Infrasonic pulses from thunderstorms

No page 53Acoustic data obtained from thunderstorm observations during 1975 at Langmuir Laboratory near Socorro, New Mexico and during 1976 at the J. F, Kennedy Space Center in Florida in conjunction with the Thunderstorm Research International Program (TRIP-76) contain many thunder records that exhibit infrasonic pulses superimposed on the higher frequency thunder signals. Only the Langmuir data were analyzed to determine frequency and amplitude. These data show infrasonic pulses with periods of about .5 sec and amplitudes of about .1 N/m. The pulse waveform is characterized by an initial compression followed by a rarefaction. Acoustic source reconstruction of both the 1975 and the 1976 data place the origins of these pulses in the cloud within essentially the same volume of space as the horizontal portions of the lightning channels. This volume is presumably the storage region for the charge that creates the lightning event. The pulses are not produced by localized heating from the main channel of the lightning flash because the pulses never were observed to come from the cloud-to-ground portion of the channel. If the pulses are wholly electrostatic in origin as proposed by the electrostatic collapse model, then to account for the initial positive pulse one must assume a rapid (.25 sec) increase in the field occurring just prior to discharge. This is a consequence of the pressure amplitude of the infrasonic pulse being directly proportional to the square of the change of the electric field