Sprite-generated Infrasonic Chirps

Sprite-generated infrasonic chirps were recorded at infrasonic arrays belonging to the present Swedish-Finish Infrasound Network during the past 13 years. The present study is concentrated on morphology of the phenomenon, in particular on its temporal and spatial distributions. The occurrence of chirps reveals a 3.2-year variation, which is in phase with the corresponding component of solar activity. Daily- and annual variations indicate that the phenomenon does not necessarily coincide with the thunderstorm activity. In addition to a conventional wavelet analysis the chirps are analyzed using a set of 6 wavelet filters covering the frequency band 0.5 to 6 Hz. A representation of the chirp in the angle-of-arrival - frequency domain describes both the source and the influence of the atmosphere on the chirp signal. It provides the information about the horizontal extent of the source. The angle-of-arrival - frequency representation may be used for categorization of the phenomenon. Also the horizontal trace velocity of the chirp signal is studied in the frequency domain. It provides the information about the angle of incidence of the ray at different frequencies and thus about the vertical extent of the source.