Locating Initial Breakdown Pulsesof Lightning Flashes
Sumedhe Karunarathne
20th April 2010
Physics and Astronomy - Atmospheric Physics
1
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Outline…
Background Sensors and sample data Time of arrival method Results, Comparison and Error results Future Plans.
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Charge Structure of Thunder-Cloud
•Lower positive charge is not present always.•Two main charges produce a positive-dipole
BACKGROUND
Upper Positive(40-400C)
Main Negative(-40-400C)
Lower Positive (3 -30C)
+
-
+
~2km~7km~12km
4
Types of Lightning
(1) Downward Negative
(2) Upward Positive
(3) Downward Positive
(4) Upward Negative
BACKGROUND
90%
Less than 10%
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Figure 4.3 of uman
Negative CG Flash
- Uman (1987 – 2001)
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Video…
http://www.youtube.com/watch?v=djCdfSl-5o8&feature=related
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Electric Field Change Meter~1.5ft
~1.5ft
Properties• Time Constant : 10s 1s 100us• Band Width : ~0Hz – 0.5Mhz• Sampling Rates: 1MHz triggered
100kHz Continuous•Resolution : 12bit
Block Diagram
Plate
Charge Amplifier
Follower
GPS
Battery Power (~11 hours)
10Ms/seconddigitizer
1s Time
Accuracy
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2010 Summer data collection Kennedy Space Center (KSC)
•5 Stations•Each station have 3 sensors(10S, 1S and 100S antennas)
• Covered about 25x40km area roughly.
•Purpose-Study the beginning of the lightning flashes and lightning propagation
S1
S2
S3
Origin
S4
S5
~20km
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What we see….
• Sensor sees the electric field change in the environment.• There are 3 major components we see as the field change
])/,(
.sin
)/,(.sin32
)/,(.sin32
[2
1),(
2
1
2
1
2
1
2
2
2
2
03
2
0
dzt
cRtzi
Rc
dzcRtzicR
dzdcRziR
tDE
H
H
H
H
tH
H
-Uman et al., 1975-Watson and Marshall , 2007
1. Electrostatic (1/R3) 2. Induction (1/R2)3. Radiation (1/R)
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What we see at 2.8km…
For closer flashes we see,• More from electrostatic and induction
terms• Less from radiation field
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What we see at 200km…
For far flashes we see,• Electrostatic and induction terms are negligible• Most of the filed change are represented by
radiation field
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Sample Data – CG flash overall
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Sample Data – CG flash Beginning
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Time of arrival
v
Rtt i
i
||
(x,y,z,t)
ith sensor
ti
Ri
v
zzyyxxt
iii
222 )()()(
Excited time of a sensor due to pulse at the location (x,y,z),
•(xi,yi,zi,ti) known•(x,y,z,ti ) unknown•v speed of light in air - known
Assumption:Ignore the refractive index effect(EM wave travel in a straight line with a constant speed)Named PBFA (Position by Fast Antenna)
• 4 sensors are enough to find x,y,z and t.• 5th sensor can be use to validate the solution
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LINET and LDAR2
S1S2
S3LDAR2 Origin
S4S5
L1
L2
L3
L6
L4
L5
L7
LINET LDAR2
LIghtning NETwork Lightning Detection And Ranging Array
VLF/LF Band (5-200kHz) VHF Narrow Band (64-66Mhz)
See long (~500 m) pulses well See short pulses (~5 m) well
Temporary mounted for 2010 summer Permanently working at KSC
7 stations 9 stations
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Results
LINET
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Results
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Errors Summery•X,Y sigma ~ ±100- 500m•Z sigma ~ ±500m – 4km•t sigma ~ ±1s - 20s.
Critical Factors for betterTriangulations•Time resolution•Sensor placement•Flash location
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Summery
• Finding pulse position using this method is strongly dependent onSensor Placement, Location of the flash, and Time resolution ofantenna.
• Since we have 5 sensors and only 4 sensors are needed to find thepulses location (x, y, x, t), the other sensor can be used to checkthe solution.
• Although the statistical error analysis gives relatively largestandard deviation, the results are reasonably consistent withLDAR2 and LINET data.
• Unlike LDAR and LINET, the PBFA method was able to find thelocation of the beginning pulses. We plan to use this informationto help determine the physics of initial lightning propagation.
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Future Plans1. Improved sensors
2009/2010 sensors 2011 sensors
GPS time 1uS accuracy 5nS accuracy
Resolution 12-14bit 12bit
Sampling rate 1MHz 10MHz
Number of stations 5 (5 Calculations) 9 (126 calculations)
Downloading data Manually Automatically to a server at UOM
Position Finding Manual post process Automatic Real Time
2. 2011 Summer 1 data CollectionPlace – Rapid City – South Dakota Purpose – Study lightning from towers
compare with high speed video data
3. 2011 Summer 2 data collectionPlace – KSC - FloridaPurpose – Study the beginning pulses and structure of lightning
Compare with high-speed video data
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Thank you!
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Results