Analysis of clap sounds recorded during the september 9-10 2011 geomagnetic storm

Strong geomagnetic storms accompanied with bright and lively aurora may produce audible sounds perceivable at ground level. However, the physical origin of these emissions is not yet known. In order to understand the phenomenon, sound recordings with simultaneous electric or magnetic field measurements have been performed at different locations in Finland during geomagnetic storms since 2000. One such measurement was performed on September 9, 2011 near the village of Fiskars in southern Finland. The main goal was to test out new instruments and to demonstrate the measurement procedure to a colleague from the University of Graz, Austria. Signals were stored in WAV format using a Zoom H4n four-channel audio recorder. One stereo signal was recorded from the internal microphones of the Zoom unit while the second stereo signal consisted of an audio signal from a separate B&K low-noise measuring microphone mounted at the focal point of a parabolic reflector. The latter signal was recorded on one channel while the other channel consisted of a VLF antenna signal. The total duration of the recording was 130.6 minutes and covered a time interval approximately from 19 to 21 UTC. Twenty-one short sound events, clearly discernible from the background noise, were detected afterwards in the recording. The best ten were selected for closer analysis. These were the loudest events (claps) and exhibited relatively high mutual cross-correlations and a low background noise. Three of these events were detected with all three microphones. This is the first time during our studies when similar sound events were recorded simultaneously with multiple microphones during a geomagnetic storm. Knowing the separation distance of the microphones (B&K vs. Zoom) it was possible to estimate the direction of the sound source. The synchronously recorded VLF signal provided data to estimate the distances of the sound sources assuming that a certain magnetic field event is associated with a certain sound event. Three independent methods to estimate distance to the sound sources yielded an average value of 70 meters. This outcome, together with the direction estimate, indicates that the average altitude of the sound sources measured from ground was only 60-70 meters.