Mechanism of lightning-associated infrasonic pulses from thunderclouds

It has been pointed by C. T. R. Wilson in 1920 that sudden reduction of the electric field inside a thundercloud immediately following a lightning discharge should produce an infrasound signature. In the present work, a model based on linearized equations of acoustics with classical viscosity and atmospheric gravitational stratification effects is employed to study electrostatic production of 0.1-1-Hz infrasonic waves from thunderclouds, with particular emphasis on the still poorly understood initial compression phase of the observed infrasonic waveforms. It is demonstrated that a growth of charge density in thundercloud prior to lightning discharge on time scales on the order of 2 to 6 s, comparable with typical documented time scales of generation of charge in thunderclouds, leads to formation of a pressure reduction in the thundercloud. This is accompanied by emission of compression waves closely resembling those observed in experiments prior to arrival of a rarefaction pulse generated in accordance with the electrostatic mechanism originally proposed by C. T. R. Wilson and further developed by Dessler in 1973 and Few in 1985. The arguments advanced in the present study agree with the ideas of Bohannon and collaborators formulated in 1977 indicating a rapid intensification of the field prior to the lightning discharge. However, we provide a quantitative demonstration that the intensification does not need to be as fast as 0.5 s proposed by these authors.