KNMI homepage Läslo Evers - A bright bolide over the North Sea

Infrasound from a bright bolide over the North Sea

1. DIA and meteoric infrasound

A bright bolide was reported by several observers in northwest Europe during the night of 2001, October 27. An origin time of 19h20m GMT was derived. Nuclear tests, sonic booms, volcanic eruptions and explosions generate infrasound within this frequency range and also meteor entries such as the bolide on 2001, October 27. The Deelen Infrasound Array (DIA) is configured with 16 KNMI microbarometers, see figure 1, and capable of measuring infrasound at frequencies between 0.002 and 40 Hz [Evers and Haak, 2001].

Figure 1: The Deelen Infrasound Array (DIA) consists of 16 KNMI microbarometers placed within an area of 2.2 km² at Airforce base Deelen. The instruments are represented by the red dots. Microbarometer number 07 was not operational during the meteor's entry.

Meteors generate infrasound during their entry in the Earth's atmosphere. Traveling at very high supersonic speeds (e.g. 35 km/s), meteors' Mach cones become cylinders and are therefore called line sources, see figure 2. Infrasound can also be generated at the end of the meteor's atmospheric trajectory. The high speeds combined with an increasing atmospheric density, while approaching the Earth, can lead to a thermal burst. These explosions can have yields similar to nuclear explosions in the order of kilotons TNT equivalent.

Figure 2: The Mach cone transforms to a cylinder with an increasing supersonic speed of the object. The speed increases from left to right as indicated by the red vector. The wave front's orientation becomes perpendicular to the propagation direction (light blue vector) with increasing speed, in other words the Mach angle becomes zero. This picture is a simplified version after [ReVelle, 1975]

2. Data analysis and detection

The infrasound recordings from the 15 operational instruments of DIA are given in figure 3. The data is band pass filtered between corner frequencies 0.5 and 10 Hz. The time axis zero time is 2001, October 27 19h35m11.4s GMT. Coherent energy appears between 40 and 80 seconds after 19h35m11.4s GMT. The signal has a slightly higher frequency contents than the surrounding noise. Furthermore, the amplitude of the coherent signal is higher than the noise level. The differential travel time of the signal across DIA can be up to 4.5 seconds, although this is not clearly visible because of the long time span plotted.

Figure 3: The recordings of the microbarometers. The time axis zero time is 2001, October 27 19h35m11.4s GMT. Coherent energy is visible between 40 and 80 seconds. Discrimination from the noise is on the basis of a slightly higher frequency contents and larger amplitude. The data is band pass filtered with corner frequencies between 0.5 an 10 Hz.

Event detection is done on the basis of coherency analysis [Smart and Flinn, 1971]. Increasing coherency corresponds to a possible infrasonic wave which traveled over the array and led to a higher signal-to-noise ratio. Figure 4 shows the result of the coherency analysis from 12 minutes of infrasound data. The time axis zero time is 19h28m47.4s GMT on 2001, 27 October. A significant increase in coherency is found around 440 seconds and between 0.1 and 3 Hz. The corresponding best beam also shows an increase in amplitude around 440 seconds. The highest coherency value (in red) of 60 is reached after 440 s at 0.6 Hz (see red circle). Other coherent energy is present throughout the recording at lower frequencies, around 0.2 Hz. This energy appears to come from the Atlantic ocean. Storm depressions on the ocean cause standing waves which through their atmospheric coupling radiate infrasound, these are so-called microbaroms.

Figure 4: Coherency analysis of 12 minutes of infrasound data. The time axis zero time is 19h28m47.4s GMT on 2001, 27 October. In the lower frame coherency is contoured as a function of time and frequency of the infrasonic energy. High coherency values correspond to possible infrasonic events. Low coherency values correspond to noise, mainly wind. The top frame shows the best beam, sum of the 15 aligned traces, for the highest coherency value. Coherent energy with the highest coherency value is found between 0.1 and 3 Hz around 440 seconds (red circle). Other coherent energy appears at lower frequencies and belong to microbaroms.

