Unknown event at EXL

Research

Figure 1: Recording of the EXL infrasound array on 2007, Feb 05. The time axis zero time is 15:43:43.819 UTC. Click to enlarge.

The LOFAR-KNMI infrasound near Exloo (EXL) was constructed for the detection of infrasound. Infrasound are air-pressure fluctuations with frequencies below the human hearing threshold of 20 Hz. Amplitudes are in the range from 1/100 to 10's of pascals (Pa).

An unusual wave was measured on 2007, February 05 around 15h52m UTC (see: Figure 1). The average amplitude is 10.4 Pa; the duration is roughly 150 s. The low frequency signal is followed by a tail that consists of higher frequencies.

Although waves of this amplitude and frequency are more often detected, the waveform is exceptional because the amplitude increase is not followed or preceded by an amplitude decrease. Waves observed by microbarometers arrays, in general, have a longitudinal character, i.e. a single set or sequence of compressions and rarefactions.

Figure 2: Array processing results. From top to bottom: the best beam, direction-of-arrival or back azimuth, apparent velocity and Fisher ratio or signal-to-noise power ratio. Click to enlarge.

Figure 2 shows the results of array processing. Coherent signal is detected on the basis of the Fisher ratio that is related to the signal-to-noise power ratio, see the lower frame.
Traveltime differences over the array are used to determine the direction-of-arrival and apparent velocity, see the middle to frames. The direction-of-arrival equals zero when the energy appears from the north and 90 if coming from the east. The horizontal fraction of the propagation velocity is called the apparent velocity, i.e. the velocity as measured by the microbarometers on the earth's surface.
The top frame shows the best beam in red, which is the sum of the phase aligned traces. The individual time shifted traces are given in black.
The event characteristics that are resolved at maximum coherency are: 7.4 m/s and 13.0 deg.

The local wind was coming from the west to northwest and temperatures were about 5.5 deg C, shown are 10 minutes averages. The synoptic analyses show the passage of a cold front over the array between 06 and 12 UT. The cold front has moved further southward at 18 UT. No lightning occurred on February 05.

See an animation of the pressure wave traveling over EXL in low (3.7 Mb) or high (21.6 Mb) resolution.

Update 2007.02.20: Explanation by Dr. Douglas R. Christie, Research School of Earth Sciences - The Australian National University.
See Comments on the Long Period Event Observed on Feb. 5, 2007 at the EXL Infrasonic Array.