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A Canadian program for dedicated monitoring of meteor generated infrasound

A Canadian program for dedicated monitoring of meteor generated infrasound. Wayne N. Edwards, Peter G. Brown Department of Physics and Astronomy, University of Western Ontario. 2008 Infrasound Technology Workshop, Bermuda. SOMN & ELFO. Proposed/In Progress. Certified - Online.

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A Canadian program for dedicated monitoring of meteor generated infrasound

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  1. A Canadian program for dedicated monitoring of meteor generated infrasound Wayne N. Edwards, Peter G. Brown Department of Physics and Astronomy, University of Western Ontario 2008 Infrasound Technology Workshop, Bermuda

  2. SOMN & ELFO Proposed/In Progress Certified - Online Elginfield Infrasound (ELFO) Where is this?

  3. Southern Ontario Meteor Network • SOMN: A multi-sensor network in Southern Ontario, Canada with the purpose of correlating observations of bright meteors across multiple technologies. • Canadian Meteor Orbit Radar ( CMOR ) • All-Sky Camera Network ( ASGARD ) • Elginfield Infrasound Array ( ELFO ) • POLARIS Seismic station ( ELFO ) • Canadian Automated Meteor Observatory ( CAMO ) • High speed radiometers  November 2008 • VLF Interferometer  Winter 2008/09 • Multi-band high speed radiometers  In development • Scientific Goals : • Calibrate relative meteoroid mass / energy scales across instruments • Determine flux of smaller near-Earth objects • Study ablation behaviour in detail: i.e. ionization, luminous, acoustic efficiencies as proxy for meteoroid physical structure • Provide observational constraints for numerical entry models • Better understand the observational biases in various meteor observations

  4. All-Sky Cameras • Currently a Network of 7 All-sky Cameras • University of Western Ontario (01) • Elginfield Observatory (02) • McMaster University (03) • CMOR: Tavistock, Ont. (04) • RASC: Collingwood, Ont. (05) • Robo-Sky: Orangeville, Ont. (06) • Kincardine, Ont. (07) • Low light cameras with “walleye” lenses. • GPS time synchronization. • Autonomous detection/storage/analysis • Daily central storage and ID at UWO • Provides an optical trigger for simultaneous meteor observations across the SOMN sensor suite. • CMOR – Meteor Patrol Radar • ELFO – Infrasound/Seismic

  5. ELFO With a ~200 km wide range for observing meteor infrasound directly, the ELFO infrasound array covers a comparable region to that of CMOR. The SOMN All-sky camera network has been distributed to visually cover the same region. Thus both CMOR & the cameras may provide triggers for infrasound searches With a ~200 km wide range for observing meteor infrasound directly, the ELFO infrasound array covers a comparable region to that of CMOR. The SOMN All-sky camera network has been distributed to visually cover the same region. Thus both CMOR & the cameras may provide triggers for infrasound searches 50 km Georgian Bay Lake Huron 5 7 6 CMOR 4 2 3 Michigan 1 New York Lake Erie Pennsylvania Ohio

  6. ELFO Seismic All-Sky Cam #2 2 ELFO HQ 4 1 3 250 m Elginfield Observatory ELFO Infrasonic Array

  7. Elginfield Infrasound Array (ELFO) Model 2.5

  8. What are we looking for? During entry, the meteoroid produces a hypervelocity ballistic shock similar to the sonic boom of a supersonic aircraft. Characteristics of this shock is related to the meteoroid’s SPEED and PHYSICAL SIZE. Observing this infrasound provides a means to determine meteoroid mass & kinetic energy

  9. Simplified Source Geometry: Blast Radius p RO Eo V This disturbance propagates with approx. cylindrical symmetry as wave period lengthens and amplitude attenuates (ReVelle 1974, 1976)

  10. Typical Form of Meteor Ballistic Wave Overpressure - Δp (Amplitude of wave) Pressure time Dominant Period - Negative Phase (suction/rarefaction) Ballistic Wave NOTE: More complicated waveforms are also possible: e.g. fragmentation

  11. The BIG vs. the small • Infrasound from large 1 – 10m sized bodies are relatively “common”, having been observed for more than a century. • Large masses, energetic, terminate at low altitude, very low frequencies produced  world wide propagation (most recent: 2008-TC3 – Sudan) • Flux rate: 1m sized meteoroids impact 1-2/month • Flux rate: 10m meteoroid sizes: ~1/decade • Flux rate: Tunguska (30-50m): ~1 every 500 – 1000 yrs. • In contrast, smaller, centimeter sized bodies are far more common, yet infrasound from these bodies has been sparse since investigation began ~30 years ago. • Flux rate: 10cm sizes: ~1 every 30 minutes • Flux rate: 1cm sizes: ~1 every 3 seconds • If only a fraction of these cm-sized bodies produce infrasound, meteor infrasound is far more plentiful than is observed. • So how do we observe it?

