Download - Infrasound from lightning
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Infrasound from lightning
Jelle Assink and Läslo Evers
Royal Netherlands Meteorological Institute Seismology Division
ITW 2007, Tokyo, Japan
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Low Frequency Array
• Astronomical initiative • Infrastructure ao. power, internet, computing and backup facilities• Dense (international) coverage
• Geophysical sensor network• Combined seismic/infrasound recording
LOFAR
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LOFAR
Objectives
• Source identification through association• Atmospheric contribution to seismic noise• Seismo-acoustics by simultaneous observations• Local noise characterization
Practicalities• Adapt KNMI microbarometer for periods up to1000 s• Construct Very Large Aperture Infrasound Array 30 KNMI-mb’s at 1 to 10s of km
• Develop low cost infrasound sensor• Construct High Density Infrasound Array 80 sensor in 100x100 meter field
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Cabauw Infrasound Array
• Combined meteo and infrasound project• Cabauw site: 215 m meteo tower• 3D sensing of the boundary layer
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Objectives
• Detect gravity waves and other atmospheric phenomena• Applying infrasound technique to non-acoustic velocities• Relation between state of the boundary layer and infrasonic signal characteristics• 3D acoustical array for signal characterization as function of height
50 km
Source: NASA
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Objectives• Detectability lightning discharges with infrasound
– To which extent– Distinction CC/CG– Source localization
• Content and behavior of related infrasound• Possible source-mechanisms• Wave propagation paths through atmosphere
• Comparison and verification KNMI lightning detection network based on EM (‘FLITS’)
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Source mechanisms
• Few (1969): thermally driven expanding channel model, blast wave
• Bowman and Bedard (1971): convective system as a whole, vortices, mass displacement
• Dessler (1973): electrostatic mechanism, reordering of charges within clouds
• Liszka (2004): transient luminous events, such as sprites
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Electromagnetic detectionKNMI FLITS network
LF antenna (around 4 MHz)
VHF array (around 110 MHz)
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Electromagnetic detection
• FLITS: Flash Localisation by Interferometry and Time of Arrival System
• LF Antenna: Time-of-Arrival– Detection and localization– Discrimination CC/CG
• VHF array: interferometry– Detection and localization
• A minimum of 4 stations for unambiguous detections
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Infrasound detection
KNMI IS network
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Electromagnetic detections
at 01-10-2006
CC
CG
Cloud-to-Clouddischarge
Cloud-to-Grounddischarge
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Infrasound & FLITS detections at DBN for 1-10-2006
CGCC
High F ISLow F IS
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All-day observation summary• Correlation in time
between (nearby) discharges and coherent infrasound detections
• Nearby discharges:– High app. velocity– High amplitude– Coherent energy
over infrasound frequency band
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Raw data
Time(s)
Pre
ssu
re(P
a)
Unfiltered data, strong front nose
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Filtered data
Time(s)
Pre
ssu
re(P
a)
Bandpass 1-10 Hz, variety of impulsive events
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Filtered data
Time(s)
Pre
ssu
re(P
a)
Bandpass 1-10 Hz, blast waves
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Atmospheric attenuationInfrasound amplitude vs. distance from array
– Normalized for discharge size– Empirical attenuation relation: exponentially decaying?
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Atmospheric attenuation
Log-log presentation
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Atmospheric attenuationPower coefficient = 1 for cylindrical spreading
= 2 for spherical spreading
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Conclusions
• CG discharges can be detected over ranges of 50 km, CC much harder to identify
• Thermally driven expanding channel model seems feasible, correlation with blast waves
• Small arrays needed for detection, 25-100 meters inter-station distance
• Attenuation: near-field infrasound indication for point source far-field cylindrical spreading
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Detection and parameter estimation results
Either high apparent velocity and large azimuthal deviation or low apparent velocity and small azimuthal deviation
What propagation path allows 0.36 km/s?
Non-tropospheric velocity of 420 m/s between DBN and DIA
Head wave like propagation in high velocity acoustic channel
Strong winds cause high propagation velocity, large azimuthal deviations and steep incident angles
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Raytracing with NRL-G2S models