Meteors are sometimes followed by certain acoustic sounds. These sounds can be divided into two groups: normal sounds (e.g. hypersonic booms) and electrophonic sounds (anomalous sounds or electrophones). The normal sound is an acoustic wave generated by the meteor airburst in the lower atmosphere, which propagates at the speed of sound. This is the reason why we hear it a few minutes after the appearance of the meteor. Electrophonic sounds are generally heard simultaneously with the meteor’s appearance. These sounds, therefore, cannot be explained by an ordinary acoustic propagation. Normal sounds have been extensively studied for several decades. In contrast, electrophonic sounds have been scarcely studied due to their non-intuitive nature and extreme rareness of the phenomenon.
The physical explanation of electrophonic sounds infers an emission of the electromagnetic waves from a fireball with the frequency of audible sound (20 Hz to 20 kHz, ELF-VLF spectral region). These waves propagate at the speed of light which means that they reach the observer without notable retardation. Finally, the sound is created by coupling the electromagnetic energy at audio frequencies with objects on the ground.
The main goal of our project is to search for correlations between meteors in the visual spectral region, and the ELF/VLF observations from our antennas.
Our instrumental setup is situated at the Tičan Observatory. With the goal to cover the whole spectral region between ELF and VLF, we use 2 different antennas. The spectral region between 3 and ~1000 Hz are covered with the “Schumann antenna”. It is a E-field ball antenna, with a ball diameter of 0.85-m, isolated from the ground on the height of about 4.4 m. The second antenna which we use is an E-field VLF receiver. It looks just like a long wire, and is covering the spectral region between 1 and ~10 kHz