Mechanism of inverted‐chirp infrasonic radiation from sprites

Abstract

Farges and Blanc (2010) reported inverted‐chirp infrasonic signals with high frequencies arriving before low frequencies, possibly emitted by sprite discharges and observed on the ground at close range (<100 km) from the source. In the present work a parallel version of a 2‐D FDTD model of infrasound propagation in a realistic atmosphere is applied to demonstrate that the observed morphology of infrasound signals is consistent with general scaling of diameters of sprite streamers inversely proportionally to the air density. The smaller structures at lower altitudes radiate higher infrasonic frequencies that arrive first at the observational point on the ground, while the low frequency components are delayed because they originate at lower air densities at higher altitudes. The results demonstrate that strong absorption of high frequency infrasonic components at high altitudes (i.e., ∼0.2 dB/km for 8 Hz at 70 km) may also contribute to formation of inverted‐chirp signals observed on the ground at close range.