Observations of the Norwegian Fireball of June 7th, 2006

 

 

 

Synopsis

Photographic Images

Short Range Infrasound

Long Range Infrasound

Local Seismic Observations

Location

Meteorite Recovery Efforts

 

 

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Synopsis

 

On June 7th, 2006 just after 00 UT  (2 am Local time) a moderate sized meteoroid impacted the Earth’s atmosphere over the northern regions of Norway, near the town of Kautokein by the Norwegian-Finland border. Soon after the fireball, the remaining dust trail was photographed by two individuals. Throughout the region the sound produced by the fireball’s decent (terminal burst/flare?) was heard, with individuals commenting on its likeness to an explosion. This sound was observed to propagate across neighbouring Sweden by the Swedish Institute of Space Physics, IRF Umeå, infrasound network. This low frequency sound was observed as far south as Freyung, Germany and as far East as Aktyubinsk, Kazakhstan. Locally the sound coupled into the ground and was observed seismically up to ~150km away at the NORSAR ARCES seismic array and two stations (KTK1, TRO) operated by the Institutt for geovitenskap, Universitetet i Bergen. An estimate of the kinetic energy of the event (based on long range infrasound observations) is 0.3 ± 0.2 kt of equivalent TNT (1 kt = 4.185 x 1012 Joules).

 

Using IRF infrasound array back azimuth intersections, a location for the sound source of:  68.860 oN,  22.204 oE  is found.

 

Using long range observations, CTBTO stations I26DE (Germany) and I31KZ (Kazakhstan), a source location of:  69.782 oN,  24.246 oW is found.

 

Using the seismic peak arrival times, the initial infrasound observation at Kiruna and assuming the observed signals correlate to the fireball’s terminal flare (point source) a 4D position is found for this point which lies at: 69.203 oN, 22.396 oE at 43 km altitude occurring at ~00:06:28 UT.

 

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Photographic Images of the Dust Trail

 

Photo by: Peter Bruvold (http://www.aftenposten.no/english/local/article1346411.ece)

 

Photo by: Martin Lyngdal (http://www.astro.uio.no/ita/nyheter/ildkule06/ildkule06.html)

 

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Short Range Infrasound

 

 

As the closest infrasonic (sub-acoustic frequencies <20 Hz) instruments to the event the airwaves produced by the fireball were particularly strong. At the closest station Kiruna the signal is so large in amplitude that it saturated the instrument and so the initial part of the observation is clipped. The same is the case for the largest amplitude portions at Jamton and Lycksele as well. Three distinct pulses are clearly visible as they propagate across the length of Sweden. Each of the instruments consist of three separate field microphones separated by 75m. Cross correlating the signals as recorded by all three microphones (beamforming) allows the direction from which the sound came from to be determined. Intersections of these projected directions can be used to estimate the position of the sounds source, the June 7th, 2006 Norwegian Fireball.

 

Beamforming Azimuths (degrees measured East from North)

 

Kiruna:

45 deg. (clipped portion of signal)

23.9 – 31.7 deg (unclipped portion of signal)

Jamton:

                        1.74 – 353.6 deg.

 

Lycksele:

                        14.63 – 17.93 deg.

 

Uppsala:

                        9.1 – 9.97 deg.

 

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Long Range Infrasound

  

 

As part of the International Monitoring System (IMS) for the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) a series of 60 infrasonic microbarometer arrays are distributed around the world. Two of these instrument arrays also recorded the low-frequency sound from the June 7th fireball, I26DE in Freyung, Germany and I31KZ in Aktyubinsk, Kazakhstan. Nearly all of the higher frequencies (>3 Hz) seen in Swedish IRF data, have attenuated away and only the lowest frequencies (0.25 – 3 Hz) remain at these large distances. Similar Beamforming procedures are possible at each of these sites.

 

Beamforming Azimuths (degrees measured East from North)

 

I26DE:             9.96 deg.

I32KZ:             332.9 deg.

 

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Local Seismic Observations

 

 

 

As the airwaves (sound) produced by the fireball propagated to the ground they were intense enough to couple with the Earth’s surface, causing the ground to move slightly. This movement was recorded by three seismic instruments located nearby. At Tromso (TRO) and Kautokeino (KTK1) this appears as a dispersed pulse, with hints of possibly the three peaks seen in the Swedish IRF data. At the NORSAR seismic array (ARCES), a 25 element seismometer array laid out in a circlular pattern just over 1 km in diameter, a complex wavetrain was also detected from the fireball. A single pulse with low frequency content (1 – 6 Hz) may be associated specifically with the airwave arrival. Similar to the infrasound, beamforming procedures can be used on this seismic arrival as it passed over the array, doing this a back azimuth of 258.6 deg. is found.

 

 

Location of the Fireball in Norway

 

 

Using the azimuth intersections from the Swedish (IRF) infrasound data a location of 68.860 oN,  22.204 oE  is found (Burst 1 – IRF intersections) just over the Norway-Finland border. The azimuth intersection from the long range CTBTO infrasound data places this position further to the NW at 69.782 oN,  24.246 oW (Burst1 – CTBTO). An average between these two positions places the terminal point of the fireball to the north of the KTK1 seismic station. A separate method that uses the arrival times of the seismic observations (and the initial observation at Kiruna) along with a reconstructed atmosphere model from June 7th, allows a 4D location for the terminal point to be found in a similar way in which earthquakes are located in the solid Earth. This position lies at 69.203oN, 22.396oW at an altitude of 43 km occurring at ~00:06:28 UT.

 

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Meteorite Recovery Efforts  

For 7 days starting on June 14th, 2006 meteorite recovery efforts were attempted. Mike Mazur and a colleague, residents of Norway, travelled to the suspected fall area in an attempt to both calibrate photographs made by eyewitnesses (above) and recover potential meteorites from the June 7th fireball. Despite hiking several kilometres into the wilderness no and searching for several days, no meteorites were found. However the conditions present where meteorites may have fallen was assessed. The easiest terrain to recover material covered no more then 20% of the total area and consisted of exposed bedrock and short, ankle-high grasses and plants (see picture below). These areas it was accessed that only larger sized meteorites (several 100 grams or larger) would be have been able to be seen from a distance. The remaining 80% of the suspected recovery zone consisted of moderate-sized lakes and peat bogs, areas where, if had meteorites had fallen, they would be generally unrecoverable.

 

 

Mike Mazur: Searching the suspected meteorite strewn field in Northern Norway. This represents the most promising area for meteorite recovery.

Panoramic view of the area for meteorite recovery

 

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