Sprite-attributed infrasonic chirps—their detection, occurrence and properties between 1994 and 2004

During the last 10 years, infrasonic chirps in the frequency range 0.5–8 Hz were occasionally observed by arrays belonging to the Swedish Infrasonic Network (SIN). These chirps have been attributed to certain types of thunderstorm activity associated with the high-altitude discharges, sprites [Liszka, L., 2004. On the possible infrasound generation by sprites, Journal of Low Frequency Noise, Vibration and Active Control 23, 85–93]. A method for automatic detection of chirps in the recorded data has been developed and applied to 10 years of data from two arrays belonging to the SIN: Jamton and Lycksele. The temporal and directional distribution of the phenomenon is described. Also, long-term variability and possible relation to the solar cycle is studied.