1. Infrasound from lightning Jelle Assink and Läslo Evers Royal Netherlands Meteorological Institute Seismology Division ITW 2007, Tokyo, Japan

2. Low Frequency Array Astronomical initiative Infrastructure ao. power, internet, computing and backup facilities Dense (international) coverage Geophysical sensor network Combined seismic/infrasound recording LOFAR

3. 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

4. Cabauw Infrasound Array Combined meteo and infrasound project Cabauw site: 215 m meteo tower 3D sensing of the boundary layer

5. 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

6. 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’)

7. 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

8. Electromagnetic detection KNMI FLITS network LF antenna (around 4 MHz) VHF array (around 110 MHz)

9. 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

10. Infrasound detection KNMI IS network

11. Electromagnetic detections at 01-10-2006 CC CG Cloud-to-Cloud discharge Cloud-to-Ground discharge

12. Infrasound & FLITS detections at DBN for 1-10-2006 CG CC High F IS Low F IS

13. 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

14. Raw data Time(s) Pressure(Pa) Unfiltered data, strong front nose

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

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

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

18. Atmospheric attenuation Log-log presentation

19. Atmospheric attenuation Power coefficient= 1 for cylindrical spreading = 2 for spherical spreading

20. 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

21. 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

22. Raytracing with NRL-G2S models