  12. The Detection Process • Using the All-Sky cameras, meteors are detected using custom motion detection software • All-Sky and Guided Automated Realtime Detection: ASGARD • Meteors are reduced and trajectories, speeds, and photometric masses determined. • Infrasound at ELFO from tobs to tobs + 15min. is inspected. • When possible, suspect detections (azimuth/trace velocity/celerity) are checked against ELFO onsite camera • Atmospheric conditions are reconstructed from UK Met Office (UARS), CMOR temp./wind measurements, and MSIS-E00 / HWM93 models. • Ray Tracing from measured trajectory to ELFO is compared to observed azimuths/trace velocity to confirm meteor source. • Only those signals surviving this process are “confirmed” meteor infrasound.

  13. The All-Sky Network Process ! ! Detection Video frames & time logged @ Set Time Upload detections to Main Server via internet Main server correlates individual camera events via observation times. EMAIL Total detections 35 Meteor detected 03:42:10 UT Cam 01, 02 Events and statistics of night’s observations are provided to user via e-mail. Requests each camera for Multi-station event raw frame data

  14. Mars 20060419c Jupiter 1-2. Meteor Identification-Reduction Camera #2 - ELFO Camera #1 - CMOR Camera #1 - UWO SOMN# 20060419c Start: 42.7396°N 81.2206°Wat 73.15 km End: 42.6950°N 80.7976°W at 48.69 km Velocity: 14.21 ± 0.07 km/s Trajectory Azimuth: 278.3° ± 0.3° Trajectory Elevation: 34.5° ± 0.4° Range to ELFO: 88.3 – 84.1 km Photo. Mass: 135g ± 74g Approx. dia: 4 cm Common Apollo-type asteroidal orbit

  15. 3-4. ELFO Candidate Infrasound Search • 20060419c • Arrive: 07:10:34.8 UT • Time delay: 4m 37.8 s • Azimuth: 144.5° • Trace Vel.: 0.420 km/s • Duration: ~0.5 sec • Δp = 0.137 ± 0.048 Pa • p2p = 0.209 ± 0.075 Pa • τ = 0.113 ± 0.031 sec • F = 9.3 ± 2.0 Hz Short duration, ballistic-looking waveform (Classic Meteor Infrasound) – 0.5 to 30 Hz

  16. 4-7. Confirmation/Source Altitude • Camera #2 Observed Azimuth: 149.1° ELFO Observed azimuth: 144.5° • Raytracing results show that infrasound from measured meteor trajectory is possible at the time and direction observed Meteor infrasound confirmed • Source Altitude Determination: Best fit based on computed az/elev/t.time • Source Height (55.6 ± 4.1 km) • Travel Time residual: 0.46 s, Azimuth missed by: 0.015°, Elevation missed by: 0.026° (Raytracing performed by InfraMap, & SUPRACENTER)

  17. Current Detections since Jan ’06 – Aug ‘08 From 1908-2000 the # of confirmed instances of <10cm meteor infrasound was: 1 Currently meteor infrasound from cm-sized meteoroids are being detected at a rate of ~1/month by SOMN. 23 – Confirmed 12 – Probable (TBC) 1 – Unconfirmed ---------------------------- 36 Total, and counting …

  18. V NON-BALLISTIC NON-BALLISTIC QUASI-BALLISTIC QUASI-BALLISTIC BALLISTIC Meteor Observational Geometry • Once observed, determining the orientation the wave has propagated assists in identifying the generation mechanism • Guided by work of Brown et al. (2007) – European meteor network/I26DE • Ballistic: Cylindrical hypersonic shock wave • Propagation ~perpendicular to trajectory • Non-Ballistic: Typical of point-like sources (e.g. fragmentation) • Omni-directional • Quasi-ballistic: Transitional zone between previous types • May possibly be eliminated once range of ballistic observations is delimited e.g. 20060419c 15° 20° 15°

  19. NON-BALLISTIC QUASI-BALLISTIC BALLISTIC QUASI-BALLISTIC NON-BALLISTIC V Observed Meteor Geometry Bulk of meteor infrasound observations fall into the BALLISTIC wave category, as predicted from cylindrical blast wave theory. (ReVelle 1974, 1976)

