Localization of sound sources in temperature inversion layer during a geomagnetic storm

The sounds that occasionally accompany the bright and active northern lights - the aurora borealis, have been studied in a small project called Auroral Acoustics at the former Helsinki University of Technology (2000-2009) and the Aalto University (since 2010). In September 2011, the project succeeded in localizing sound sources in the open sky some 70 meters above the ground. On March 17-18, 2013 during an intense auroral display over southern Finland, hundreds of short duration sound events were recorded with a threemicrophone array accompanied by a loop antenna for magnetic field measurements. In sixty cases the sound source was estimated to be directly above the microphone array. A pulse in the magnetic field preceded each of the strongest sounds, indicating an impulsive current in the vicinity of the sound source. Rank correlation between the RMS values of the magnetic pulses and the sounds showed a 99.9% probability that the sounds had a causal relation to discharging currents (possibly corona) about 75 meters above the ground. Concurrent atmospheric measurement data also show that a temperature inversion layer was located at the same altitude. This paper presents detailed analysis of one sound type in the March 2013 data. The inversion layer hypothesis (published in 2016) is described and tested by fitting a model to the data. The obtained results support the hypothesis and estimates that the altitude of the active inversion layer on that evening was at 78-80 meters. Moreover, the analysis revealed a new noise producing process that is associated with those low frequency sound events in both audio and magnetic field (VLF) components. This noise producing process likely takes place in the inversion layer, too.