval of the order of a few milliseconds. Such
behavior is indicative of a residual charge (and associated electrostatic field energy)
located near the attachment point and a process other than the return stokes being at work
to neutralize this residual charge. E-field waveforms with ∆ERS (t) < ∆EL and no
flattening with 20 µs were used for computing only the peak power, which occur before
the “abnormal” behavior of the return-stroke E-field begins. The statistics of the peak
current and ∆EL associated with this type of waveforms are given in Table 3-3.
Table 3-3: Summary of peak current and ∆EL statistics for 18 strokes with ∆ERS (t) < ∆EL and no flattening within 20 µs
Peak Current, kA
∆EL, kV/m
Min Max Mean GM Min Max Mean GM
5.1
26.4
11.4
10.5
175
1150
554
474
3.4 Analysis of V-Shaped E-Field Signatures with ∆ERS = ∆EL
The product of channel-base current and close longitudinal electric field, each as a
function of time, yields the power per unit channel length vs. time waveform. Since we
have the current record for the return stroke only, the following results represent
processes following the initiation of the return stroke (leader/return stroke transition). The
energy per unit length is obtained by the integration over time of the power waveform, as
discussed in Section 3.1.
3.4.1 Data Processing
E-field waveforms are typically noisy (see Figure 3-8 a) and hence some sort of
filtering (averaging) has to be performed to make the electric field tractable. Only the
40
portion of the electric field record following the initiation of the return stroke was
filtered, since only this portion was needed for estimating power and energy input. A
moving-averaging window of 100 data points, which acts as a low-pass filter, was used
for this purpose. Averaging was done after a suitable time interval after the beginning of
the return stroke, so that the initial (fast-varying) portion of the return stroke is not
modified, as illustrated in Figure. 3.8 b. In this example the E-field waveform is averaged
2.5 µs after the start of the return stroke. One can see that the main features of the
waveform are preserved, while the noise is significantly reduced. Such filtering was
performed for all the strokes analyzed here. The resultant electric fields are shown in
Figures 3-10 to 3-17.
Leader Return Stroke A
Time-AveragedOriginal
2.5µs B
Figure 3-8: Stroke S0013-1. A) Original E-field record. B) Filtered (100-µs moving-window time averaged) version of the E-field waveform shown in A).
Time, µs
In 2000, lightning currents were measured using two methods. In the first method,
the total lightning current was measured using a current viewing resistor, CVR (shunt),
41
placed at the base of the strike rod. In the second method, currents entering the launcher
grounding system (ground screen and ground rod) were measured and summed to obtain
the total lightning current. In this latter case, the current into the 70 × 70m2 buried
metallic grid (ground screen) was measured using two CVR’s and the ground rod current
was measured using two P 110A’s current transformers (CT). The current range of P
110A’s is from a few amperes to 20 kA, when terminated with a 50-ohm resistor. A
passive combiner was used to sum the two signals from the ground rod CT’s to a total
ground rod current. The ground screen current was measured by two separate
instrumentation systems IIS-S (south ground screen current) and IIS-N (north ground
screen current). The sum of the ground rod current and north and south screen currents
gives the total screen
IR = 0.85+1.02 IS R2 = 0.8 n = 36
Figure.3-9: Scatter plot of screen current, IS vs. strike rod current, IR, for 2000. [V. Kodali, “Characterization and analysis of close lightning electromagnetic fields,” Master’s thesis, University of Florida; 2003].
current. A scatter plot of ground screen current (the sum of the ground screen and ground
rod currents to be exact) vs. strike rod current is shown in Figure 3-9. With a few
42
exceptions, the two current values are very close to each other. In the following sections
(also in Tables 3-1 to 3-3), we used the strike-rod current, although in Table 3-4 the
power and energy were also computed using the ground-screen current, when available.
3.4.2 Power and Input Energy
Power as a function of time, obtained as the product of longitudinal E-field and
strike-rod current, and energy, the integral of the power curve, are shown in Figures 3-10
to 3-17, for the eight strokes having the V-shaped E-field signatures with ∆ERS = ∆EL.
The estimated peak power and energy values are given in Table 3-4. Histograms of the
various quantities and associated scatter plots are found in Section 3.4.3. Error analysis is
presented in Section 3.4.4.
Figure 3-10: Time variation of electric field, current, power, and energy for stroke
S006-4.
µs
43
µs
Figure 3-11: Same as Figure. 3-10, but for Stroke S008-4. Negative values in the variation of power with time are due to residual noise in the electric field waveform.
µs Figure 3-12: Same as Figure. 3-10, but for Stroke S0013-1.
44
Time, µs
Figure 3-13: Same as Figure. 3-10, but for Stroke S0013-4. Negative values in the variation of power with time are due to residual noise in the electric field waveform.
Time, µs
Figure 3-14: Same as Figure. 3-10, but for Stroke S0015-2. Negative values in the variation of power with time are due to residual noise in the electric field waveform.
45
µs
Figure 3-15: Same as Figure. 3-10, but for Stroke S0015-4.
Power and energy are estimated for both strike-rod and ground-screen currents,
when both currents are available. As seen from Table 3-4, the peak power varies from 2.2
×108 W/m to 25.1×108 W/m and input energy at 10 to 50 µs from 0.9 ×103 J/m to 6.35
×103 J/m. The peak power values are consistent with 12 ×108 W/m reported by Krider et
al. (1968), and the energy values are in agreement with predictions (of the order of 103
J/m) of gas-dynamic models (Sections 2.5.1 and 2.5.3, respectively).
46
Figure 3-16: Same as Figure. 3-10, but for Stroke S0015-6. The time scale here is different from that in Figures. 3-10 to 3-15 and 3-17 (50 µs vs. 55 µs). The energy is obtained at 10 µs after the return stroke, because at later times the electric field becomes positive causing the power waveform to change polarity (become negative), which is physically unreasonable.
Figure 3-17: Same as Figure. 3-10, but for Stroke S0023-3.
Time, µs
Time, µs
Leader Return Stroke
47
Table 3-4: Power and energy estimates for strokes having V- shaped E-field signatures with ∆EL= ∆ERS.Peak current,
kA
Peak power, ×108 W/m
Energy, ×103 J/m
Date FlashID
Stroke order
Termination point
Rod Screen
∆EL, kV/m
Rod Screen Rod Screen
Remarks
6/13
S0006
4
Rod
14.3
10.2
52.5
2.35
2. 9
1.8 (at 45.7 µs)
1.7(at 41.3 µs)
Classical trigger, 5
RSs
6/17 S0008 4 Ring 20.9 19.3 60.0 2.2 4.8 0.9 (at45.6 µs)
0.8 (at 45.5 µs)
Classical trigger, >8
RSs 6/18 1 Rod 11.6 - 125.0 5.2 - 2.57 (at
45.7
µs)
-
6/18
S0013 4 Ring 11.6 - 122.5 8.6 - 1.34 (at
45.5 -
µs)
Classical trigger, 6
RSs
6/23 2 Rod 19.3 Noisy 105.0 9.9 - 6.35 (at45.5 µs)
-
6/23 4 Rod 21.5 Noisy 112.5 8.7 - 5.0 (at45.5 µs)
-
6/23
S0015
6 Rod 20.0 Noisy 92.5 14.5 - 1.3 (at 10.0 µs)*
-
Classical trigger, 6
RSs
7/11 S0023 3 Ring 9.85 16.3 305.0 25.1 22.0 6.2 (at 45.7 µs)
5.4 (at 47.3 µs)
Classical trigger, 3
RSs
* For this stroke, after 10 µs ∆ERS > ∆EL causing the power waveform to change polarity (become negative) which is physically unreasonable.