The coherent energy, which traveled over the microbarometers configuring DIA, is more exactly located in figure 5. The white slowness vector points to the normalized spectral maximum and resolves the meteor's source characteristics. The angle with respect to the North is called back azimuth and has a value of 296 degrees. The length of the vector gives the apparent sound velocity of the infrasonic waves, being 350 m/s.

Figure 5: The result of a high resolution frequency-slowness analysis. The infrasonic energy arrived at DIA with a dominant frequency of 0.6 Hz around 19h35m30s. The white vector pointing to the normalized spectral maximum, resolves a back azimuth with respect to the North of 296 degrees with an apparent sound speed velocity of 350 m/s (length of the vector).

The traces are aligned following the resolved event characteristics. Doing so, differential travel times over the array become 0 seconds. The aligned traces are summed to derive the best beam (see figure 6). The best beam has a higher signal-to-noise ratio than the individual traces, since incoherent signal (noise) doesn't add up constructively. The best beam is the best representation of the infrasonic wave. Several clear waveforms can be seen in the best beam in figure 6, e.g between 50 and 55 seconds.

Figure 6: The recordings of the microbarometers aligned following the resolved apparent sound velocity of 350 m/s and back azimuth of 296 degrees. The traces represent the meteor's infrasound with 0 seconds differential travel times over DIA. The aligned traces are summed to derive the best beam, the trace in red. The summation increases the signal-to-noise ratio. Incoherent energy (noise) will not contribute to the best beam, while coherent energy will positively add up to higher amplitude values.

3. Identification of the meteor

3.1 Cross bearing results

The infrasonic energy appears to come from the northwest, as follows from the coherency analysis combined with frequency-slowness analysis. As described, the meteor's infrasonic energy can either be considered as coming from line source or as a more or less point source when the thermal burst is considered. Therefore, resolving an exact point in time and space can only be done with respect to the described simplifications.
Colleague Alexis le Pichon from the Commissariat à l'Energie Atomique (France) reported infrasound from the bolide at their infrasound array in Flers. During three minutes, starting at 19h52m02s, infrasound was recorded with an average back azimuth of 26 degrees. Figure 7 displays the results from cross bearing the DIA and Flers back azimuths. The infrasonic energy appears to have come from longitude 2.9 degrees and latitude 52.9 degrees.

Figure 7: Map displaying the meteor's location from which the infrasound was emitted (brown star) above the North Sea and the location of DIA and Flers (red diamonds). The resolved back azimuths are plotted as red lines. DIA recorded the energy with a back azimuth of 296 degrees at 222 km distance. The infrasound appeared in Flers with a back azimuth of 26 degrees with a source receiver distance of 518 km.
The contribution of Alexis le Pichon from the Commissariat à l'Energie Atomique is acknowledged.

3.2 Comparison with independent observations

The Dutch Meteor Society (DMS) has received detailed observations from the fireball and reconstructed a path of meteor through the atmosphere. Figure 8 shows this reconstruction as green vector, constructed of observations at the red squares. The resolved azimuths of DIA and Flers and cross bearing fit the reconstruction. The comparison of the two independent methods gives confidence in the resolved location.

Figure 8: Map showing the results of the reconstruction done by the Dutch Meteor Society (green line) from several observations (red squares). The bearings from DIA and Flers and resolved meteor's location fit the DMS path.
The contribution of the Dutch Meteor Society is acknowledged.

References

Evers, L.G., and H.W. Haak, Listening to sounds from an exploding meteor and oceanic waves, Geoph. Res. Lett., 28, 41-44, 2001.

ReVelle, D.O., Studies of sounds from meteors, Sky and Telescope, 49, 87-91, 1975.

Smart, E., and E.A. Flinn, Fast frequency-wavenumber analysis and Fisher signal detection in real-time infrasonic array data processing, Geoph. J. R. Astron. Soc., 26, 279-284, 1971.

October 2001
Läslo Evers