Lightning and its generating mechanism
Thunder has been described for more than two thousand years, but it was not until 1963 that Malan (1963) first described the sound of nearby lightning in modern terms. Latham (1964), Nakano and Tacuti (1970), Uman and Evans (1977) have all measured thunder. The general description of lightning is that when lightning strikes within the range of 100m from the observer, the sound first shows a "click", then a whip-like crack, and finally becomes a continuous thunder rumble. Ma Lan (1963) thinks that the "click" sound is caused by the upward pilot discharge of the main wiring on the ground. The popping sound is caused by the shock wave generated by the part of the return channel closest to the observer. The rumble comes from the upper part of the curved discharge channel. When the lightning strike point is hundreds of meters away from the observer, before the first clap, the first sound heard by the human ear is similar to the sound of tearing cloth, which lasts for nearly one second, and then there is a loud clap. The sound of tearing cloth comes from (1) vertical discharge channel, and its length is close to the distance from the observer. (2) Multiple connections from bottom to top lead this process. Hill (1977) once chose seven of the twelve facts about thunder summarized by Ramilad (1960):
(1) Cloud lightning usually produces the most thunder.
(2) Thunder can only be heard occasionally ten miles away.
(3) The lightning distance can be estimated by the time interval between seeing lightning and hearing the first thunder.
(4) Atmospheric turbulence will reduce the audibility of thunder.
(5) It rained cats and dogs immediately after the heavy thunder.
(6) The intensity of thunder seems to vary from place to place.
(7) As the rumble continues, the tone of thunder becomes deeper.
As we all know, the propagation speed of sound in air is about 330m/s, the propagation speed of light is 3× 108m/s, and the channel development speed is above105 m/s. Therefore, the distance between the nearest lightning channel and the observer can be roughly estimated by using the time difference between the arrival of sound and light. For example, if the acousto-optic difference to the observer is 10s, the distance from the nearest lightning channel to the observer is 330m/s×10s = 3.3km. This method is often used for field observation.
So, how is the thunder formed? According to the generally accepted theory of the cause of lightning, the lightning that people can hear originates from the high-voltage shock wave generated by the initial rapid expansion of the lightning channel, and degenerates into sound waves at a long distance. The spectral analysis of the return channel shows that the temperature of the return channel will reach 30000K in less than 10 μ s, and the pressure in the channel will rise rapidly due to the increase of temperature because there is not enough time to make the particle concentration in the channel change significantly. The average channel pressure in the first 5μs can reach 10 bar. The overpressure of this channel will lead to a strong shock wave and make the channel expand rapidly.
Abramson et al. (1947) first pointed out theoretically that when spark breakdown and temperature rise occur in gas, plasma will suddenly expand with shock wave. On this basis, an analytical method for solving hydrodynamic problems under the ideal condition of instantaneous energy release along an infinite narrow line source is developed. This analysis method was subsequently extended by Drabkina( 195 1) to the case that energy gradually accumulates in the breakdown channel. Later, this theory was further extended by Braginskii( 1958) and applied to lightning. Sakurai (1953) and Hayashi (1954) gave similar analytical solutions of instantaneous energy release along an infinite narrow line source.
The perfect description of lightning channel growth involves many factors, such as radiation transmission, initial conditions in the channel before the main return current, time distribution of input current, physical characteristics such as the conversion of electric energy into heat energy in channel plasma, channel loss, and geometric characteristics such as channel length and bending. Although trautmann (1969), Colgate and McKee (1969), Hill (197 1), Proust (197 1a) and a few (/kloc). 198 1) have tried to discuss the channel growth closer to the lightning channel, but so far all the processing methods only consider the symmetrical distribution of initial energy in the cylinder, and have not tried to simulate the real curved lightning channel. But for the finite line source, all the results confirm that when the lightning channel gathers extremely high energy per unit length, it will produce overpressure shock wave.
A few (1969, 198 1) suggest that the power spectrum of lightning has the characteristics of spherical symmetric expansion shock wave. It is assumed that the average lEngth of a short channel which is a "point source" is equal to 3/4 times of the characteristic radius R0 of the channel, where R0=(En/πP0) 1/2, where en is the energy dissipation per unit length of the channel and P0 is the ambient pressure. The frequency of the maximum power spectrum FM = 0.63C0 (P0/e), where C0 is the speed of sound.
Although there are not enough experiments on shock wave propagation caused by lightning, Holmes et al. (197 1A), Dawson et al. (1968) and Uman et al. (1970) measured the attenuation of shock wave caused by long spark discharge in the laboratory, and the experiments basically confirmed the above shock wave theories.
Different from the thermal channel mechanism that produces the above-mentioned audible thunder, infrasound may be related to the relaxation of electrostatic field in the cloud caused by lightning changing the charge distribution of the cloud (less, 1985). In fact, up to now, although there is a physical model to describe the mechanism of these two processes, what is the direct evidence of these two mechanisms, how these two mechanisms contribute to the observed mine pressure changes, and so on. , have not been solved.
Reconstruction of lightning channel by lightning
If three or more microphones not in a straight line record the main features of thunder at the same time, the position of the sound source can be determined by the acousto-optic difference reaching each microphone. There are usually two different methods. The more accurate method is ray tracing, which can give multiple sound source points in a lightning event, thus reconstructing the lightning discharge channel. In this way, the distance between microphones is relatively close, generally tens of meters. The direction of incident sound wave can be determined by the time difference between the main characteristics of sound wave and each microphone, and then the position of discharge source can be determined by mathematical regression of directional ray through the acousto-optic difference between lightning and microphone array. The reconstruction of lightning discharge channel by this method can be found in the articles of Shaohe Teer (1974), Nakano (1976) and MacGoman et al. (198 1).
Another acoustic positioning method is called thunder ranging. In this method, the three microphones are far apart, generally in the order of kilometers, and the measured position generally has a large error. According to the minority theory (198 1), when an acoustic signal reaches two microphones with a distance of more than 100m, it will become irrelevant due to different propagation paths, but some rough features are still relevant at two microphones with a distance of kilometers. For mine explosion, the acousto-optic difference reaching a station can be used to determine the spherical surface of the energy location. The intersection of three spheres obtained by three microphones is the location where the thunder occurs. The article by Uman et al. (1978) can be used to reconstruct the lightning channel in this way.