48
3.4.3 Statistical Analysis
Histograms displaying the distributions of peak current, ∆EL, peak power, energy,
and action integral for the V-shaped E-field waveforms with ∆ERS = ∆EL are shown in
Figures 3-18 through 3-22. The means and standard deviations are given separately for
different lightning channel termination points (rod or ring) and for all data combined.
Additionally presented in Figure 3-23 and Figure 3-24 are histograms of risetimes for
current and power.
Num
ber
Peak Current, kA ring n = 3 Mean = 14.1 kA St. Dev. = 5.9 kA
rod n = 5 Mean = 18.1 kA St. Dev. = 3.8 kA
n = 8 Mean = 16.1 kA St. Dev. = 4.8 kA Min = 9.9 kA Max = 21.5 kA
Figure 3-18: Histogram of peak current for strokes characterized by V- shaped electric
field signatures with ∆ERS = ∆EL.
49
Num
ber
∆EL, kV/m ring n = 3 Mean = 162.5 kV/m St. Dev = 127.3 kV/m
rod n = 5 Mean = 97.5 kV/m St. Dev = 27.8 kV/m
n = 8 Mean = 121.9 kV/m St. Dev = 78.8 kV/m Min = 52.5 kV/m Max = 305 kV/m
Figure 3-19: Histogram of ∆EL for strokes characterized by V- shaped electric field
signatures with ∆ERS = ∆EL.
50
Num
ber
Peak Power, × 108 W/m ring n = 3 Mean =11.9 × 108 W/m St. Dev. = 11.7 × 108 W/m
rod n = 5 Mean =8.2 × 108 W/m St. Dev. = 4.4 × 108 W/m
n =8 Mean =9.6 × 108 W/m St. Dev. = 7.4 × 108 W/m
Min = 2.2 × 108 W/m St. Dev = 25 × 108 W/m Figure 3-20: Histogram of peak power for strokes characterized by V- shaped electric
field signatures with ∆ERS = ∆EL.
51
Num
ber
Energy, × 103 J/m ring n = 3 Mean = 2.8× 103 J/m St. Dev. = 2.9 × 103 J/m
rod n = 5 Mean = 3.6 × 103 J/m St. Dev. = 2.5 × 103 J/m
rod n = 8 Mean = 3.6 × 103 J/m St. Dev. = 2.5 × 103 J/m Min = 0.9 × 103 J/m St. Dev. = 6.2 × 103 J/m
Figure 3-21: Histogram for input energy for strokes characterized by V- shaped electric
field signatures with ∆ERS = ∆EL.
52
Num
ber
Action Integral, × 103 A2s
ring n = 3 Mean = 2.14 × 103 A2s St. Dev. = 1.63 × 103 A2s rod n = 5 Mean = 1.89 × 103 A2s St. Dev. = 1.37 × 103 A2s
n = 8 Mean = 2.04 × 103 A2s St. Dev = 1.44 × 103 A2s Min = 0.56 × 103 A2s Max = 4.68× 103 A2s
Figure 3-22: Histogram for action integral for strokes characterized by V- shaped electric field signatures with ∆ERS = ∆EL.
53
Mean = 0.85 µs Std. dev = 0.37 µs Min = 0.40 µs Max = 1.6 µs
Num
ber
Risetime, µs Figure 3-23: Histogram of the risetime of current for strokes characterized by V-shaped
electric field signatures with ∆ERS = ∆EL. The risetime is defined as the time taken by current to rise from 0% to 100% of the peak value.
Mean = 0.43 µs Std.dev = 0.12 µs Min = 0.28 µs Max = 0.60 µs
Num
ber
Risetime, µs Figure 3-24: Histogram of the 0-100 % risetime of power per unit length for strokes
characterized by V-shaped electric field signatures with ∆ERS = ∆EL.
54
Scatter plots showing correlation between the various parameters are presented in
Figures 3-25 through 3-29.
P = -0.49 I + 17.6 R2 = 0.1
Peak
Pow
er, ×
108 W
/m
Peak Current, kA
Figure 3-25: Peak power vs. peak current for strokes characterized by V- shaped electric field signatures with ∆ERS = ∆EL. The filled circles and the hollow circles represent the strokes for which the lightning channel terminated on the rod and ring, respectively.
It can be observed from Figures 3-25 and 3-26 that the determination coefficient,
R2, which is the square of the correlation coefficient, R, in both cases is close to zero.
Two possible reasons for the low correlation coefficients are the following: 1) the
influence of the electric field is more significant than that of current and 2) the electric
field decays to a negligible value before the current attains its peak magnitude. Indeed,
the mean risetime to peak current is 0.85 µs (see Figure. 3-23), and the average risetime
of power per unit length to its peak 0.43 µs (see Figures. 3-23 and 3-24). Hence, electric
55
field has a more pronounced effect on the peak power value, since the current attains its
peak value only after the power does, resulting in the lack of dependence of the peak
power on the peak current value. It is likely that both reasons listed above can contribute
to the observed lack of correlation between the peak power and peak current. There
appears to weak positive correlation between the energy per unit length and action
integral. The latter was computed as the integral of the square of current over the same
time interval (typically between 40 and 50 µs) as the corresponding power per unit
length. The unit for action integral is A2s, which is the same as J/Ω.
W = -0.02 I + 3.6 R2 = 2.3 × 10-3
Ener
gy, ×
103 J/
m
Peak Current, kA Figure 3-26: Energy vs. peak current for strokes characterized by V- shaped electric field
signatures with ∆ERS = ∆EL. The filled circles and the hollow circles represent the strokes for which the lightning channel terminated on the rod and ring, respectively.
56
P = 0.08 ∆EL – 0.66 R2 = 0.78
Peak
Pow
er, ×
108 W
/m
∆EL, kV/m Figure 3-27: Peak power vs. ∆EL for strokes characterized by V- shaped electric field
signature with ∆ERS = ∆EL.
W = 0.02 ∆EL + 1 R2 = 0.37
Ener
gy, ×
103 J/
m
Figure 3-28: Energy vs. ∆EL for strokes characterized by V- shaped electric field
signature with ∆ERS = ∆EL.
∆EL, kV/m
57
Energy, J/m
Act
ion
Inte
gral
, kJ/Ω
AI = 348 W + 936.21 R2 = 0.31
5 4 3 2 1 0
0 1000 2000 3000 4000 5000 6000 7000
Figure 3-29: Energy vs. Action Integral for strokes characterized by V- shaped electric field signature with ∆ERS = ∆EL.
3.4.4 Error Analysis
In this section, three sources of uncertainty involved in power and energy estimates
are examined: 1) as noted in Section 3.2.1, electric fields measured using Pockels sensors
may be underestimated by 40% or so due to the insufficient upper frequency response of
1 MHz of the measuring system (Miki et al., 2002), (2) the sampling interval for electric
field records was 0.5 µs vs. 20 ns for current records (see section 3.2.2), and (3) electric
field and current records were aligned manually using the bottom of the V-shaped electric
filed signature and the beginning of the current waveform. There is little ambiguity in
selecting the return-stroke current starting point, while the bottom of the V is somewhat
uncertain within ± 0.5 µs due to insufficient sampling rate for electric field. Sensitivity of
power and energy estimates to changes in the electric field peak magnitude and its
58
position on the time scale are examined using the following procedure. The peak power
and energy are calculated after applying a correction factor of 1.6 at the instant of peak
electric field (to take into account the 40% potential error due to the insufficient upper
frequency response of the measuring system). Then the electric field waveform is
modified in three different ways: (1) keeping the position of the negative field maximum
intact (2) moving the negative maximum (the bottom of the V) 0.24 µs to the left, and (3)
moving the maximum, 0.24 µs to the right from its original position, in order to partially
account for the ± 0.5 µs uncertainty noted above. These three steps are illustrated in
Figures 3-30 to 3-32 for stroke S0013-1. Similar procedure was applied to all the strokes
analyzed here, and results are summarized in Table 3-5, along with the peak power and
energy values estimated from the original records without correction.
