On the origin of thunder: Reconstruction of lightning flashes, statistical analysis, and modeling

Thunder from 27 natural lightning flashes of three thunderstorms has been recorded in 2012 in Southern France in the 0.1–180 Hz frequency bandwidth, using a 50 m-wide triangular array of 4 recalibrated microphones in the 0.3–20 km distance range from lightning. Source reconstruction allows to separate, within the acoustical signal, Cloud-to-Ground (CG) from Intra-Cloud (IC) parts of the discharge. The possibility to separate nearby CG events is shown. A total of 36 CG signals and associated spectra is obtained, along with some IC signals. The combination of reconstruction, separation, and frequency analysis provides new insights on the origin of thunder. Thunder infrasound is shown unambiguously to originate dominantly from return strokes. Spectra of CGs and ICs are similar, but of higher amplitude for CGs. No sharp frequency peaks can be clearly evidenced. The influence of distance, therefore of propagation effects, is pointed out. Best fits of energy and frequency gravity center dependence with distance are in agreement with a nonlinear line source propagation. A link between acoustic energy and impulse Charge Moment Change (iCMC) is also indicated. Lightning is modeled as a randomly tortuous line source, and the resulting spectra are compared to observations.