  20. 20061104 29.93 km/s 20071004b 16.26 km/s 20070125 68.63 km/s 20060213 12.17 km/s 20071021 68.0 km/s 20070511 64.72 km/s Meteor Ballistic Wave Observations Classical Meteor Ballistic waves “N-waves” Reverbatory-type Ballistic waves “Double/Triple Bangs”

  21. Non-Ballistic wave Observations 20060813 69 km/s 20070102 41 km/s Often seen with fragmenting/flaring meteors, non-ballistic waves often do not display a repeating structure (as in ballistic waves) and so are “unique”. Likely a result of the individuality of the gross fragmentation process.

  22. Quasi-Ballistic wave Observations 20070723 26.1 km/s 20061101 57 km/s As might be expected, quasi-ballistic observations show similarities to both proceeding types. With further quasi-ballistic observations, the ballistic region will become better defined and this category may be revised (eliminated).

  23. Putting Theory to the Test … With the accumulating number of meteor infrasound observations, this has provided a means of testing the theoretical predictions of cylindrical blast wave theory as applied to meteors developed by ReVelle (1974, 1976). We see that currently the growth of ballistic & quasi-ballistic fundamental periods is underestimated, with amplitudes overestimated by factors of 2-3.

  24. Observed Source Altitudes • Source altitude distribution shows peak between 75-85 km • typical of cm-sized meteor end heights • Infrasound produced from ~100 km altitudes is not uncommon! (Brown et al. 2007) • Observed Δp decreases with increasing source altitude • Increased attenuation • Conservation of energy as wave propagates into denser air

  25. Infrasonic Mass & Luminous Efficiency With close agreement between theory & observation, we can use theoretical predictions (constrained by observations) to infer the physical size of the source meteoroid, and assuming a density, its mass! Up to ~100g masses, agreement is good. Thus we can INDEPENDANTLY infer a luminous efficiency ( often only possible through dynamical measurements ).

  26. Rates of Detection & Meteor Infrasound Flux • Back of the envelope (Optimistic): • Current detection rates show that infrasound from cm-sized meteoroids at ELFO occurs at a rate of ~1/month. • Over the same period (01/2006 – 10/2008), SOMN recorded ~2180 meteors • Infrasound producing meteors represent: ~1.65% • ~25% Detection Efficiency rating: • Night observing only, 50% clear nights to see meteors • With ~107 meteors @ 1 cm impacting Earth/year … that’s 661,000 cm-sized meteors producing infrasound/year. • That’s meteor infrasound at the surface every ~48 seconds! • Back of the envelope (Pessimistic): • ELFO covers ~200 km radius area: ~125,700 km2 • ~25% Detection Efficiency • 36 meteors over 34 months • Earth’s total surface area: 510,000,000 km2  ELFO covers 0.024% of Earth’s surface • This means 584,000 meteors at 1 – 10 cm producing infrasound globally over 34 months. • That’s meteor infrasound at the surface every 2min 33 seconds!

  27. Future Work • The observing of meteor infrasound from centimetre sized meteoroids continues … • Better define the limits of ballistic deviations from perpendicular • Determine limit to which meteor infrasound cannot reach the surface • Update of ReVelle (1974,1976) analytical theory using modern acoustic attenuation (e.g. Sutherland and Bass 2003) • Revision to wave period growth  ? source period adjustment ? • Investigation of non-linear to linear wave transitions • Constrain with observation, modern shock experiments • Investigate propagation/attenuation using modern methods • PE propagation models using line source approximation • finite-difference atmospheric hydro-code modelling

  28. Acknowledgements • Data and Funding • National Science and Engineering Research Council ( NSERC ) • United Kingdom Meteorological Office ( UKMO ) • Natural Resources Canada ( NRCan ) • Array Operations etc. • David McCormack, Philip Munro, Catherine Woodgold, Paul Street, Robert Schieman, ( NRCan ) • Data Reduction and Analysis • Douglas ReVelle, Los Alamos National Laboratory ( LANL ) • Robert Weryk, Zybszek Krezminski, Sean Kohut, Elizabeth Silber, Andrew Weatherbee, ( UWO )

  29. Questions?

  30. THE END

  31. Period a strong function of velocity 20071004b 16.26 km/s Ro = 2.4m Observed Periods @ Surface 20060213 12.17 km/s Ro = 4.5m 20071021 68.0 km/s Ro = 5.7m Theoretical period @ source

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