Identification of infrasound produced by sprites during the Sprite2003 campaign

Abstract

During the summer of 2003, complementary measurements were taken in Europe during the Sprite2003 campaign to study sprites and associated processes. On July 21, 28 sprites observed by light‐sensitive optical cameras located at the Observatoire du Pic du Midi in the Pyrenees mountains were found to correlate with chirp‐like signals of several tenth to few minutes duration, measured by an infrasound station at Flers about 400 km from the thunderstorm. The sprite activity identified by the infrasound signature continued past sunrise into the daytime when optical systems cannot be used, documenting the occurrence of daytime sprites. Using an acoustic wave propagation model, the observed propagation delays, frequency dispersion, and duration of the infrasound bursts are found to be consistent with source altitudes at 60–80 km with horizontal dimensions equal to the optical width of the sprites. The chirp‐like dispersion is a result of the propagation properties in the earth‐thermosphere waveguide.