First images of thunder: Acoustic imaging of triggered lightning

Abstract

An acoustic camera comprising a linear microphone array is used to image the thunder signature of triggered lightning. Measurements were taken at the International Center for Lightning Research and Testing in Camp Blanding, FL, during the summer of 2014. The array was positioned in an end‐fire orientation thus enabling the peak acoustic reception pattern to be steered vertically with a frequency‐dependent spatial resolution. On 14 July 2014, a lightning event with nine return strokes was successfully triggered. We present the first acoustic images of individual return strokes at high frequencies (>1 kHz) and compare the acoustically inferred profile with optical images. We find (i) a strong correlation between the return stroke peak current and the radiated acoustic pressure and (ii) an acoustic signature from an M component current pulse with an unusual fast rise time. These results show that acoustic imaging enables clear identification and quantification of thunder sources as a function of lightning channel altitude.

Acoustical Measurement of Natural Lightning Flashes: Reconstructions and Statistical Analysis of Energy Spectra

“…In particular, events 2 and 5 located about 300 m from the acoustic array provide, to our knowledge, the closest measurement of natural thunder spectra. Acoustic imaging of a single‐triggered lightning producing nine CGs was recently achieved by Dayeh et al () at about 100 m from the lightning ground impact in the range [20 − 20,000] Hz. Comparing their frequency spectrum (their Figure 3d) to our spectra for events 2 and 5 in the common frequency range [20 − 180] Hz, one observes a relatively flat behavior.…”

“…Signal spectrograms were also shown but were limited to the range of [0.1 − 40] Hz. Other spectral analyses were provided at long distances (up to 100 km) in the infrasonic range (<10 Hz) by Assink et al () and Farges and Blanc (), thus deducing that (we quote Farges & Blanc, ) “most of infrasound from lightning is probably produced by the same mechanism as audible thunder.” Dayeh et al () presented wide band spectrum from a single‐triggered lightning recorded at 100 m of the microphone array, showing a flat spectrum below 100 Hz. Finally, the most recent thunder study is proposed by Haney et al () who analyzed lightning occurring during two explosive eruptions at Bogoslof volcano, Alaska.…”

Characteristics of acoustic response from simulated impulsive lightning current discharge

“…The distributions describing lightning discharges from different acoustic detection positions were explored and compared with electromagnetic detection [8]. Dayeh et al obtained the acoustic images of the triggered lightning based on the near-field acoustic array at the International Center for Lightning Research and Testing, Florida, USA [9]. There are few reports on the characteristics of lightning acoustic waveforms and their quantitative correlation analysis of lightning discharge intensity.…”

Introduction to lightning detection

“…The transport of electrons with an average total charge of ~20C in a lightning channel with a diameter of ~1–3 cm causes a substantial heating of the air molecules up to temperatures ~20 000–30 000 K. The rapidly expanding air emanating from the plasma channel results in a shock wave that is perceived at distances up to ~20 km as audible thunder (Teer and Few, ; Johnson et al ., ). Nearby CG discharges generate thunder with a ‘clap’ sound, such that the discharge can be imaged with an acoustic camera (Dayeh et al ., ), a technology that was originally developed to study the vibrations of car and aircraft chassis. The sound waves from distant lightning discharges exhibit significant dispersion and scatter from topographic features, which results in a deep rumbling sound that extends from the audible part of the spectrum down to infrasound frequencies <20 Hz (Farges and Blanc, ).…”