59
E-Fi
eld,
kV
/m
µs
Figure 3-30: Flash 0013, stroke 1; Correction factor of 1.6 is applied at the instant of negative E-field peak. The filled circles and the arrows indicate the original data points (the sampling interval of the electric field record was 0.5 µs). The hollow symbols represent fictitious data points based on linear interpolation so as to match the sampling interval of 0.02 µs of the current record. The circles represent the original E-field waveform and the triangles represent the E-field waveform after the correction factor of 1.6 is applied. Calculated values of peak power and energy before and after correction are given.
60
E-Fi
eld,
kV
/m
µs
Figure 3-31: Flash 0013, stroke 1; Correction factor of 1.6 is applied at the instant of negative E-field peak, which is shifted by 0.24 µs to the left in order to partially account for the ± 0.5 µs uncertainty in the position of the peak. The filled circles and the arrows indicate the original data points. The hollow symbols represent fictitious data points based on linear interpolation so as to match the sampling interval of 0.02 µs of the current record. Calculated values of peak power and energy before and after correction are given.
61
Figure 3-32: Flash 0013, stroke 1; Correction factor of 1.6 is applied at the instant of
negative E-field peak, which is shifted by 0.24 µs to the right in order to partially account for the ± 0.5 µs uncertainty in the position of the peak. The filled circles and the arrows indicate the original data points. The hollow symbols represent fictitious data points based on linear interpolation so as to match the sampling interval of 0.02 µs of the current record. Calculated values of peak power and energy before and after correction are given.
igure 3-32: Flash 0013, stroke 1; Correction factor of 1.6 is applied at the instant of negative E-field peak, which is shifted by 0.24 µs to the right in order to partially account for the ± 0.5 µs uncertainty in the position of the peak. The filled circles and the arrows indicate the original data points. The hollow symbols represent fictitious data points based on linear interpolation so as to match the sampling interval of 0.02 µs of the current record. Calculated values of peak power and energy before and after correction are given.
Time, µs
62
Table 3-5: Dependence of peak power and energy on errors in the value of E-field peak and its position on the time scale. Original record
E-Field peak multiplied by 1.6
E-Field peak multiplied by 1.6 and shifted by 0.24 µs to the left
E-Field peak multiplied by 1.6 and shifted by 0.24 µs to the right
Flash ID
Stroke order
P.P, ×108, W/m
E, ×103, J/m
P.P, ×108, W/m
∆ %
E, ×103, J/m
∆ %
P.P, ×108, W/m
∆ %
E, ×103, J/m
∆ %
P.P, ×108, W/m
∆ %
E, ×103, J/m
∆ %
S006 4 2.3 6.4 2.6 13 1.8 0.0 4.5 96 1.92 6.7 1.74 -24 1.77 -1.7
S008 4 1.97 0.8 2.83 44 0.82 2.5 6.58 234 0.96 20 1.96 -0.5 0.76 -5.0
S0013 1 5.2 2.6 6.1 17 2.6 0.0 8.0 54 2.8 7.7 4.1 -21 2.4 -7.7 S0013 4 8.6 1.34 8.6 0 2.3 -3.0 13 51 1.7 27 5.9 -31 1.2 -10
S0015 2 9.9 6.35 12.7 28 6.43 1.0 14 41 6.2 -2.0 8.1 -18 6.1 -4.0 S0015 4 8.7 5.0 11.9 37 5.1 2.0 13.9 60 5.38 8.0 5.9 -32 4.86 -3.0
S0015 6 14.5 1.3 14.5 0.0 1.33 2.3 17.5 21 1.7 31 10.7 26 0.94 -28
S0023 3 25.1 6.2 31.5 25 6.4 3.0 26.9 7.0 6.9 11 23.8 -5.0 5.6 -10
Mean 9.53 3.75 11.3 21 3.35 1.0 13 71 3.5 14 7.8 -20 2.9 -9.0
St. deviation 7.53 2.5 9.3 16.1 2.3 1.96 7.1 71.2 2.3 11.3 7.1 19.4 2.2 8.4
63
Table 3-5 suggests that the peak power estimates are sensitive to the considered
uncertainties, the maximum mean error being 71%. Intuitively it makes sense, because
the peak power occurs in the first few microseconds or less of the beginning of the return
stroke. On the other hand, energy estimates are relatively insensitive to the uncertainties
examined here, the variation not exceeding 31% (14% on average for the 8 strokes
analyzed).
3.4.5 Channel Resistance and Radius
The expression, R (t) = E (t) / I (t), gives the evolution of resistance per unit channel
length with time. Since we cannot measure leader currents, R (t) can be evaluated only
for the return stroke. The evolution of channel radius can be estimated from the channel
resistance using the expression, r (t) = [σ π R (t)] –0.5, where σ is the electrical
conductivity of the channel, assuming σ = 104 S/m. In reality, σ increases with time (as
the channel temperature increases), but this variation is rather weak for the expected
temperature range (≥ 20,000° K or so) (e g., Rakov, 1998). The assumption of σ =
constant implies that R (t) decreases only due to expansion of channel (increase in r (t)).
In principle, the channel radius and resistance can be evaluated for the entire length
of the field record. But the results become dominated by noise once the electric field
magnitude decreases below 20 kV/m because of the limitations on the dynamic range of
the Pockels sensor. The evolution of resistance and channel radius along with
corresponding E-field, current, and power profiles, for eight strokes exhibiting V-shaped
E-field signatures with ∆ERS = ∆EL for the time interval when the E-field magnitude is
greater than 20 kV/m is shown in Figures 3-33 to 3-40. Table 3-6 shows the resistance
and channel radius at the instant of peak power. The time scale over which the evolution
64
is shown differs from that of other Figures in Section 3.4.2 because as the magnitude of
E(t) falls below about 20 kV/m, it attains small values during zero crossings forcing R(t)
to very small values, which in turn causes r (t) to go to unreasonably high values
(Figure 3-41).
a) b) c) d) e)
S006-4
Figure 3-33: Evolution of the various quantities for the first 0.58 µs for Flash S006, stroke 4. a) E-field; b) current; c) power per unit length; d) channel resistance per unit length; e) channel radius.
65
a) b) c) d) e)
Figure 3-34: Evolution of the various quantities for the first 0.4 µs for Flash S008, stroke 4. a) E-field; b) current; c) power per unit length; d) channel resistance per unit length; e) channel radius.
