Bolide over Bavaria (Germany)
By Läslo Evers
The Deelen Infrasound Array (DIA) recorded
energy from a huge bolide above Bavaria (Germany). Very bright flashes from
the bolide were observed by several people. A photograph is displayed in figure 1.
Figure 1: A photograph of the bolide as made by the Deutschen
zentrum für Luft- und Raumfahrt (DLR).
Figure 2 shows the results of time domain Fisher analysis of DIA's infrasound
data. An apparent sound speed of 365 m/s coming from 135 deg are resolved,
at maximum signal likelihood around 460 seconds.
Figure 2: Results of the analysis of infrasound data. The time
axis zero time is 2002, April 06 20h55m38.9s GMT. The lower frame
shows signal likelihood (Fisher value) as function time. The top
frame displays the best beam in the direction of the signal with
maximum likelihood. The two middle frames show the resolved apparent
sound speed and back azimuth at maximum Fisher value. Around 460 seconds
a strong increase in Fisher value is found, corresponding to the
arrival of infrasound from the bolide (see the top frame). Resolved
back azimuth and apparent sound speed are respectively 135 degrees
and 365 m/s. The calculated Fisher value of 15.5 corresponds to a
signal-to-noise ratio of 0.9. The data in this analyis are bandpass
filtered between 0.05 and 2.0 Hz.
The best beam of the bolide has a dominant frequency content of 0.3
Hz, see figure 3.
Figure 3: Coherency analysis of the infrasound signal. Maximum
coherency, of the signal traveling over DIA, is found around 0.3 Hz
at 460 seconds. The top frame shows the corresponding best beam in the
direction of the bolide (i.e. 135 degrees). The data
are bandpass filtered between 0.05 and 2.0 Hz.
The event was beautifully recorded by the nearby infrasound array
IS26.
Results of the Fisher analysis for IS26 are shown in figure 4.
Figure 4: Fisher analysis of the IS26 data. The bolide's energy arrives
from 234.5 degrees with an apparent sound speed of 328.8 m/s. The data
are band pass filtered between 0.1 and 5 Hz. The time axis zero time is
2002, April 06 20h19m59.0s GMT.
Localisation of the bolide through cross bearing is done by combining the
observations from DIA and IS26. Figure 5 shows the observed bearings in red
and an estimation of the wind corrected bearings in grey. The bolide's location
is given by the light blue star.
Figure 5: The location of the bolide as found through cross bearing. The
observed bearings are in red and an estimate of the wind corrected bearings
is given in grey. The light blue star gives the estimated location of the bolide.
April 2002
Läslo Evers