A Canadian program for dedicated monitoring of meteor generated infrasound

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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
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Where is this?
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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

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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

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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
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7
6
CMOR
4
2
3
Michigan
1
New York
Lake Erie
Pennsylvania
Ohio
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ELFO Seismic
All-Sky Cam 2
2
ELFO HQ
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1
3
250 m
Elginfield Observatory
ELFO Infrasonic Array
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Elginfield Infrasound Array (ELFO)
Model 2.5
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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
meteoroids SPEED and PHYSICAL SIZE. Observing
this infrasound provides a means to determine
meteoroid mass kinetic energy
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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)
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Typical Form of Meteor Ballistic Wave
Overpressure - ?p (Amplitude of wave)
Pressure
time
Dominant Period -
Negative Phase (suction/rarefaction)
Ballistic Wave
NOTE More complicated waveforms are also
possible e.g. fragmentation
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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?

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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.

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The All-Sky Network Process
!
!
Detection Video frames time logged
_at_ Set Time Upload detections to Main Server via
internet
Main server correlates individual camera events
via observation times.
EMAIL Total detections 35 Meteor
detected 034210 UT Cam 01, 02
Events and statistics of nights observations are
provided to user via e-mail.
Requests each camera for Multi-station event raw
frame data
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1-2. Meteor Identification-Reduction
Camera 1 - UWO
Camera 2 - ELFO
Camera 1 - CMOR
SOMN 20060419c Start 42.7396N 81.2206W at
73.15 km End 42.6950N 80.7976W 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
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3-4. ELFO Candidate Infrasound Search

• 20060419c
• Arrive 071034.8 UT
• Time delay 4m 37.8 s
• Azimuth 144.5
• Trace Vel. 0.420 km/s
• Duration 0.5 sec
• ?p 0.137 0.048 Pa
• p2p 0.209 0.075 Pa
• t 0.113 0.031 sec
• F 9.3 2.0 Hz

Short duration, ballistic-looking waveform
(Classic Meteor Infrasound) 0.5 to 30 Hz
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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)
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Current Detections since Jan 06 Aug 08
From 1908-2000 the of confirmed instances of
lt10cm meteor infrasound was 1 Currently meteor
infrasound from cm-sized meteoroids are being
detected at a rate of 1/month by SOMN.
20060213 Confirmed RHE/SS 20070511 Confirmed MS
20060302 Probable SS 20070521 Unconfirmed
20060305 Confirmed MS 20070725 Confirmed MS
20060405 Confirmed MS 20070727 Confirmed MS
20060419 Confirmed MS 20070916 Probable MS (Poor Geometry)
20060419c Confirmed MS 20070917 Confirmed MS
20060805 Confirmed MS 20071004 Confirmed MS
20060813 Confirmed MS 20071004b Confirmed MS
20060901 Probable SS 20071021 Confirmed MS
20061021 Probable RHE/BE 20071130 Confirmed MS
20061101 Confirmed MS 20071215 Confirmed MS
20061104 Confirmed MS 20080325 Probable MS (TBD)
20061121 Confirmed MS 20080511 Probable MS (TBD)
20061223 Confirmed MS 20080520 Probable MS (TBD)
20070102 Confirmed MS 20080602 Probable MS (TBD)
20070125 Confirmed MS 20080612c Probable MS (TBD)
20070129 Probable RE/SS 20080623 Probable MS (TBD)
20070421 Confirmed MS 20080714 Probable MS (TBD)
20060213 Confirmed RHE/SS 20070511 Confirmed MS
20060302 Probable SS 20070521 Unconfirmed
20060305 Confirmed MS 20070725 Confirmed MS
20060405 Confirmed MS 20070727 Confirmed MS
20060419 Confirmed MS 20070916 Probable MS (Poor Geometry)
20060419c Confirmed MS 20070917 Confirmed MS
20060805 Confirmed MS 20071004 Confirmed MS
20060813 Confirmed MS 20071004b Confirmed MS
20060901 Probable SS 20071021 Confirmed MS
20061021 Probable RHE/BE 20071130 Confirmed MS
20061101 Confirmed MS 20071215 Confirmed MS
20061104 Confirmed MS 20080325 Probable MS (TBD)
20061121 Confirmed MS 20080511 Probable MS (TBD)
20061223 Confirmed MS 20080520 Probable MS (TBD)
20070102 Confirmed MS 20080602 Probable MS (TBD)
20070125 Confirmed MS 20080612c Probable MS (TBD)
20070129 Probable RE/SS 20080623 Probable MS (TBD)
20070421 Confirmed MS 20080714 Probable MS (TBD)
23 Confirmed 12 Probable (TBC) 1
Unconfirmed ---------------------------- 36
Total, and counting
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Meteor Observational Geometry
e.g. 20060419c
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• 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

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Observed Meteor Geometry
Bulk of meteor infrasound observations fall into
the BALLISTIC wave category, as predicted from
cylindrical blast wave theory. (ReVelle 1974,
1976)
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Meteor Ballistic Wave Observations
Classical Meteor Ballistic waves N-waves
Reverbatory-type Ballistic waves Double/Triple
Bangs
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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.
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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).
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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.
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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 ?p decreases with increasing source
  altitude
• Increased attenuation
• Conservation of energy as wave propagates into
  denser air

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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 ).
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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 _at_ 1 cm impacting Earth/year
  thats 661,000 cm-sized meteors producing
  infrasound/year.
• Thats 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
• Earths total surface area 510,000,000 km2 ?
  ELFO covers 0.024 of Earths surface
• This means 584,000 meteors at 1 10 cm producing
  infrasound globally over 34 months.
• Thats meteor infrasound at the surface every
  2min 33 seconds!

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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

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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 )

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Questions?
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THE END
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Period a strong function of velocity
20071004b 16.26 km/s Ro 2.4m
Observed Periods _at_ Surface
20060213 12.17 km/s Ro 4.5m
20071021 68.0 km/s Ro 5.7m
Theoretical period _at_ source