66
Figure 3-35: Evolution of the various quantities for the first 1.4 µs for Flash S0013,
stroke 1. a) E-field; b) current; c) power per unit length; d) channel resistance per unit length; e) channel radius
67
a) b) c) d) e)
S0013-4
Figure 3-36: Evolution of the various quantities for the first 1.2 µs for Flash S0013, stroke 4. a) E-field; b) current; c) power per unit length; d) channel resistance per unit length; e) channel radius
68
a) b) c) d) e)
Figure 3-37: Evolution of the various quantities for the first 2.1 µs for Flash S0015, stroke 2. a) E-field; b) current; c) power per unit length; d) channel resistance per unit length; e) channel radius
69
a) b) c) d) e)
Figure 3-38: Evolution of the various quantities for the first 1.5 µs for Flash S0014, stroke 4. a) E-field; b) current; c) power per unit length; d) channel resistance per unit length; e) channel radius
70
a) b) c) d) e)
Figure 3-39: Evolution of the various quantities for the first 1.3 µs for Flash S0015, stroke 6. a) E-field; b) current; c) power per unit length; d) channel resistance per unit length; e) channel radius
71
a) b) c) d) e)
S0023-3
Figure 3-40: Evolution of the various quantities for the first 5 µs for Flash S0023, stroke 3. a) E-field; b) current; c) power per unit length; d) channel resistance per unit length; e) channel radius
72
Cha
nnel
Rad
ius,
cm
Time, µs
Figure 3-41: Evolution of channel radius for S0008-4. The peaks are formed due to small values attained by the noisy electric field waveforms.
Table 3-6: Resistance and channel radius for strokes having V- shaped E-field signatures
with ∆ERS = ∆EL
Flash ID
Stroke order
Resistance*, Ω/m
Channel radius*, cm
Termination point
S006 4 0.67 0.69 Rod S008 4 1.3 0.49 Ring
1 5.1 0.25 Rod S0013 4 8.0 0.22 Ring 2 4.5 0.31 4 5.1 0.25
S0015
6 4.3 0.27
Rod
S0023
3
30.8 0.10
Ring
Mean value 7.5 0.32
* Resistance and channel radius are evaluated at the instant of peak power
73
As expected, the channel resistance decreases and channel radius increases with time as
the return-stroke current heats the channel.
In Rakov (1998), the propagation mechanisms of dart leaders and return strokes are
analyzed by comparing the behavior of traveling waves on a lossy transmission line and
the observed characteristics of these two lightning processes. The R in the transmission
line model is assumed to be a constant but different ahead of and behind either the dart-
leader or the return-stroke front with any nonlinear effects occurring at the front. The
channel radius and resistance ahead of return-stroke front are estimated to be around 0.3
cm and 3.5 Ω/m. The channel radius and resistance in Table 3-6 are obtained at the
instant of peak power which occurs at around 0.4 µs. The mean values for channel radius
and resistance in Table 3-6 are 0.32 cm and 7.5 Ω/m. In Rakov (1998), the channel radius
and resistance behind the return-stroke front are estimated to be around 3 cm and 0.035
Ω/m. For the pre-dart-leader channel, these two quantities are estimated to be around 3
cm and 18 kΩ/m, respectively.
3.5 Analysis of V-shaped E-field Signatures with ∆ERS < ∆EL and Field Flattening within 20 µs
An example of such a waveform exhibiting the residual electric field, ∆EL - ∆ERS, is
shown in Figure. 3-5 b. The product of channel-base current and close vertical
(longitudinal) electric field, each as a function of time, yields the power per unit channel
length vs. time waveform. The energy per unit length is obtained by integration over time
of the power waveform, as discussed in section 3.1. The electric field waveform due to
the return stroke was adjusted by subtracting the residual electric field, ∆EL- ∆ERS. ∆ERS
is defined as the average value of the electric field from 44 µs to 50 µs after the return
stroke. This time frame is selected since most of the energy estimates for most of the
__ __
74
events analyzed in Section 3.4 was obtained at around 45 µs after the beginning of the
return stroke. The adjustment was needed (see Section 3.3.2) to eliminate the electrostatic
energy not involved in the return stroke process. Figure 3-42 shows this procedure for the
stroke S0008-3. This methodology enables us to reasonably compare the energy estimates
for the two classes of electric field waveforms. Table 3-7 gives the peak power and
energy values for the 5 strokes exhibiting V-shaped E-field signatures with ∆ERS < ∆EL
and field flattening within 20 µs. Histograms giving the distribution of peak current, ∆EL,
peak power, energy, and action integral are shown in Figures 3-43 to 3-47.
75
Table 3-7: Power and energy estimates for strokes having V- shaped E-Field Signatures with ∆ERS < ∆EL and field flattening within 20 µs or so.
Peak current, kA
Peak power, ×108 W/m
Energy, ×103 J/m
(∆EL- ∆ERS)*
kV/m Date
FlashID
Stroke order
Termination point
Screen Rod
∆EL, kV/m
Screen Rod Screen Rod Screen Rod
6/13
S0008
3
Ring
13.9
17.8
180.5
8.69
12.7
1 (at 20 µs)
8.1 (at 50 µs)
20.0
18.1
6/18 S0012 1 Ring - 21.0 189.9 - 14.9 - 2.9 (at50 µs)
- 36.7
2 - 12.3 124.9 - 9.40 - 1.9 (at47.5 µs)
17.9-
3 - 26.3 219.9 - 16.8 - 3.66 (at50 µs)
62.0-
6/18
S0013
5
Ring
- 22.7 277.3 - 20.2 - 3.3 (at45.5 µs)
- 47.5
* ∆ERS in (∆EL- ∆ERS) is the average value of electric field from 44 µs to 50 µs after the return stroke.
76
∆ERS
__ ∆EL
Figure.3-42: V-shaped signature with ∆EL> ∆ERS. ∆ERS represents the average electric field between 44 µs to 50 µs after the beginning of the return stroke (at 50 µs). The electric field waveform due to the return stroke was adjusted by subtracting the residual electric field, ∆EL- ∆ERS.
__ __ Time, µs
Min = 12.3 kA Max = 26.3 kA Mean = 20 kA Std. Dev. = 5.3 kA n =5
Num
ber
Peak Current, kA
Figure 3-43: Histogram of peak current for strokes characterized by V-shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs.
77
Min = 107.0 kV Max = 229.8 kV Mean = 162.1 kV Std. Dev. = 43.90 kV n = 5
Num
ber
∆EL, kV Figure 3-44: Histogram of ∆EL for strokes characterized by V-shaped electric field
signatures with ∆ERS < ∆EL and flattening within 20 µs.
Min = 9.4 × 108 W/m Max = 20.2 × 108 W/m Mean = 14.8 × 108 W/m Std. Dev. = 4.08 × 108 W/m n = 5
Num
ber
Peak Power, × 108 W/m
Figure 3-45: Histogram of peak power per unit length for strokes characterized by V-shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs.
78
Min = 1.9 × 103 J/m Max = 8.1 × 103 J/m Mean = 3.9 × 103 J/m Std. Dev. = 2.4 × 103 J/m n = 5
Num
ber
Energy, × 103 J/m Figure 3-46: Histogram of energy per unit length for strokes characterized by V-shaped
electric field signatures with ∆ERS < ∆EL and flattening within 20 µs.
Min = 2.05 × 103 A2s Max = 6.18 × 103 A2s Mean = 3.92 × 103 A2s Std. Dev. = 1.79 × 103 A2s n = 5
Num
ber
Action Integral, × 103A2s
Figure 3-47: Histogram of action integral for strokes characterized by V-shaped electric field signatures with ∆ERS < ∆EL and flattening within 20 µs.
79
The mean values of peak power and energy for this category of strokes are similar
to their counterparts for the first category (see Section 3.4). Scatter plots showing
correlation between the different quantities are presented in Figures 3-48 to 3-52.
P = 0.56 I + 3.17 R2 = 0.72
Peak
Pow
er, ×
108 W
/m
Figure 3-48: Peak power vs. peak current for strokes characterized by V- shaped electric
field signatures with ∆ERS < ∆EL and flattening within 20 µs.
