Infrasound from lightning

Jelle Assink and Läslo Evers

Royal Netherlands Meteorological Institute Seismology Division

ITW 2007, Tokyo, Japan

Low Frequency Array

• Astronomical initiative • Infrastructure ao. power, internet, computing and backup facilities• Dense (international) coverage

• Geophysical sensor network• Combined seismic/infrasound recording

LOFAR

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

Cabauw Infrasound Array

• Combined meteo and infrasound project• Cabauw site: 215 m meteo tower• 3D sensing of the boundary layer

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

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

• 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 detectionKNMI FLITS network

LF antenna (around 4 MHz)

VHF array (around 110 MHz)

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

KNMI IS network

Electromagnetic detections

at 01-10-2006

CC

CG

Cloud-to-Clouddischarge

Cloud-to-Grounddischarge

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

CGCC

High F ISLow F IS

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

Time(s)

Pre

ssu

re(P

a)

Unfiltered data, strong front nose

Filtered data

Time(s)

Pre

ssu

re(P

a)

Bandpass 1-10 Hz, variety of impulsive events

Filtered data

Time(s)

Pre

ssu

re(P

a)

Bandpass 1-10 Hz, blast waves

Atmospheric attenuationInfrasound amplitude vs. distance from array

– Normalized for discharge size– Empirical attenuation relation: exponentially decaying?

Atmospheric attenuation

Log-log presentation

Atmospheric attenuationPower coefficient = 1 for cylindrical spreading

= 2 for spherical spreading

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

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

Raytracing with NRL-G2S models