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Infrasound from lightning. Jelle Assink and Läslo Evers Royal Netherlands Meteorological Institute Seismology Division. ITW 2007, Tokyo, Japan. Lo w F requency Ar ray. Astronomical initiative Infrastructure ao. power, internet, computing and backup facilities

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Infrasound from lightning

Infrasound from lightning

Jelle Assink and Läslo Evers

Royal Netherlands Meteorological Institute

Seismology Division

ITW 2007, Tokyo, Japan


Lo w f requency ar ray
Low Frequency Array

  • Astronomical initiative

  • Infrastructure ao. power, internet,

  • computing and backup facilities

  • Dense (international) coverage

  • Geophysical sensor network

  • Combined seismic/infrasound

  • recording

LOFAR


Objectives
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

LOFAR


C abauw i nfrasound a rray
Cabauw Infrasound Array

  • Combined meteo and

  • infrasound project

  • Cabauw site: 215 m meteo

  • tower

  • 3D sensing of the boundary

  • layer


Objectives1
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


Objectives2
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’)


Source mechanisms
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


Electromagnetic detection

LF antenna (around 4 MHz)

VHF array (around 110 MHz)

Electromagnetic detection

KNMI FLITS network


Electromagnetic detection1
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


Infrasound detection
Infrasound detection

KNMI IS network


Electromagnetic detections
Electromagnetic detections

at 01-10-2006

Cloud-to-Cloud

discharge

CC

Cloud-to-Ground

discharge

CG


Infrasound flits detections at dbn for 1 10 2006

CG

CC

High F IS

Low F IS

Infrasound & FLITS detections at DBN for 1-10-2006


All day observation summary
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


Raw data

Unfiltered data, strong front nose

Raw data

Pressure(Pa)

Time(s)


Filtered data

Bandpass 1-10 Hz, variety of impulsive events

Filtered data

Pressure(Pa)

Time(s)


Filtered data1

Bandpass 1-10 Hz, blast waves

Filtered data

Pressure(Pa)

Time(s)


Atmospheric attenuation
Atmospheric attenuation

Infrasound amplitude vs. distance from array

  • Normalized for discharge size

  • Empirical attenuation relation: exponentially decaying?


Atmospheric attenuation1
Atmospheric attenuation

Log-log presentation


Atmospheric attenuation2
Atmospheric attenuation

Power coefficient = 1 for cylindrical spreading

= 2 for spherical spreading


Conclusions
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


Detection and parameter estimation results

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

Detection and parameter estimation results

Either high apparent velocity and large azimuthal deviation or low apparent velocity and small azimuthal deviation


Raytracing with nrl g2s models
Raytracing with NRL-G2S models


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