Peak Current, kA
80
W = 0.02 I + 3.6 R2 = 1.6 × 10-3
En
ergy
, × 1
03 J/m
Peak Current, kA Figure 3-49: Energy vs. peak current for strokes characterized by V- shaped electric field
signatures with ∆ERS < ∆EL and flattening within 20 µs.
P = 0.08 ∆EL + 0.98 R2 = 0.83
Peak
Pow
er, ×
108 W
/m
Figure 3-50: Energy vs. ∆EL for strokes characterized by V- shaped electric field
signatures with ∆ERS < ∆EL and flattening within 20 µs.
∆EL, kV/m
81
100 120 140 160 180 200 220 2401
2
3
4
5
6
7
8
9
W = 0.01 ∆EL + 2.28 R2 = 0.04
Ener
gy, ×
103 J/
m
Figure 3-51: Energy vs. ∆EL for strokes characterized by V- shaped electric field signatures with ∆EL< ∆ERS and flattening within 20 µs.
∆EL, kV/m
AI = -0.05 W + 4.13 R2 = 5.2 × 10-3
Act
ion
Inte
gral
, kJ/Ω
Energy, J/m 1000 2000 3000 4000 5000 6000 7000 8000 9000
Figure 3-52: Energy vs. Action integral for strokes characterized by V- shaped electric field signatures with ∆EL< ∆ERS and flattening within 20 µs.
82
In contrast with the strokes for which ∆ERS = ∆EL, there appears to be moderate
positive correlation between the peak power and peak current (or ∆EL) and essentially no
correlation between the energy and action integral, although the sample size is small.
3.6 Analysis of V-Shaped E-field Signatures with ∆ERS (t) < ∆EL and no Field Flattening within 20 µs
An example of such a signature is shown in Figure 3-5 (c). Table 3-8 gives the
values of the strike-rod peak current (no usable ground-screen currents are available),
∆EL, and peak power for the 18 strokes of this type. Since there is no field flattening
within 20 µs (in fact, the field varied on the millisecond time scale) of the start of the
return stroke, energy computation was not performed for the events of this type.
Table 3-8: Power estimates for strokes having V- shaped E-Field Signatures with ∆ERS (t) < ∆EL (t) and no flattening within 20 µs
Date Flash ID
Stroke order
Termination point
Peak current,
kA
∆EL, kV/m
Peak power, ×109 W/m
Remarks
1 11.8 492 4.4 7/11 S0022
3 Rod
8.9 486.6 3.7 Classical
trigger, 3 RSs
1 11.5 451.5 3.4 7/11 S0023
2 Ring
15.2 308.2 3.9 Classical
trigger, 3 RSs
1 15.0 743.4 11.0 2 9.4 763 5.9
3 7.1 432 2.7 7/16 S0025
4
Ring
26.4 1149 25.1
Classical trigger, 4 RSs
1 11.4 865.1 9
2 17.0 1102 17.1 7/16 S0027
3
Ring
15.3 1108 12.8
Classical trigger, 9 RSs
83
Continued Table3-8.
Date Flash ID
Stroke order
Termination point
Peak current,
kA
∆EL, kV/m
Peak power,
×109 W/m Remarks
1 6.7 256.8 1.6
2 5.1 358.8 1.8
3 5.9 175.7 0.64
4 11.3 446.1 4.8
5 8.2 209.5 1.3
6 12.0 364.9 4.1
7/20 S0029
7
Rod
6.9 263.6 1.8
Classical trigger, 9 RSs
Num
ber
Peak Current, kA
Ring
Rod
n = 9 Mean = 14.3 kA Std. Dev. = 5.6 kA n = 9 Mean = 8.5 kA Std. Dev. = 2.63 kA
Mean = 11.4 kA Std.Dev = 5.1 kA Min = 5.1 kA Max = 26.4 kA
n = 18
Figure 3-53: Histogram of ∆EL for strokes characterized by V- shaped electric field
signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs.
84
Num
ber
∆EL, kV n = 9 Mean = 769.1 kV Std. Dev. = 317.6 kV
n = 9 Mean = 339.3 kV Std. Dev. = 119.2 kV
n = 18 Mean = 554.2 kV Std. Dev. = 321 kV
Min = 175.7 kV Max = 1149 kV
Figure 3-54: Histogram of ∆EL for strokes characterized by V- shaped electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs.
85
Num
ber
Peak Power, × 109 W/m Ring n = 9 Mean = 10.1 × 109 W/m Std.Dev = 7.4 × 109 W/m
Rod n = 9 Mean = 2.7 × 109 W/m Std.Dev = 1.6 × 109 W/m
Mean = 6.4 × 109 W/m Std.Dev = 6.4 × 109 W/m Min = 0.64 × 109 W/m Max = 25.1 × 109 W/m
n = 18 Figure 3-55: Histogram of peak power for strokes characterized by V- shaped electric
field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs.
86
Num
ber
Action Integral, × 103A2s Ring n = 9 Mean = 2.14 × 103A2s St. Dev. = 1.63 × 103A2s Rod n = 9 Mean = 1.89 × 103A2s St. Dev. = 1.37 × 103A2s
n = 18 Mean = 2.41 × 103A2s St. Dev = 2.06 × 103A2s Min = 0.3 × 103A2s Max = 7.02 × 103A2s
Figure 3-56: Histogram of action integral for strokes characterized by electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs.
As seen in Figure. 3-53, the mean peak power for this stroke category is several
times larger than for the first two categories considered in Sections 3.4 and 3.5. Scatter
plots showing correlation between the different quantities are presented in Figures. 3-55
and 3-56.
87
P = 1.1 I – 6.23 R2 = 0.83
Peak
Pow
er, ×
109 W
/m
Peak Current, kA
Figure 3-57: Peak power vs. peak current for strokes characterized by electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs. The filled circles and the hollow circles represent strokes for which the lightning channel was attached to the rod and the ring, respectively.
88
P = 0.02 ∆EL – 3.5 R2 = 0.83
Peak
Pow
er, ×
109 W
/m
∆EL, kV
Figure 3-58: Peak power vs. ∆EL for strokes characterized by electric field signatures with ∆ERS (t) < ∆EL and no field flattening within 20 µs. The filled circles and the hollow circles represent the strokes for which the lightning channel was attached to the rod and the ring, respectively.
Similar to the strokes considered in Section 3.5, there appears to be moderate
positive correlation between the peak power and peak current (or ∆EL).
CHAPTER 4 CHARACTERIZATION OF PULSES SUPERIMPOSED ON STEADY CURRENTS
4.1 Initial Stage in Rocket-Triggered Lightning
4.1.1 Introduction
Rocket-triggered lightning is initiated by an upward leader propagating from the
upper end of a vertical grounded wire extended below the charged cloud by a small
rocket. The upward-leader stage, including explosion of the triggering wire and its
replacement by an upward-leader plasma channel, is followed by an initial continuous
current (ICC). ICC has duration of some hundreds of milliseconds and amplitude of some
tens to some thousands of amperes. The upward leader and the ICC constitute the initial
stage (IS) of rocket-triggered lightning. ICC pulses are the current pulses superimposed
on the slowly varying continuous current of the initial stage. After the cessations of the
ICC, one or more downward leader/upward return stroke sequences may occur. The
magnitudes of ICC pulses are smaller than those of return-stroke pulses. Overall, the
initial stage in rocket-triggered lightning is apparently similar to that of lightning initiated
from tall structures (e.g., Rakov, 1999). This chapter analyzes the action integral (energy
per unit resistance at the channel termination point) and other characteristics of the ICC
pulses, including duration, rise-time, pulse peak, half-peak width, and charge in rocket-
triggered lightning The following comparisons are made: (a) ICC pulses in triggered
lightning recorded at the ICLRT in 2002 and 2003 (relatively high sampling rate) vs.
their counterparts recorded earlier (relatively low sampling rate), (b) ICC pulses in
triggered lightning vs. those in object-initiate lightning, and (c) ICC pulses in triggered
89
90
lightning vs. M-component (pulses superimposed on the continuing current that follows
the return strokes) pulses in triggered lightning.
