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

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 detection KNMI 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 CC CG Cloud-to-Cloud discharge Cloud-to-Ground discharge

Infrasound & FLITS detections at DBN for CG CC High F IS Low 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) Pressure(Pa) Unfiltered data, strong front nose

Filtered data Time(s) Pressure(Pa) Bandpass 1-10 Hz, variety of impulsive events

Filtered data Time(s) Pressure(Pa) Bandpass 1-10 Hz, blast waves

Atmospheric attenuation Infrasound amplitude vs. distance from array –Normalized for discharge size –Empirical attenuation relation: exponentially decaying?

Atmospheric attenuation Log-log presentation

Atmospheric attenuation Power 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, 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