A typical initial stage current in rocket-triggered lightning is shown in Figure 4-1.
RS2
Wire explosion
ICC pulses Initial stage
RS1
Figure 4-1: Flash 03-31, bipolar flash. The negative initial stage is followed by 2 return strokes (RS1 and RS2) of opposite polarity. In the inset the instant at which wire explosion takes place is shown.
In Figure 4-1, the initial stage contains a number of ICC pulses superimposed on a
slowly varying current waveform. Figure 4-2 illustrates ICC pulses with relatively short
(ICC1 and ICC2) and relatively long (ICC3 and ICC4) risetimes, as well as a typical
return-stroke pulse (RS1). Definitions of the characteristics of ICC pulses analyzed here
are illustrated in Figure 4-3.
91
ICC4 ICC3
ICC2
ICC1
RS1
Figure 4-2: Flash 03-31. Comparison of ICC pulses with relatively short (ICC1 and ICC2) and relatively long (ICC3 and ICC4) risetimes and a typical return-stroke pulse (RS1).
While computing the charge and action integral for the ICC pulses, the background,
slowly-varying current is approximated by an imaginary line (dashed line in Figure 4-4),
the background is subtracted, and charge and action integral are computed using the
modified current pulse shown in the inset of Figure 4-4.
92
4.1.2 Statistical Characteristics of ICC Pulses
The ICC pulses analyzed here occurred in 11 flashes triggered at the ICLRT at
Camp Blanding, Florida, in 2002 and 2003. Histograms of the distributions of the
parameters are shown in Figures 4-5 to 4-22. These are compared with statistics, found in
Miki et al. (2004), of the ICC pulses in rocket-triggered lightning analyzed previously
and ICC pulses in object-initiated lightning derived from current measurements on 1) the
Gaisberg tower (100 m, Austria), 2) the Peissenberg tower (160 m, Germany), and 3) the
Fukui chimney (200 m, Japan). Miki et al. (2004) found that the characteristics of ICC
pulses in object-initiated lightning are similar within a factor of two, but differ more
Charge = ∫i(t) dt Action Integral = ∫ i(t)2dt
Flash 03-31
Figure 4-3: Definitions of parameters (peak, duration, rise time, half-peak width; additionally shown is the preceding continuous current level) of ICC pulses. Integration in evaluating charge and action integral is over the duration of the pulse. Peak= 0.31 kA, duration=1.9 ms, risetime=137 µs, half-peak width=0.43 ms, charge = 86 mC, Action integral = 6.1 A2s.
93
Cur
rent
, A
Cur
rent
, A
Flash F029
Time, s
Time, s Figure 4-4: Illustration of the removal of the background continuous current in computing
charge and action integral. The dashed line represents the imaginary line, which is used as reference. The charge and action integral are computed by integrating the modified current (or the square of current) as shown in the inset. In this example, charge = 80 mC, action integral = 7.8 A2s.
significantly from their counterparts in triggered lightning. The triggered-lightning data
analyzed by Miki et al. (2004) were acquired at the ICLRT in 1996, 1997, 1999, and
2000. The ICC pulses in object-initiated lightning exhibit larger peaks, shorter rise times,
and shorter half-peak widths than do the ICC pulses in 1996, 1997, 1999, and 2000
rocket-triggered lightning. The rocket-triggered lightning currents were recorded on a
magnetic tape and later digitized. The sampling interval was 40 µs for the years 1996 and
1997, 80 µs for the year 1999, and few microseconds for 2000. In contrast, sampling
intervals for the years 2002 and 2003 considered here were 1 µs and 0.5 µs, respectively.
The original objective of the analysis whose results are presented in Figure 4-5 through
94
Figure 4-22 was to examine the dependence of the statistics of the parameters of ICC
pulses in rocket-triggered lightning on the sampling interval. This is particularly
important for the rise time that can be smaller than the sampling intervals used in 1996
and 1997 (40 µs) and 1999 (80 µs).
GM=232 A Max=1076 A Min=6.3 A Sample size=66
0 32 64 128 256 512 1024 2048 Pulse Peak, A
18 16
14 12
10 N
umbe
r
8 4 2 0
6
Figure 4-5: Histograms of the peak of the ICC pulses for 2002.
95
GM=76.8 A Max=490.6 A Min=17.7 A Sample size=50
0 32 64 128 256 512 1024 2048
Num
ber
18 16 10 8
14 12
4
6
2 0 Pulse Peak, A
Figure 4-6: Histograms of the peak of ICC pulses for 2003.
5
25 20 15 10
GM=144.1 A Max=2082 A Min=6.3 A Sample size=116
Num
ber
2002 2003
0 32 64 128 256 512 1024 2048 0
Pulse Peak, A
Figure 4-7: Histogram of the peak of ICC pulses for 2002 and 2003.
96
GM=3.7 ms Max=16.3 ms Min=0.8 ms Sample size=66
0.5 1 2 4 8 16 32
25 20 15 10 5 0
Duration, ms
Num
ber
Figure 4-8: Histogram of the duration of ICC pulses for 2002.
30 GM=4.6 ms Max=15.4 ms Min=0.76 ms Sample size=50
Num
ber
25 20 15 10 5 0
F 0.5 1 2 4 8 16 32 Duration, ms
Figure 4-9: Histogram of the duration of ICC pulses for 2003.
97
2002 2003
GM=4.1 ms Max=16.3 ms Min=0.76 ms Sample size=116
0.5 1 2 4 8 16 32
50 45 40 35 30 25 20 15 10 5 0
Duration, ms
Num
ber
Figure 4-10: Histogram of the duration of ICC pulses for 2002 and 2003.
Risetime, µs
GM=0.36 ms Max=2.1 ms Min=0.04 ms Sample size=66
4
Num
ber
20 18 16 14 12 10 8 6 4 2 0
8 16 32 64 128 256 512 1024 2048 4096
Figure 4-11: Histogram of the risetime of ICC pulses for 2002.
98
Num
ber
20 18 16 14 12 10 8 6 4 2 0
GM=0.46 ms Max=2.9 ms Min=0.06 ms Sample size=50
Risetime,µs 8 16 32 64 128 256 512 1024 2048 4096 4
Figure 4-12: Histogram of the risetime of ICC pulses for 2003.
4
5
10
0
40 35 30 25 20
15
GM=0.40 ms Max=2.9 ms Min=0.04 ms Sample size=116
2002 2003
Num
ber
Risetime, µs 8 16 32 64 128 256 512 1024 2048 4096
Figure 4-13: Histogram of the risetime of ICC pulses for 2002 and 2003.
99
GM=0.83 ms Max=4 ms Min=0.18 ms Sample size=66
16 32 64 128 256 512 1024 2048 4096 8192
25 20 15 10 5 0
Num
ber
Half-peak width, µs
Figure 4-14: Histogram of the half-peak width of ICC pulses for 2002.
Half-peak width, µs
GM=1.4 ms Max=5.4 ms Min=0.26 ms Sample size=50
16 32 64 128 256 512 1024 2048 4096 8192
25 20 15 10 5 0
Num
ber
Figure 4-15: Histogram of the half-peak width of ICC pulses for 2003.
100
Half-peak width, µs
GM=0.98 ms Max=5.4 ms Min=0.18 ms Sample size=116
2002 2003
16 32 64 128 256 512 1024 2048 4096 8192Half-peak width, µs
Num
ber
40 35 30 25 20
15 10 5 0
Figure 4-16: Histogram of the half-peak width of ICC pulses for years 2002 and 2003.
Charge, mC
Num
ber
20 18 16 14 12 10 8 6 4 2 0
0 50 100 150 200 250 300 350 400 450 500 550
GM=100 mC Max=550 mC Min=4 mC Sample size=66
Figure 4-17: Histogram of the charge of ICC pulses for 2002.
101
GM=90 mC Max=450 mC Min=10 mC Sample size=50
Num
ber
0 50 100 150 200 250 300 350 400 450 500 550 Charge, mC
Figure 4-18: Histogram of the charge of ICC pulses for 2003.
GM=96 mC Max=550 mC Min=4 mC Sample size=116
2002 2003
Num
ber
Figure 4-19: Histogram of the charge of ICC pulses for years 2002 and 2003.
0 50 100 150 200 250 300 350 400 450 500 550 Charge, mC
102
GM=15.2 A2s Max=378.8 A2s Min= 0.08 A2s Sample size=66
Num
ber
0 50 100 150 200 250 300 350 400
Action Integral, A2sFigure 4-20: Histogram of the action integral of ICC pulses for years 2002.
GM=4.5 A2s Max=63.1 A2s Min= 0.11 A2s Sample size=50
Num
ber
0 50 100 150 200 250 300 350 400
Action Integral, A2s Figure 4-21: Histogram of the action integral of ICC pulses for years 2003.
103
GM=9.0 A2s Max=378.8 A2s Min= 0.08 A2s Sample size=116
2002 2003
Num
ber
Action Integral, A2s 0 50 100 150 200 250 300 350 400
Figure 4-22: Histogram of the action integral of ICC pulses for years 2002 and 2003.
Histograms of the peak, duration, rise time and, half-peak width of ICC pulses in
lightning triggered from tall objects and in rocket-triggered lightning reported by Miki et
al., (2004) are shown in Figures 4-23 to 4-26. They did not present any charge and action
integral statistics for ICC pulses. As stated earlier, the statistics for the rocket-triggered
lightning were obtained from experiments conducted at the ICLRT in 1996, 1997, 1999,
and 2000.
104
Figure 4-23: Histograms of the peak of ICC pulses. The geometric mean (GM), maximum (MAX), and minimum (min) values are indicated on each histogram. The rocket-triggered lightning data presented in this figure were obtained at the ICLRT at Camp Blanding, Florida, in 1996, 1997, 1999, and 2000.
105
Figure 4-24: Histograms of the duration of ICC pulses. The geometric mean (GM), maximum (MAX), and minimum (min) values are indicated on each histogram. The rocket-triggered lightning data presented in this figure were obtained at the ICLRT at Camp Blanding, Florida, in 1996, 1997, 1999, and 2000.
106
Figure 4-25: Histograms of the risetime of ICC pulses. The geometric mean (GM), maximum (MAX), and minimum (min) values are indicated on each histogram. The rocket-triggered lightning data presented in this figure were obtained at the ICLRT at Camp Blanding, Florida, in 1996, 1997, 1999, and 2000.
107
Table 4-1 summarizes the ICC pulse parameters (geometric mean values) for 2002 and
2003 rocket-triggered lightning experiments and compares those with their counterparts
for 1996, 1997, 1999, and 2000, analyzed by Miki et al. (2004).
Figure 4-26: Histograms of the half-peak width of ICC pulses. The geometric mean (GM), maximum (MAX), and minimum (min) values are indicated on each histogram.
108
Table 4-1: Summary of parameters (geometric means) of ICC pulses. Type of
Lightning Experimental
Site Source Year
Sample
Size Peak,
A Duration,
ms Risetime, µs
Half-peak
width, us
Charge, mC
Action Integral,
A2s
1996,1997,1999
247-296 113N=296
2.59 N=254
464 N=267
943 N=247
- - Miki et al. (2004)
2000 110 76.6 3.18 517 1079 - -
2002 66 232 3.7 360 800 100 15.2
2003 50 76.8
4.6
460 1400 90 4.5
2002+2003 116 144 4.1 400 1000 96 8.9
Rocket- Triggered Lightning
ICLRT
This Study
1996-2003 473-522 111.4 3.0 461 987 - -
Gaisberg Tower
2000 344 377 1.2 10 276 - -
Peissenberg Tower
1996-1999 124 512 0.83 61 153 - -
Object- Initiated Lightning
Fukui Chimney
Miki et al. (2004)
1996-1999 231 781 0.51 44 141 - -
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As seen in Table 4-1, the geometric means of the parameters of the ICC pulses in
rocket-triggered lightning from 2002 and 2003 are consistent with those for 1996, 1997,
1999, and 2000, suggesting that they are not influenced by different sampling intervals.
ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter
half-peak widths than do the initial-stage pulses in rocket-triggered lightning.
Two possible reasons are proposed in Miki et al. (2004). First, multiple upward
branches could have facilitated the simultaneous occurrence of a continuous current in
one branch and a downward leader in another branch in object-initiated flashes, as
observed for Monte San Salvatore and Ostankino tower flashes (Berger, 1967; Gorin et
al., 1975). Second, the charge sources for initial-stage current in thunderclouds over the
tall objects in Austria, Germany, and Japan might be located closer to the lightning
attachment point than the sources of initial-stage current pulses in Florida. Hence,
because of the shorter propagation path between the in-cloud source and the lightning
attachment point, the fronts of the downward-propagating current waves in object-
initiated flashes might have suffered less degradation due to dispersion and attenuation
than their counterparts in Florida rocket-triggered flashes. Table 4-2 which compares the
parameters of ICC pulses in Gaisberg tower flashes in winter and summer supports the
latter hypothesis. It is known that the cloud charge sources in winter are lower than those
in summer.
Hence, it is expected that the ICC pulses in winter flashes should exhibit larger
peaks, shorter rise times, and shorter half-peak widths (HPW) than the ICC pulses in
summer flashes. As seen in Table 4-2, all parameters except for the risetime are similar.
More data are needed to arrive at a more decisive conclusion. Charge and action integral
110
for ICC pulses are considerably smaller than their counterparts for return strokes (e.g.,
Rakov, 1999).
Table 4-2: Parameters of ICC pulses in Gaisberg tower flashes as a function of season. Adapted from Miki et al. (2004)
Season Sample size Peak, A Duration, ms Risetime, µs HPW, µs
Winter 36 319 1.03 74.9 298
Summer 38 368 1.25 134 269
4.2 M-Components
4.2.1 Introduction
M-components are impulsive processes that occur during the continuing current
following the return strokes. In this chapter, statistics are compiled for the following
parameters of the M-component pulse: magnitude, rise time, duration, half-peak width,
charge, and action integral. The purpose of this is to compare these statistics to their
counterparts for the ICC pulses occurring during the initial stage. The same triggered-
lightning current records as in Section 4.1 are used here. More information on the return
stroke and M-component pulses can be found in Fisher et al. (1993) and Rakov and
Uman (2003).
4.2.2 Statistical Characteristics of M-Components
An example of a triggered-lightning flash, which contains the initial stage, return
strokes, and M-components, is shown in Figure 4-27. Definitions of the characteristics of
M-components analyzed here are illustrated in Figure 4-28. A total of 72 M-components
in 14 flashes triggered at the ICLRT at Camp Blanding, Florida, in 2002 and 2003 are
used for analysis here.
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Initial stage
Cur
rent
, A
Cur
rent
, A
M
RS1
Time, s RS1 RS2 RS3
Time, s Figure 4-27: Flash F0213. The initial stage is followed by 3 return strokes (RS1, RS2 and
RS3). In the inset, 3 M-components (labeled M) following return stroke RS1 are shown.
The histograms of duration, half peak width, peak and risetime of the M-
components are shown in Figures 4-29 to 4-34. Table 4-3 compares the characteristics of
M-components obtained at ICLRT, NASA Kennedy Space Center, Florida, and in
Alabama with those of ICC pulses obtained at ICLRT during 1996 to 2003.
112
Duration
Half-peak width
Puls
e Pe
ak
Cur
rent
, A
Charge = ∫i(t) dt Action Integral = ∫ i(t)2dt
Risetime (10-90%)
Time, ms
Figure 4-28: Flash F0213. Definitions of parameters (peak, duration, rise time, half-peak width) of M-components. Peak= 70.3 A, duration=15.2 ms, rise time=0.9 ms, half-peak width=1.8 ms.
0 2 4 6 8 10 12 14 16 18 Duration, ms
Num
ber
30 25 20 15 10 5 0
2002 GM = 2.2ms N =32 2003 GM = 2.4 ms N = 40
GM = 2.3 ms Min = 0.06 ms Max = 17.2 ms N = 72
Figure 4-29: Histogram of duration of M-component pulse.
113
0 16 32 64 128 256 512 1024 2048 4096
Num
ber
14 12 10 8 6 4 2 0
2002 GM = 198 A N = 32
2003 GM = 125 A N = 40 GM = 154 A Min = 10.8 A Max = 3578 A N = 72
Peak, kA Figure 4-30: Histogram of peak of M-component pulse.
Num
ber
10 8
16 14 12
6 4 2 0
0 64 128 256 512 1024 2048 4096 Risetime, µs
2002 GM = 220.3 µs N =32 2003 GM = 252 µs N = 40
GM = 237 µs Min = 23 µs Max = 2640 µs N = 72
Figure 4-31: Histogram of risetime of M-component pulse.
114
0 128 512 1024 2048 4096 Half-peak width, µs
Num
ber
35 30 25 20 15 10 5 0
2002 GM = 426 µs N =32 2003 GM = 452 µs N = 40 GM = 440 µs
Min = 44 µs Max = 3600 µs N = 72
Figure 4-32: Histogram of the half-peak width of M-component pulse.
0 100 200 300 400 500 600 700 800 900 1000 Charge, mC
35 30 25 20 15 10 5 0
Num
ber
30 2002 GM =70 mC N = 32
2003 GM = 117 mC N = 40
GM =91.5 mC Min = 1 mC Max = 909 mC N =72
Figure 4-33: Histogram of the charge of M-component pulse.
115
0 200 400 600 800 1000 1200 1400 1600 1800 2000 Action Integral, A2s
Num
ber
70 60 50 40 30 20 10 0
2002 GM = 11.0 A2s N = 32
2003 GM =13.0 A2s N = 40
GM = 12.1 A2s Min = 0.03 A2s Max = 1940 A2s N = 72
Figure 4-34: Action integral of M-component pulse.
The geometric means of M-component current peak and duration obtained here are
similar to those previously reported by Thottappillil et al. (1995). The geometric means
of risetime and half-peak width are smaller by a factor of about two compared to those
obtained by Thottappillil et al. (1995). The probable reason for this discrepancy might be
the fact that in Thottappillil et al. (1995), some overlapping M components which do not
allow unambiguous measurement of such parameters as risetime, duration, and half-peak
width were not used while compiling the statistics. These overlapping M components
usually occur during the first 5 ms following the beginning of the return stroke and have a
faster rise time. Added to this, sample sizes are very small compared to those used in
116
Thottappillil et al. (1995). Table 4-3 gives the statistics of the characteristics of M
components along with those of ICC pulses.
117
Table 4-3: Geometric means of the various parameters of M-components and ICC pulses. M- components
Experimental Site Source Year
Sample
Size Peak,
A Duration,
ms Risetime,µs
Half-Peak
width, us
Charge, mC
Action Integral,
A2s
Lightning triggering
sites in Florida and Alabama
Thottappillilet al. (1995) 1990,1991 113-
124 117
N=24 2.10
N=114 422
N=124 800
N=113129
N=104 -
2002 32 198 2.2 220 426 70 112003 40 125 2.4 252 452 117 13ICLRT This
Study 2002-2003 72 154 2.3 237 440 92 12
ICC pulses
1996, 97,99
247-296
113 N=296
2.6 N=254
464 N=267
943 N=247 - -
2000 110 76.6 3.2 517 1079 - -
Miki et al. (2004)
2002 66 232 3.7 360 800 100 15.22003 50 76.8 4.6 460 1400 90 4.52002-2003 116 144 4.1 400 1000 96 9.0
ICLRT
This study (Table 4.1) 1996-
2003 473-522 111 3.0 461 987 - -
118
As seen Table 4-3, the duration, risetime, and half-peak width of the ICC pulses are
approximately twice those of M-components, suggesting a frequent occurrence of slower
pulses in the initial stage compared to the pulses superimposed on the continuing currents
following return strokes. The charge and action integral of M-components and ICC
pulses are similar.
CHAPTER 5 RECOMMENDATIONS FOR FUTURE RESEARCH
1. The frequency range of the Pockels sensor measuring system used in 2000 was relatively narrow, from 50 Hz to 1 MHz. This led to underestimation of the peaks of the fast electric field waveforms by about 40%. Pockels sensors with higher upper frequency response should be used to overcome this limitation.
2. The sampling interval of the vertical electric field was 0.5 µs compared to 20 ns of the high current records. As seen in Section 3.4.4, peak power estimates are sensitive to the uncertainties related to the relatively large electric field sampling interval. Hence, smaller sampling intervals (higher sampling rates) should be used in future experiments to get more accurate estimates of the peak power.
3. The high-current record length was 50 µs. Longer current records are needed to obtain the power and energy curves for later times.
4. The estimation of power and energy in this study assumes that the lightning channel is vertical, but in reality the lightning channel is tortuous and drifts because of wind. Hence, optical sensors should be placed around the strike rod to identify those flashes for which the channel is relatively straight and vertical, so that more accurate peak power and energy estimates could be obtained.
5. Along with the vertical electric fields, horizontal electric fields were also measured for eight strokes. Unfortunately, the unavailability of high-current records for those flashes prevented the estimation of the Poynting vector associated with the upward-moving wave. It would be interesting to obtain such estimates in the future.
6. Electric and magnetic field measurements at 15 and 30 m should be used to compute the electromagnetic power and energy radiated from the channel for comparison with the input power and energy.
7. Sample sizes in this study were rather small. Additional measurements are needed to obtain a larger sample that would allow one to draw more statistically significant conclusions.
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BIOGRAPHICAL SKETCH
Vinod Jayakumar was born in Vellore, India, in 1980. He graduated with a
bachelor’s degree in electronics and communications from P.S.G college of technology at
Coimbatore, Tamil Nadu, India in 2002. In 2002, he went to the USA to pursue graduate
studies at the University of Florida.
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